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INTERNET-DRAFT Clifford Neuman
Obsoletes: 1510 USC-ISI
Tom Yu
Sam Hartman
Ken Raeburn
MIT
February 15, 2004
Expires 15 August, 2004
The Kerberos Network Authentication Service (V5)
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
The distribution of this memo is unlimited. It is filed as draft-
ietf-krb-wg-kerberos-clarifications-05.txt, and expires 15 August
2004. Please send comments to: ietf-krb-wg@anl.gov
ABSTRACT
This document provides an overview and specification of Version 5 of
the Kerberos protocol, and updates RFC1510 to clarify aspects of the
protocol and its intended use that require more detailed or clearer
explanation than was provided in RFC1510. This document is intended
to provide a detailed description of the protocol, suitable for
implementation, together with descriptions of the appropriate use of
protocol messages and fields within those messages.
February 2004 [Page 1]
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OVERVIEW
This document describes the concepts and model upon which the
Kerberos network authentication system is based. It also specifies
Version 5 of the Kerberos protocol. The motivations, goals,
assumptions, and rationale behind most design decisions are treated
cursorily; they are more fully described in a paper available in IEEE
communications [NT94] and earlier in the Kerberos portion of the
Athena Technical Plan [MNSS87].
This document is not intended to describe Kerberos to the end user,
system administrator, or application developer. Higher level papers
describing Version 5 of the Kerberos system [NT94] and documenting
version 4 [SNS88], are available elsewhere.
BACKGROUND
The Kerberos model is based in part on Needham and Schroeder's
trusted third-party authentication protocol [NS78] and on
modifications suggested by Denning and Sacco [DS81]. The original
design and implementation of Kerberos Versions 1 through 4 was the
work of two former Project Athena staff members, Steve Miller of
Digital Equipment Corporation and Clifford Neuman (now at the
Information Sciences Institute of the University of Southern
California), along with Jerome Saltzer, Technical Director of Project
Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other
members of Project Athena have also contributed to the work on
Kerberos.
Version 5 of the Kerberos protocol (described in this document) has
evolved from Version 4 based on new requirements and desires for
features not available in Version 4. The design of Version 5 of the
Kerberos protocol was led by Clifford Neuman and John Kohl with much
input from the community. The development of the MIT reference
implementation was led at MIT by John Kohl and Theodore Ts'o, with
help and contributed code from many others. Since RFC1510 was issued,
extensions and revisions to the protocol have been proposed by many
individuals. Some of these proposals are reflected in this document.
Where such changes involved significant effort, the document cites
the contribution of the proposer.
Reference implementations of both version 4 and version 5 of Kerberos
are publicly available and commercial implementations have been
developed and are widely used. Details on the differences between
Kerberos Versions 4 and 5 can be found in [KNT94].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
February 2004 [Page 2]
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Table of Contents
1. Introduction ................................................... 7
1.1. Cross-realm operation ........................................ 9
1.2. Choosing a principal with which to communicate ............... 10
1.3. Authorization ................................................ 11
1.4. Extending Kerberos Without Breaking Interoperability ......... 12
1.4.1. Compatibility with RFC 1510 ................................ 12
1.4.2. Sending Extensible Messages ................................ 13
1.5. Environmental assumptions .................................... 14
1.6. Glossary of terms ............................................ 14
2. Ticket flag uses and requests .................................. 17
2.1. Initial, pre-authenticated, and hardware authenticated
tickets ..................................................... 18
2.2. Invalid tickets .............................................. 18
2.3. Renewable tickets ............................................ 18
2.4. Postdated tickets ............................................ 19
2.5. Proxiable and proxy tickets .................................. 20
2.6. Forwardable tickets .......................................... 21
2.7. Transited Policy Checking .................................... 21
2.8. OK as Delegate ............................................... 22
2.9. Other KDC options ............................................ 23
2.9.1. Renewable-OK ............................................... 23
2.9.2. ENC-TKT-IN-SKEY ............................................ 23
2.9.3. Passwordless Hardware Authentication ....................... 23
3. Message Exchanges .............................................. 23
3.1. The Authentication Service Exchange .......................... 23
3.1.1. Generation of KRB_AS_REQ message ........................... 25
3.1.2. Receipt of KRB_AS_REQ message .............................. 25
3.1.3. Generation of KRB_AS_REP message ........................... 25
3.1.4. Generation of KRB_ERROR message ............................ 28
3.1.5. Receipt of KRB_AS_REP message .............................. 28
3.1.6. Receipt of KRB_ERROR message ............................... 29
3.2. The Client/Server Authentication Exchange .................... 30
3.2.1. The KRB_AP_REQ message ..................................... 30
3.2.2. Generation of a KRB_AP_REQ message ......................... 30
3.2.3. Receipt of KRB_AP_REQ message .............................. 31
3.2.4. Generation of a KRB_AP_REP message ......................... 33
3.2.5. Receipt of KRB_AP_REP message .............................. 33
3.2.6. Using the encryption key ................................... 34
3.3. The Ticket-Granting Service (TGS) Exchange ................... 34
3.3.1. Generation of KRB_TGS_REQ message .......................... 36
3.3.2. Receipt of KRB_TGS_REQ message ............................. 37
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3.3.3. Generation of KRB_TGS_REP message .......................... 38
3.3.3.1. Checking for revoked tickets ............................. 40
3.3.3.2. Encoding the transited field ............................. 41
3.3.4. Receipt of KRB_TGS_REP message ............................. 42
3.4. The KRB_SAFE Exchange ........................................ 43
3.4.1. Generation of a KRB_SAFE message ........................... 43
3.4.2. Receipt of KRB_SAFE message ................................ 43
3.5. The KRB_PRIV Exchange ........................................ 44
3.5.1. Generation of a KRB_PRIV message ........................... 45
3.5.2. Receipt of KRB_PRIV message ................................ 45
3.6. The KRB_CRED Exchange ........................................ 46
3.6.1. Generation of a KRB_CRED message ........................... 46
3.6.2. Receipt of KRB_CRED message ................................ 47
3.7. User-to-User Authentication Exchanges ........................ 47
4. Encryption and Checksum Specifications ......................... 49
5. Message Specifications ......................................... 50
5.1. Specific Compatibility Notes on ASN.1 ........................ 52
5.1.1. ASN.1 Distinguished Encoding Rules ......................... 52
5.1.2. Optional Integer Fields .................................... 52
5.1.3. Empty SEQUENCE OF Types .................................... 52
5.1.4. Unrecognized Tag Numbers ................................... 53
5.1.5. Tag Numbers Greater Than 30 ................................ 53
5.2. Basic Kerberos Types ......................................... 53
5.2.1. KerberosString ............................................. 53
5.2.2. Realm and PrincipalName .................................... 55
5.2.3. KerberosTime ............................................... 56
5.2.4. Constrained Integer types .................................. 56
5.2.5. HostAddress and HostAddresses .............................. 57
5.2.6. AuthorizationData .......................................... 57
5.2.6.1. IF-RELEVANT .............................................. 59
5.2.6.2. KDCIssued ................................................ 59
5.2.6.3. AND-OR ................................................... 60
5.2.6.4. MANDATORY-FOR-KDC ........................................ 60
5.2.7. PA-DATA .................................................... 61
5.2.7.1. PA-TGS-REQ ............................................... 62
5.2.7.2. Encrypted Timestamp Pre-authentication ................... 62
5.2.7.3. PA-PW-SALT ............................................... 62
5.2.7.4. PA-ETYPE-INFO ............................................ 63
5.2.7.5. PA-ETYPE-INFO2 ........................................... 63
5.2.8. KerberosFlags .............................................. 64
5.2.9. Cryptosystem-related Types ................................. 65
5.3. Tickets ...................................................... 67
5.4. Specifications for the AS and TGS exchanges .................. 74
5.4.1. KRB_KDC_REQ definition ..................................... 74
5.4.2. KRB_KDC_REP definition ..................................... 82
5.5. Client/Server (CS) message specifications .................... 85
5.5.1. KRB_AP_REQ definition ...................................... 85
5.5.2. KRB_AP_REP definition ...................................... 89
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5.5.3. Error message reply ........................................ 90
5.6. KRB_SAFE message specification ............................... 90
5.6.1. KRB_SAFE definition ........................................ 90
5.7. KRB_PRIV message specification ............................... 92
5.7.1. KRB_PRIV definition ........................................ 92
5.8. KRB_CRED message specification ............................... 92
5.8.1. KRB_CRED definition ........................................ 93
5.9. Error message specification .................................. 95
5.9.1. KRB_ERROR definition ....................................... 95
5.10. Application Tag Numbers ..................................... 97
6. Naming Constraints ............................................. 98
6.1. Realm Names .................................................. 98
6.2. Principal Names .............................................. 99
6.2.1. Name of server principals .................................. 101
7. Constants and other defined values ............................. 101
7.1. Host address types ........................................... 101
7.2. KDC messaging - IP Transports ................................ 103
7.2.1. UDP/IP transport ........................................... 103
7.2.2. TCP/IP transport ........................................... 103
7.2.3. KDC Discovery on IP Networks ............................... 104
7.2.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names ....... 105
7.2.3.2. Specifying KDC Location information with DNS SRV
records ..................................................... 105
7.2.3.3. KDC Discovery for Domain Style Realm Names on IP
Networks .................................................... 106
7.3. Name of the TGS .............................................. 106
7.4. OID arc for KerberosV5 ....................................... 106
7.5. Protocol constants and associated values ..................... 106
7.5.1. Key usage numbers .......................................... 107
7.5.2. PreAuthentication Data Types
............................................................. 108
7.5.3. Address Types
............................................................. 109
7.5.4. Authorization Data Types
............................................................. 109
7.5.5. Transited Encoding Types
............................................................. 109
7.5.6. Protocol Version Number
............................................................. 110
7.5.7. Kerberos Message Types
............................................................. 110
7.5.8. Name Types
............................................................. 110
7.5.9. Error Codes
............................................................. 110
8. Interoperability requirements .................................. 112
8.1. Specification 2 .............................................. 112
8.2. Recommended KDC values ....................................... 115
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9. IANA considerations ............................................ 115
10. Security Considerations ....................................... 116
11. Author's Addresses ............................................ 120
12. Acknowledgements .............................................. 121
13. REFERENCES .................................................... 122
13.1 NORMATIVE REFERENCES ......................................... 122
13.2 INFORMATIVE REFERENCES ....................................... 123
14. Copyright Statement ........................................... 124
15. Intellectual Property ......................................... 125
A. ASN.1 module ................................................... 125
B. Changes since RFC-1510 ......................................... 133
END NOTES ......................................................... 136
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1. Introduction
Kerberos provides a means of verifying the identities of
principals, (e.g. a workstation user or a network server) on an
open (unprotected) network. This is accomplished without relying
on assertions by the host operating system, without basing trust
on host addresses, without requiring physical security of all the
hosts on the network, and under the assumption that packets
traveling along the network can be read, modified, and inserted at
will[1]. Kerberos performs authentication under these conditions
as a trusted third-party authentication service by using
conventional (shared secret key [2]) cryptography. Kerberos
extensions (outside the scope of this document) can provide for
the use of public key cryptography during certain phases of the
authentication protocol [@RFCE: if PKINIT advances concurrently
include reference to the RFC here]. Such extensions support
Kerberos authentication for users registered with public key
certification authorities and provide certain benefits of public
key cryptography in situations where they are needed.
The basic Kerberos authentication process proceeds as follows: A
client sends a request to the authentication server (AS)
requesting "credentials" for a given server. The AS responds with
these credentials, encrypted in the client's key. The credentials
consist of a "ticket" for the server and a temporary encryption
key (often called a "session key"). The client transmits the
ticket (which contains the client's identity and a copy of the
session key, all encrypted in the server's key) to the server. The
session key (now shared by the client and server) is used to
authenticate the client, and may optionally be used to
authenticate the server. It may also be used to encrypt further
communication between the two parties or to exchange a separate
sub-session key to be used to encrypt further communication.
Implementation of the basic protocol consists of one or more
authentication servers running on physically secure hosts. The
authentication servers maintain a database of principals (i.e.,
users and servers) and their secret keys. Code libraries provide
encryption and implement the Kerberos protocol. In order to add
authentication to its transactions, a typical network application
adds calls to the Kerberos library directly or through the Generic
Security Services Application Programming Interface, GSSAPI,
described in separate document [ref to GSSAPI RFC]. These calls
result in the transmission of the necessary messages to achieve
authentication.
The Kerberos protocol consists of several sub-protocols (or
exchanges). There are two basic methods by which a client can ask
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a Kerberos server for credentials. In the first approach, the
client sends a cleartext request for a ticket for the desired
server to the AS. The reply is sent encrypted in the client's
secret key. Usually this request is for a ticket-granting ticket
(TGT) which can later be used with the ticket-granting server
(TGS). In the second method, the client sends a request to the
TGS. The client uses the TGT to authenticate itself to the TGS in
the same manner as if it were contacting any other application
server that requires Kerberos authentication. The reply is
encrypted in the session key from the TGT. Though the protocol
specification describes the AS and the TGS as separate servers,
they are implemented in practice as different protocol entry
points within a single Kerberos server.
Once obtained, credentials may be used to verify the identity of
the principals in a transaction, to ensure the integrity of
messages exchanged between them, or to preserve privacy of the
messages. The application is free to choose whatever protection
may be necessary.
To verify the identities of the principals in a transaction, the
client transmits the ticket to the application server. Since the
ticket is sent "in the clear" (parts of it are encrypted, but this
encryption doesn't thwart replay) and might be intercepted and
reused by an attacker, additional information is sent to prove
that the message originated with the principal to whom the ticket
was issued. This information (called the authenticator) is
encrypted in the session key, and includes a timestamp. The
timestamp proves that the message was recently generated and is
not a replay. Encrypting the authenticator in the session key
proves that it was generated by a party possessing the session
key. Since no one except the requesting principal and the server
know the session key (it is never sent over the network in the
clear) this guarantees the identity of the client.
The integrity of the messages exchanged between principals can
also be guaranteed using the session key (passed in the ticket and
contained in the credentials). This approach provides detection of
both replay attacks and message stream modification attacks. It is
accomplished by generating and transmitting a collision-proof
checksum (elsewhere called a hash or digest function) of the
client's message, keyed with the session key. Privacy and
integrity of the messages exchanged between principals can be
secured by encrypting the data to be passed using the session key
contained in the ticket or the sub-session key found in the
authenticator.
The authentication exchanges mentioned above require read-only
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access to the Kerberos database. Sometimes, however, the entries
in the database must be modified, such as when adding new
principals or changing a principal's key. This is done using a
protocol between a client and a third Kerberos server, the
Kerberos Administration Server (KADM). There is also a protocol
for maintaining multiple copies of the Kerberos database. Neither
of these protocols are described in this document.
1.1. Cross-realm operation
The Kerberos protocol is designed to operate across organizational
boundaries. A client in one organization can be authenticated to a
server in another. Each organization wishing to run a Kerberos
server establishes its own "realm". The name of the realm in which
a client is registered is part of the client's name, and can be
used by the end-service to decide whether to honor a request.
By establishing "inter-realm" keys, the administrators of two
realms can allow a client authenticated in the local realm to
prove its identity to servers in other realms[3]. The exchange of
inter-realm keys (a separate key may be used for each direction)
registers the ticket-granting service of each realm as a principal
in the other realm. A client is then able to obtain a ticket-
granting ticket for the remote realm's ticket-granting service
from its local realm. When that ticket-granting ticket is used,
the remote ticket-granting service uses the inter-realm key (which
usually differs from its own normal TGS key) to decrypt the
ticket-granting ticket, and is thus certain that it was issued by
the client's own TGS. Tickets issued by the remote ticket-granting
service will indicate to the end-service that the client was
authenticated from another realm.
A realm is said to communicate with another realm if the two
realms share an inter-realm key, or if the local realm shares an
inter-realm key with an intermediate realm that communicates with
the remote realm. An authentication path is the sequence of
intermediate realms that are transited in communicating from one
realm to another.
Realms may be organized hierarchically. Each realm shares a key
with its parent and a different key with each child. If an inter-
realm key is not directly shared by two realms, the hierarchical
organization allows an authentication path to be easily
constructed. If a hierarchical organization is not used, it may be
necessary to consult a database in order to construct an
authentication path between realms.
Although realms are typically hierarchical, intermediate realms
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may be bypassed to achieve cross-realm authentication through
alternate authentication paths (these might be established to make
communication between two realms more efficient). It is important
for the end-service to know which realms were transited when
deciding how much faith to place in the authentication process. To
facilitate this decision, a field in each ticket contains the
names of the realms that were involved in authenticating the
client.
The application server is ultimately responsible for accepting or
rejecting authentication and SHOULD check the transited field. The
application server may choose to rely on the KDC for the
application server's realm to check the transited field. The
application server's KDC will set the TRANSITED-POLICY-CHECKED
flag in this case. The KDCs for intermediate realms may also check
the transited field as they issue ticket-granting tickets for
other realms, but they are encouraged not to do so. A client may
request that the KDCs not check the transited field by setting the
DISABLE-TRANSITED-CHECK flag. KDCs SHOULD honor this flag.
1.2. Choosing a principal with which to communicate
The Kerberos protocol provides the means for verifying (subject to
the assumptions in 1.5) that the entity with which one
communicates is the same entity that was registered with the KDC
using the claimed identity (principal name). It is still necessary
to determine whether that identity corresponds to the entity with
which one intends to communicate.
When appropriate data has been exchanged in advance, this
determination may be performed syntactically by the application
based on the application protocol specification, information
provided by the user, and configuration files. For example, the
server principal name (including realm) for a telnet server might
be derived from the user specified host name (from the telnet
command line), the "host/" prefix specified in the application
protocol specification, and a mapping to a Kerberos realm derived
syntactically from the domain part of the specified hostname and
information from the local Kerberos realms database.
One can also rely on trusted third parties to make this
determination, but only when the data obtained from the third
party is suitably integrity protected while resident on the third
party server and when transmitted. Thus, for example, one should
not rely on an unprotected domain name system record to map a host
alias to the primary name of a server, accepting the primary name
as the party one intends to contact, since an attacker can modify
the mapping and impersonate the party with which one intended to
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communicate.
Implementations of Kerberos and protocols based on Kerberos MUST
NOT use insecure DNS queries to canonicalize the hostname
components of the service principal names (i.e. MUST NOT use
insecure DNS queries to map one name to another to determine the
host part of the principal name with which one is to communicate).
In an environment without secure name service, application authors
MAY append a statically configured domain name to unqualified
hostnames before passing the name to the security mechanisms, but
should do no more than that. Secure name service facilities, if
available, might be trusted for hostname canonicalization, but
such canonicalization by the client SHOULD NOT be required by KDC
implementations.
Implementation note: Many current implementations do some degree
of canonicalization of the provided service name, often using DNS
even though it creates security problems. However there is no
consistency among implementations about whether the service name
is case folded to lower case or whether reverse resolution is
used. To maximize interoperability and security, applications
SHOULD provide security mechanisms with names which result from
folding the user-entered name to lower case, without performing
any other modifications or canonicalization.
1.3. Authorization
As an authentication service, Kerberos provides a means of
verifying the identity of principals on a network. Authentication
is usually useful primarily as a first step in the process of
authorization, determining whether a client may use a service,
which objects the client is allowed to access, and the type of
access allowed for each. Kerberos does not, by itself, provide
authorization. Possession of a client ticket for a service
provides only for authentication of the client to that service,
and in the absence of a separate authorization procedure, it
should not be considered by an application as authorizing the use
of that service.
Such separate authorization methods MAY be implemented as
application specific access control functions and may utilize
files on the application server, or on separately issued
authorization credentials such as those based on proxies [Neu93],
or on other authorization services. Separately authenticated
authorization credentials MAY be embedded in a ticket's
authorization data when encapsulated by the KDC-issued
authorization data element.
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Applications should not accept the mere issuance of a service
ticket by the Kerberos server (even by a modified Kerberos server)
as granting authority to use the service, since such applications
may become vulnerable to the bypass of this authorization check in
an environment if they interoperate with other KDCs or where other
options for application authentication are provided.
1.4. Extending Kerberos Without Breaking Interoperability
As the deployed base of Kerberos implementations grows, extending
Kerberos becomes more important. Unfortunately some extensions to
the existing Kerberos protocol create interoperability issues
because of uncertainty regarding the treatment of certain
extensibility options by some implementations. This section
includes guidelines that will enable future implementations to
maintain interoperability.
Kerberos provides a general mechanism for protocol extensibility.
Some protocol messages contain typed holes -- sub-messages that
contain an octet-string along with an integer that defines how to
interpret the octet-string. The integer types are registered
centrally, but can be used both for vendor extensions and for
extensions standardized through the IETF.
In this document, the word "extension" means an extension by
defining a new type to insert into an existing typed hole in a
protocol message. It does not mean extension by addition of new
fields to ASN.1 types, unless explicitly indicated otherwise in
the text.
1.4.1. Compatibility with RFC 1510
It is important to note that existing Kerberos message formats can
not be readily extended by adding fields to the ASN.1 types.
Sending additional fields often results in the entire message
being discarded without an error indication. Future versions of
this specification will provide guidelines to ensure that ASN.1
fields can be added without creating an interoperability problem.
In the meantime, all new or modified implementations of Kerberos
that receive an unknown message extension SHOULD preserve the
encoding of the extension but otherwise ignore the presence of the
extension. Recipients MUST NOT decline a request simply because an
extension is present.
There is one exception to this rule. If an unknown authorization
data element type is received by a server other than the ticket
granting service either in an AP-REQ or in a ticket contained in
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an AP-REQ, then authentication MUST fail. One of the primary uses
of authorization data is to restrict the use of the ticket. If the
service cannot determine whether the restriction applies to that
service then a security weakness may result if the ticket can be
used for that service. Authorization elements that are optional
SHOULD be enclosed in the AD-IF-RELEVANT element.
The ticket granting service MUST ignore but propagate to
derivative tickets any unknown authorization data types, unless
those data types are embedded in a MANDATORY-FOR-KDC element, in
which case the request will be rejected. This behavior is
appropriate because requiring that the ticket granting service
understand unknown authorization data types would require that KDC
software be upgraded to understand new application-level
restrictions before applications used these restrictions,
decreasing the utility of authorization data as a mechanism for
restricting the use of tickets. No security problem is created
because services to which the tickets are issued will verify the
authorization data.
Implementation note: Many RFC 1510 implementations ignore unknown
authorization data elements. Depending on these implementations to
honor authorization data restrictions may create a security
weakness.
1.4.2. Sending Extensible Messages
Care must be taken to ensure that old implementations can
understand messages sent to them even if they do not understand an
extension that is used. Unless the sender knows an extension is
supported, the extension cannot change the semantics of the core
message or previously defined extensions.
For example, an extension including key information necessary to
decrypt the encrypted part of a KDC-REP could only be used in
situations where the recipient was known to support the extension.
Thus when designing such extensions it is important to provide a
way for the recipient to notify the sender of support for the
extension. For example in the case of an extension that changes
the KDC-REP reply key, the client could indicate support for the
extension by including a padata element in the AS-REQ sequence.
The KDC should only use the extension if this padata element is
present in the AS-REQ. Even if policy requires the use of the
extension, it is better to return an error indicating that the
extension is required than to use the extension when the recipient
may not support it; debugging why implementations do not
interoperate is easier when errors are returned.
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1.5. Environmental assumptions
Kerberos imposes a few assumptions on the environment in which it
can properly function:
* "Denial of service" attacks are not solved with Kerberos. There
are places in the protocols where an intruder can prevent an
application from participating in the proper authentication steps.
Detection and solution of such attacks (some of which can appear
to be not-uncommon "normal" failure modes for the system) is
usually best left to the human administrators and users.
* Principals MUST keep their secret keys secret. If an intruder
somehow steals a principal's key, it will be able to masquerade as
that principal or impersonate any server to the legitimate
principal.
* "Password guessing" attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to
successfully mount an offline dictionary attack by repeatedly
attempting to decrypt, with successive entries from a dictionary,
messages obtained which are encrypted under a key derived from the
user's password.
* Each host on the network MUST have a clock which is "loosely
synchronized" to the time of the other hosts; this synchronization
is used to reduce the bookkeeping needs of application servers
when they do replay detection. The degree of "looseness" can be
configured on a per-server basis, but is typically on the order of
5 minutes. If the clocks are synchronized over the network, the
clock synchronization protocol MUST itself be secured from network
attackers.
* Principal identifiers are not recycled on a short-term basis. A
typical mode of access control will use access control lists
(ACLs) to grant permissions to particular principals. If a stale
ACL entry remains for a deleted principal and the principal
identifier is reused, the new principal will inherit rights
specified in the stale ACL entry. By not re-using principal
identifiers, the danger of inadvertent access is removed.
1.6. Glossary of terms
Below is a list of terms used throughout this document.
Authentication
Verifying the claimed identity of a principal.
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Authentication header
A record containing a Ticket and an Authenticator to be presented
to a server as part of the authentication process.
Authentication path
A sequence of intermediate realms transited in the authentication
process when communicating from one realm to another.
Authenticator
A record containing information that can be shown to have been
recently generated using the session key known only by the client
and server.
Authorization
The process of determining whether a client may use a service,
which objects the client is allowed to access, and the type of
access allowed for each.
Capability
A token that grants the bearer permission to access an object or
service. In Kerberos, this might be a ticket whose use is
restricted by the contents of the authorization data field, but
which lists no network addresses, together with the session key
necessary to use the ticket.
Ciphertext
The output of an encryption function. Encryption transforms
plaintext into ciphertext.
Client
A process that makes use of a network service on behalf of a user.
Note that in some cases a Server may itself be a client of some
other server (e.g. a print server may be a client of a file
server).
Credentials
A ticket plus the secret session key necessary to successfully use
that ticket in an authentication exchange.
Encryption Type (etype)
When associated with encrypted data, an encryption type identifies
the algorithm used to encrypt the data and is used to select the
appropriate algorithm for decrypting the data. Encryption type
tags are communicated in other messages to enumerate algorithms
that are desired, supported, preferred, or allowed to be used for
encryption of data between parties. This preference is combined
with local information and policy to select an algorithm to be
used.
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KDC
Key Distribution Center, a network service that supplies tickets
and temporary session keys; or an instance of that service or the
host on which it runs. The KDC services both initial ticket and
ticket-granting ticket requests. The initial ticket portion is
sometimes referred to as the Authentication Server (or service).
The ticket-granting ticket portion is sometimes referred to as the
ticket-granting server (or service).
Kerberos
The name given to the Project Athena's authentication service, the
protocol used by that service, or the code used to implement the
authentication service. The name is adopted from the three-headed
dog which guards Hades.
Key Version Number (kvno)
A tag associated with encrypted data identifies which key was used
for encryption when a long lived key associated with a principal
changes over time. It is used during the transition to a new key
so that the party decrypting a message can tell whether the data
was encrypted using the old or the new key.
Plaintext
The input to an encryption function or the output of a decryption
function. Decryption transforms ciphertext into plaintext.
Principal
A named client or server entity that participates in a network
communication, with one name that is considered canonical.
Principal identifier
The canonical name used to uniquely identify each different
principal.
Seal
To encipher a record containing several fields in such a way that
the fields cannot be individually replaced without either
knowledge of the encryption key or leaving evidence of tampering.
Secret key
An encryption key shared by a principal and the KDC, distributed
outside the bounds of the system, with a long lifetime. In the
case of a human user's principal, the secret key MAY be derived
from a password.
Server
A particular Principal which provides a resource to network
clients. The server is sometimes referred to as the Application
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Server.
Service
A resource provided to network clients; often provided by more
than one server (for example, remote file service).
Session key
A temporary encryption key used between two principals, with a
lifetime limited to the duration of a single login "session". In
the Kerberos system, a session key is generated by the KDC. The
session key is distinct from the sub-session key, described next..
Sub-session key
A temporary encryption key used between two principals, selected
and exchanged by the principals using the session key, and with a
lifetime limited to the duration of a single association. The sub-
session key is also referred to as the subkey.
Ticket
A record that helps a client authenticate itself to a server; it
contains the client's identity, a session key, a timestamp, and
other information, all sealed using the server's secret key. It
only serves to authenticate a client when presented along with a
fresh Authenticator.
2. Ticket flag uses and requests
Each Kerberos ticket contains a set of flags which are used to
indicate attributes of that ticket. Most flags may be requested by
a client when the ticket is obtained; some are automatically
turned on and off by a Kerberos server as required. The following
sections explain what the various flags mean and give examples of
reasons to use them. With the exception of the INVALID flag
clients MUST ignore ticket flags that are not recognized. KDCs
MUST ignore KDC options that are not recognized. Some
implementations of RFC 1510 are known to reject unknown KDC
options, so clients may need to resend a request without new KDC
options if the request was rejected when sent with options added
since RFC 1510. Since new KDCs will ignore unknown options,
clients MUST confirm that the ticket returned by the KDC meets
their needs.
Note that it is not, in general, possible to determine whether an
option was not honored because it was not understood or because it
was rejected either through configuration or policy. When adding a
new option to the Kerberos protocol, designers should consider
whether the distinction is important for their option. In cases
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where it is, a mechanism for the KDC to return an indication that
the option was understood but rejected needs to be provided in the
specification of the option. Often in such cases, the mechanism
needs to be broad enough to permit an error or reason to be
returned.
2.1. Initial, pre-authenticated, and hardware authenticated tickets
The INITIAL flag indicates that a ticket was issued using the AS
protocol, rather than issued based on a ticket-granting ticket.
Application servers that want to require the demonstrated
knowledge of a client's secret key (e.g. a password-changing
program) can insist that this flag be set in any tickets they
accept, and thus be assured that the client's key was recently
presented to the application client.
The PRE-AUTHENT and HW-AUTHENT flags provide additional
information about the initial authentication, regardless of
whether the current ticket was issued directly (in which case
INITIAL will also be set) or issued on the basis of a ticket-
granting ticket (in which case the INITIAL flag is clear, but the
PRE-AUTHENT and HW-AUTHENT flags are carried forward from the
ticket-granting ticket).
2.2. Invalid tickets
The INVALID flag indicates that a ticket is invalid. Application
servers MUST reject tickets which have this flag set. A postdated
ticket will be issued in this form. Invalid tickets MUST be
validated by the KDC before use, by presenting them to the KDC in
a TGS request with the VALIDATE option specified. The KDC will
only validate tickets after their starttime has passed. The
validation is required so that postdated tickets which have been
stolen before their starttime can be rendered permanently invalid
(through a hot-list mechanism) (see section 3.3.3.1).
2.3. Renewable tickets
Applications may desire to hold tickets which can be valid for
long periods of time. However, this can expose their credentials
to potential theft for equally long periods, and those stolen
credentials would be valid until the expiration time of the
ticket(s). Simply using short-lived tickets and obtaining new ones
periodically would require the client to have long-term access to
its secret key, an even greater risk. Renewable tickets can be
used to mitigate the consequences of theft. Renewable tickets have
two "expiration times": the first is when the current instance of
the ticket expires, and the second is the latest permissible value
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for an individual expiration time. An application client must
periodically (i.e. before it expires) present a renewable ticket
to the KDC, with the RENEW option set in the KDC request. The KDC
will issue a new ticket with a new session key and a later
expiration time. All other fields of the ticket are left
unmodified by the renewal process. When the latest permissible
expiration time arrives, the ticket expires permanently. At each
renewal, the KDC MAY consult a hot-list to determine if the ticket
had been reported stolen since its last renewal; it will refuse to
renew such stolen tickets, and thus the usable lifetime of stolen
tickets is reduced.
The RENEWABLE flag in a ticket is normally only interpreted by the
ticket-granting service (discussed below in section 3.3). It can
usually be ignored by application servers. However, some
particularly careful application servers MAY disallow renewable
tickets.
If a renewable ticket is not renewed by its expiration time, the
KDC will not renew the ticket. The RENEWABLE flag is reset by
default, but a client MAY request it be set by setting the
RENEWABLE option in the KRB_AS_REQ message. If it is set, then the
renew-till field in the ticket contains the time after which the
ticket may not be renewed.
2.4. Postdated tickets
Applications may occasionally need to obtain tickets for use much
later, e.g. a batch submission system would need tickets to be
valid at the time the batch job is serviced. However, it is
dangerous to hold valid tickets in a batch queue, since they will
be on-line longer and more prone to theft. Postdated tickets
provide a way to obtain these tickets from the KDC at job
submission time, but to leave them "dormant" until they are
activated and validated by a further request of the KDC. If a
ticket theft were reported in the interim, the KDC would refuse to
validate the ticket, and the thief would be foiled.
The MAY-POSTDATE flag in a ticket is normally only interpreted by
the ticket-granting service. It can be ignored by application
servers. This flag MUST be set in a ticket-granting ticket in
order to issue a postdated ticket based on the presented ticket.
It is reset by default; it MAY be requested by a client by setting
the ALLOW-POSTDATE option in the KRB_AS_REQ message. This flag
does not allow a client to obtain a postdated ticket-granting
ticket; postdated ticket-granting tickets can only by obtained by
requesting the postdating in the KRB_AS_REQ message. The life
(endtime-starttime) of a postdated ticket will be the remaining
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life of the ticket-granting ticket at the time of the request,
unless the RENEWABLE option is also set, in which case it can be
the full life (endtime-starttime) of the ticket-granting ticket.
The KDC MAY limit how far in the future a ticket may be postdated.
The POSTDATED flag indicates that a ticket has been postdated. The
application server can check the authtime field in the ticket to
see when the original authentication occurred. Some services MAY
choose to reject postdated tickets, or they may only accept them
within a certain period after the original authentication. When
the KDC issues a POSTDATED ticket, it will also be marked as
INVALID, so that the application client MUST present the ticket to
the KDC to be validated before use.
2.5. Proxiable and proxy tickets
At times it may be necessary for a principal to allow a service to
perform an operation on its behalf. The service must be able to
take on the identity of the client, but only for a particular
purpose. A principal can allow a service to take on the
principal's identity for a particular purpose by granting it a
proxy.
The process of granting a proxy using the proxy and proxiable
flags is used to provide credentials for use with specific
services. Though conceptually also a proxy, users wishing to
delegate their identity in a form usable for all purpose MUST use
the ticket forwarding mechanism described in the next section to
forward a ticket-granting ticket.
The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
When set, this flag tells the ticket-granting server that it is OK
to issue a new ticket (but not a ticket-granting ticket) with a
different network address based on this ticket. This flag is set
if requested by the client on initial authentication. By default,
the client will request that it be set when requesting a ticket-
granting ticket, and reset when requesting any other ticket.
This flag allows a client to pass a proxy to a server to perform a
remote request on its behalf (e.g. a print service client can give
the print server a proxy to access the client's files on a
particular file server in order to satisfy a print request).
In order to complicate the use of stolen credentials, Kerberos
tickets are usually valid from only those network addresses
specifically included in the ticket[4]. When granting a proxy, the
client MUST specify the new network address from which the proxy
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is to be used, or indicate that the proxy is to be issued for use
from any address.
The PROXY flag is set in a ticket by the TGS when it issues a
proxy ticket. Application servers MAY check this flag and at
their option they MAY require additional authentication from the
agent presenting the proxy in order to provide an audit trail.
2.6. Forwardable tickets
Authentication forwarding is an instance of a proxy where the
service that is granted is complete use of the client's identity.
An example where it might be used is when a user logs in to a
remote system and wants authentication to work from that system as
if the login were local.
The FORWARDABLE flag in a ticket is normally only interpreted by
the ticket-granting service. It can be ignored by application
servers. The FORWARDABLE flag has an interpretation similar to
that of the PROXIABLE flag, except ticket-granting tickets may
also be issued with different network addresses. This flag is
reset by default, but users MAY request that it be set by setting
the FORWARDABLE option in the AS request when they request their
initial ticket-granting ticket.
This flag allows for authentication forwarding without requiring
the user to enter a password again. If the flag is not set, then
authentication forwarding is not permitted, but the same result
can still be achieved if the user engages in the AS exchange
specifying the requested network addresses and supplies a
password.
The FORWARDED flag is set by the TGS when a client presents a
ticket with the FORWARDABLE flag set and requests a forwarded
ticket by specifying the FORWARDED KDC option and supplying a set
of addresses for the new ticket. It is also set in all tickets
issued based on tickets with the FORWARDED flag set. Application
servers may choose to process FORWARDED tickets differently than
non-FORWARDED tickets.
If addressless tickets are forwarded from one system to another,
clients SHOULD still use this option to obtain a new TGT in order
to have different session keys on the different systems.
2.7. Transited Policy Checking
In Kerberos, the application server is ultimately responsible for
accepting or rejecting authentication and SHOULD check that only
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suitably trusted KDCs are relied upon to authenticate a principal.
The transited field in the ticket identifies which realms (and
thus which KDCs) were involved in the authentication process and
an application server would normally check this field. If any of
these are untrusted to authenticate the indicated client principal
(probably determined by a realm-based policy), the authentication
attempt MUST be rejected. The presence of trusted KDCs in this
list does not provide any guarantee; an untrusted KDC may have
fabricated the list.
While the end server ultimately decides whether authentication is
valid, the KDC for the end server's realm MAY apply a realm
specific policy for validating the transited field and accepting
credentials for cross-realm authentication. When the KDC applies
such checks and accepts such cross-realm authentication it will
set the TRANSITED-POLICY-CHECKED flag in the service tickets it
issues based on the cross-realm TGT. A client MAY request that the
KDCs not check the transited field by setting the DISABLE-
TRANSITED-CHECK flag. KDCs are encouraged but not required to
honor this flag.
Application servers MUST either do the transited-realm checks
themselves, or reject cross-realm tickets without TRANSITED-
POLICY-CHECKED set.
2.8. OK as Delegate
For some applications a client may need to delegate authority to a
server to act on its behalf in contacting other services. This
requires that the client forward credentials to an intermediate
server. The ability for a client to obtain a service ticket to a
server conveys no information to the client about whether the
server should be trusted to accept delegated credentials. The OK-
AS-DELEGATE provides a way for a KDC to communicate local realm
policy to a client regarding whether an intermediate server is
trusted to accept such credentials.
The copy of the ticket flags in the encrypted part of the KDC
reply may have the OK-AS-DELEGATE flag set to indicates to the
client that the server specified in the ticket has been determined
by policy of the realm to be a suitable recipient of delegation.
A client can use the presence of this flag to help it make a
decision whether to delegate credentials (either grant a proxy or
a forwarded ticket-granting ticket) to this server. It is
acceptable to ignore the value of this flag. When setting this
flag, an administrator should consider the security and placement
of the server on which the service will run, as well as whether
the service requires the use of delegated credentials.
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2.9. Other KDC options
There are three additional options which MAY be set in a client's
request of the KDC.
2.9.1. Renewable-OK
The RENEWABLE-OK option indicates that the client will accept a
renewable ticket if a ticket with the requested life cannot
otherwise be provided. If a ticket with the requested life cannot
be provided, then the KDC MAY issue a renewable ticket with a
renew-till equal to the requested endtime. The value of the renew-
till field MAY still be adjusted by site-determined limits or
limits imposed by the individual principal or server.
2.9.2. ENC-TKT-IN-SKEY
In its basic form the Kerberos protocol supports authentication in
a client-server
setting and is not well suited to authentication in a peer-to-
peer environment because the long term key of the user does not
remain on the workstation after initial login. Authentication of
such peers may be supported by Kerberos in its user-to-user
variant. The ENC-TKT-IN-SKEY option supports user-to-user
authentication by allowing the KDC to issue a service ticket
encrypted using the session key from another ticket-granting
ticket issued to another user. The ENC-TKT-IN-SKEY option is
honored only by the ticket-granting service. It indicates that the
ticket to be issued for the end server is to be encrypted in the
session key from the additional second ticket-granting ticket
provided with the request. See section 3.3.3 for specific details.
2.9.3. Passwordless Hardware Authentication
The OPT-HARDWARE-AUTH option indicates that the client wishes to
use some form of hardware authentication instead of or in addition
to the client's password or other long-lived encryption key. OPT-
HARDWARE-AUTH is honored only by the authentication service. If
supported and allowed by policy, the KDC will return an errorcode
KDC_ERR_PREAUTH_REQUIRED and include the required METHOD-DATA to
perform such authentication.
3. Message Exchanges
The following sections describe the interactions between network
clients and servers and the messages involved in those exchanges.
3.1. The Authentication Service Exchange
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Summary
Message direction Message type Section
1. Client to Kerberos KRB_AS_REQ 5.4.1
2. Kerberos to client KRB_AS_REP or 5.4.2
KRB_ERROR 5.9.1
The Authentication Service (AS) Exchange between the client and
the Kerberos Authentication Server is initiated by a client when
it wishes to obtain authentication credentials for a given server
but currently holds no credentials. In its basic form, the
client's secret key is used for encryption and decryption. This
exchange is typically used at the initiation of a login session to
obtain credentials for a Ticket-Granting Server which will
subsequently be used to obtain credentials for other servers (see
section 3.3) without requiring further use of the client's secret
key. This exchange is also used to request credentials for
services which must not be mediated through the Ticket-Granting
Service, but rather require a principal's secret key, such as the
password-changing service[5]. This exchange does not by itself
provide any assurance of the identity of the user[6].
The exchange consists of two messages: KRB_AS_REQ from the client
to Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for
these messages are described in sections 5.4.1, 5.4.2, and 5.9.1.
In the request, the client sends (in cleartext) its own identity
and the identity of the server for which it is requesting
credentials, other information about the credentials it is
requesting, and a randomly generated nonce which can be used to
detect replays, and to associate replies with the matching
requests. This nonce MUST be generated randomly by the client and
remembered for checking against the nonce in the expected reply.
The response, KRB_AS_REP, contains a ticket for the client to
present to the server, and a session key that will be shared by
the client and the server. The session key and additional
information are encrypted in the client's secret key. The
encrypted part of the KRB_AS_REP message also contains the nonce
which MUST be matched with the nonce from the KRB_AS_REQ message.
Without pre-authentication, the authentication server does not
know whether the client is actually the principal named in the
request. It simply sends a reply without knowing or caring whether
they are the same. This is acceptable because nobody but the
principal whose identity was given in the request will be able to
use the reply. Its critical information is encrypted in that
principal's key. However, an attacker can send a KRB_AS_REQ
message to get known plaintext in order to attack the principal's
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key. Especially if the key is based on a password, this may create
a security exposure. So, the initial request supports an optional
field that can be used to pass additional information that might
be needed for the initial exchange. This field SHOULD be used for
pre-authentication as described in sections 3.1.1 and 5.2.7.
Various errors can occur; these are indicated by an error response
(KRB_ERROR) instead of the KRB_AS_REP response. The error message
is not encrypted. The KRB_ERROR message contains information which
can be used to associate it with the message to which it replies.
The contents of the KRB_ERROR message are not integrity-protected.
As such, the client cannot detect replays, fabrications or
modifications. A solution to this problem will be included in a
future version of the protocol.
3.1.1. Generation of KRB_AS_REQ message
The client may specify a number of options in the initial request.
Among these options are whether pre-authentication is to be
performed; whether the requested ticket is to be renewable,
proxiable, or forwardable; whether it should be postdated or allow
postdating of derivative tickets; and whether a renewable ticket
will be accepted in lieu of a non-renewable ticket if the
requested ticket expiration date cannot be satisfied by a non-
renewable ticket (due to configuration constraints).
The client prepares the KRB_AS_REQ message and sends it to the
KDC.
3.1.2. Receipt of KRB_AS_REQ message
If all goes well, processing the KRB_AS_REQ message will result in
the creation of a ticket for the client to present to the server.
The format for the ticket is described in section 5.3.
Because Kerberos can run over unreliable transports such as UDP,
the KDC MUST be prepared to retransmit responses in case they are
lost. If a KDC receives a request identical to one it has recently
successfully processed, the KDC MUST respond with a KRB_AS_REP
message rather than a replay error. In order to reduce ciphertext
given to a potential attacker, KDCs MAY send the same response
generated when the request was first handled. KDCs MUST obey this
replay behavior even if the actual transport in use is reliable.
3.1.3. Generation of KRB_AS_REP message
The authentication server looks up the client and server
principals named in the KRB_AS_REQ in its database, extracting
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their respective keys. If the requested client principal named in
the request is not known because it doesn't exist in the KDC's
principal database, then an error message with a
KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.
If required, the server pre-authenticates the request, and if the
pre-authentication check fails, an error message with the code
KDC_ERR_PREAUTH_FAILED is returned. If pre-authentication is
required, but was not present in the request, an error message
with the code KDC_ERR_PREAUTH_REQUIRED is returned and a METHOD-
DATA object will be stored in the e-data field of the KRB-ERROR
message to specify which pre-authentication mechanisms are
acceptable. Usually this will include PA-ETYPE-INFO and/or PA-
ETYPE-INFO2 elements as described below. If the server cannot
accommodate any encryption type requested by the client, an error
message with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise the
KDC generates a 'random' session key[7].
When responding to an AS request, if there are multiple encryption
keys registered for a client in the Kerberos database, then the
etype field from the AS request is used by the KDC to select the
encryption method to be used to protect the encrypted part of the
KRB_AS_REP message which is sent to the client. If there is more
than one supported strong encryption type in the etype list, the
KDC SHOULD use the first valid strong etype for which an
encryption key is available.
When the user's key is generated from a password or pass phrase,
the string-to-key function for the particular encryption key type
is used, as specified in [@KCRYPTO]. The salt value and additional
parameters for the string-to-key function have default values
(specified by section 4 and by the encryption mechanism
specification, respectively) that may be overridden by pre-
authentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO, PA-
ETYPE-INFO2, etc). Since the KDC is presumed to store a copy of
the resulting key only, these values should not be changed for
password-based keys except when changing the principal's key.
When the AS server is to include pre-authentication data in a KRB-
ERROR or in an AS-REP, it MUST use PA-ETYPE-INFO2, not PA-ETYPE-
INFO, if the etype field of the client's AS-REQ lists at least one
"newer" encryption type. Otherwise (when the etype field of the
client's AS-REQ does not list any "newer" encryption types) it
MUST send both, PA-ETYPE-INFO2 and PA-ETYPE-INFO (both with an
entry for each enctype). A "newer" enctype is any enctype first
officially specified concurrently with or subsequent to the issue
of this RFC. The enctypes DES, 3DES or RC4 and any defined in
[RFC1510] are not "newer" enctypes.
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It is not possible to reliably generate a user's key given a pass
phrase without contacting the KDC, since it will not be known
whether alternate salt or parameter values are required.
The KDC will attempt to assign the type of the random session key
from the list of methods in the etype field. The KDC will select
the appropriate type using the list of methods provided together
with information from the Kerberos database indicating acceptable
encryption methods for the application server. The KDC will not
issue tickets with a weak session key encryption type.
If the requested start time is absent, indicates a time in the
past, or is within the window of acceptable clock skew for the KDC
and the POSTDATE option has not been specified, then the start
time of the ticket is set to the authentication server's current
time. If it indicates a time in the future beyond the acceptable
clock skew, but the POSTDATED option has not been specified then
the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise the
requested start time is checked against the policy of the local
realm (the administrator might decide to prohibit certain types or
ranges of postdated tickets), and if acceptable, the ticket's
start time is set as requested and the INVALID flag is set in the
new ticket. The postdated ticket MUST be validated before use by
presenting it to the KDC after the start time has been reached.
The expiration time of the ticket will be set to the earlier of
the requested endtime and a time determined by local policy,
possibly determined using realm or principal specific factors. For
example, the expiration time MAY be set to the earliest of the
following:
* The expiration time (endtime) requested in the KRB_AS_REQ
message.
* The ticket's start time plus the maximum allowable lifetime
associated with the client principal from the authentication
server's database.
* The ticket's start time plus the maximum allowable lifetime
associated with the server principal.
* The ticket's start time plus the maximum lifetime set by the
policy of the local realm.
If the requested expiration time minus the start time (as determined
above) is less than a site-determined minimum lifetime, an error
message with code KDC_ERR_NEVER_VALID is returned. If the requested
expiration time for the ticket exceeds what was determined as above,
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and if the 'RENEWABLE-OK' option was requested, then the 'RENEWABLE'
flag is set in the new ticket, and the renew-till value is set as if
the 'RENEWABLE' option were requested (the field and option names are
described fully in section 5.4.1).
If the RENEWABLE option has been requested or if the RENEWABLE-OK
option has been set and a renewable ticket is to be issued, then the
renew-till field MAY be set to the earliest of:
* Its requested value.
* The start time of the ticket plus the minimum of the two
maximum renewable lifetimes associated with the principals'
database entries.
* The start time of the ticket plus the maximum renewable
lifetime set by the policy of the local realm.
The flags field of the new ticket will have the following options set
if they have been requested and if the policy of the local realm
allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
If the new ticket is postdated (the start time is in the future), its
INVALID flag will also be set.
If all of the above succeed, the server will encrypt the ciphertext
part of the ticket using the encryption key extracted from the server
principal's record in the Kerberos database using the encryption type
associated with the server principal's key (this choice is NOT
affected by the etype field in the request). It then formats a
KRB_AS_REP message (see section 5.4.2), copying the addresses in the
request into the caddr of the response, placing any required pre-
authentication data into the padata of the response, and encrypts the
ciphertext part in the client's key using an acceptable encryption
method requested in the etype field of the request, or in some key
specified by pre-authentication mechanisms being used.
3.1.4. Generation of KRB_ERROR message
Several errors can occur, and the Authentication Server responds
by returning an error message, KRB_ERROR, to the client, with the
error-code and e-text fields set to appropriate values. The error
message contents and details are described in Section 5.9.1.
3.1.5. Receipt of KRB_AS_REP message
If the reply message type is KRB_AS_REP, then the client verifies
that the cname and crealm fields in the cleartext portion of the
reply match what it requested. If any padata fields are present,
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they may be used to derive the proper secret key to decrypt the
message. The client decrypts the encrypted part of the response
using its secret key, verifies that the nonce in the encrypted
part matches the nonce it supplied in its request (to detect
replays). It also verifies that the sname and srealm in the
response match those in the request (or are otherwise expected
values), and that the host address field is also correct. It then
stores the ticket, session key, start and expiration times, and
other information for later use. The last-req field (and the
deprecated key-expiration field) from the encrypted part of the
response MAY be checked to notify the user of impending key
expiration. This enables the client program to suggest remedial
action, such as a password change.
Upon validation of the KRB_AS_REP message (by checking the
returned nonce against that sent in the KRB_AS_REQ message) the
client knows that the current time on the KDC is that read from
the authtime field of the encrypted part of the reply. The client
can optionally use this value for clock synchronization in
subsequent messages by recording with the ticket the difference
(offset) between the authtime value and the local clock. This
offset can then be used by the same user to adjust the time read
from the system clock when generating messages [DGT96].
This technique MUST be used when adjusting for clock skew instead
of directly changing the system clock because the KDC reply is
only authenticated to the user whose secret key was used, but not
to the system or workstation. If the clock were adjusted, an
attacker colluding with a user logging into a workstation could
agree on a password, resulting in a KDC reply that would be
correctly validated even though it did not originate from a KDC
trusted by the workstation.
Proper decryption of the KRB_AS_REP message is not sufficient for
the host to verify the identity of the user; the user and an
attacker could cooperate to generate a KRB_AS_REP format message
which decrypts properly but is not from the proper KDC. If the
host wishes to verify the identity of the user, it MUST require
the user to present application credentials which can be verified
using a securely-stored secret key for the host. If those
credentials can be verified, then the identity of the user can be
assured.
3.1.6. Receipt of KRB_ERROR message
If the reply message type is KRB_ERROR, then the client interprets
it as an error and performs whatever application-specific tasks
are necessary to recover.
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3.2. The Client/Server Authentication Exchange
Summary
Message direction Message type Section
Client to Application server KRB_AP_REQ 5.5.1
[optional] Application server to client KRB_AP_REP or 5.5.2
KRB_ERROR 5.9.1
The client/server authentication (CS) exchange is used by network
applications to authenticate the client to the server and vice
versa. The client MUST have already acquired credentials for the
server using the AS or TGS exchange.
3.2.1. The KRB_AP_REQ message
The KRB_AP_REQ contains authentication information which SHOULD be
part of the first message in an authenticated transaction. It
contains a ticket, an authenticator, and some additional
bookkeeping information (see section 5.5.1 for the exact format).
The ticket by itself is insufficient to authenticate a client,
since tickets are passed across the network in cleartext[8], so
the authenticator is used to prevent invalid replay of tickets by
proving to the server that the client knows the session key of the
ticket and thus is entitled to use the ticket. The KRB_AP_REQ
message is referred to elsewhere as the 'authentication header.'
3.2.2. Generation of a KRB_AP_REQ message
When a client wishes to initiate authentication to a server, it
obtains (either through a credentials cache, the AS exchange, or
the TGS exchange) a ticket and session key for the desired
service. The client MAY re-use any tickets it holds until they
expire. To use a ticket the client constructs a new Authenticator
from the system time, its name, and optionally an application
specific checksum, an initial sequence number to be used in
KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used
in negotiations for a session key unique to this particular
session. Authenticators MAY NOT be re-used and SHOULD be rejected
if replayed to a server[9]. If a sequence number is to be
included, it SHOULD be randomly chosen so that even after many
messages have been exchanged it is not likely to collide with
other sequence numbers in use.
The client MAY indicate a requirement of mutual authentication or
the use of a session-key based ticket (for user-to-user
authentication - see section 3.7) by setting the appropriate
flag(s) in the ap-options field of the message.
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The Authenticator is encrypted in the session key and combined
with the ticket to form the KRB_AP_REQ message which is then sent
to the end server along with any additional application-specific
information.
3.2.3. Receipt of KRB_AP_REQ message
Authentication is based on the server's current time of day
(clocks MUST be loosely synchronized), the authenticator, and the
ticket. Several errors are possible. If an error occurs, the
server is expected to reply to the client with a KRB_ERROR
message. This message MAY be encapsulated in the application
protocol if its raw form is not acceptable to the protocol. The
format of error messages is described in section 5.9.1.
The algorithm for verifying authentication information is as
follows. If the message type is not KRB_AP_REQ, the server returns
the KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the
Ticket in the KRB_AP_REQ is not one the server can use (e.g., it
indicates an old key, and the server no longer possesses a copy of
the old key), the KRB_AP_ERR_BADKEYVER error is returned. If the
USE-SESSION-KEY flag is set in the ap-options field, it indicates
to the server that user-to-user authentication is in use, and that
the ticket is encrypted in the session key from the server's
ticket-granting ticket rather than in the server's secret key. See
section 3.7 for a more complete description of the effect of user-
to-user authentication on all messages in the Kerberos protocol.
Since it is possible for the server to be registered in multiple
realms, with different keys in each, the srealm field in the
unencrypted portion of the ticket in the KRB_AP_REQ is used to
specify which secret key the server should use to decrypt that
ticket. The KRB_AP_ERR_NOKEY error code is returned if the server
doesn't have the proper key to decipher the ticket.
The ticket is decrypted using the version of the server's key
specified by the ticket. If the decryption routines detect a
modification of the ticket (each encryption system MUST provide
safeguards to detect modified ciphertext), the
KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that
different keys were used to encrypt and decrypt).
The authenticator is decrypted using the session key extracted
from the decrypted ticket. If decryption shows it to have been
modified, the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name
and realm of the client from the ticket are compared against the
same fields in the authenticator. If they don't match, the
KRB_AP_ERR_BADMATCH error is returned; this normally is caused by
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a client error or attempted attack. The addresses in the ticket
(if any) are then searched for an address matching the operating-
system reported address of the client. If no match is found or the
server insists on ticket addresses but none are present in the
ticket, the KRB_AP_ERR_BADADDR error is returned. If the local
(server) time and the client time in the authenticator differ by
more than the allowable clock skew (e.g., 5 minutes), the
KRB_AP_ERR_SKEW error is returned.
Unless the application server provides its own suitable means to
protect against replay (for example, a challenge-response sequence
initiated by the server after authentication, or use of a server-
generated encryption subkey), the server MUST utilize a replay
cache to remember any authenticator presented within the allowable
clock skew. Careful analysis of the application protocol and
implementation is recommended before eliminating this cache. The
replay cache will store at least the server name, along with the
client name, time and microsecond fields from the recently-seen
authenticators and if a matching tuple is found, the
KRB_AP_ERR_REPEAT error is returned [10]. If a server loses track
of authenticators presented within the allowable clock skew, it
MUST reject all requests until the clock skew interval has passed,
providing assurance that any lost or replayed authenticators will
fall outside the allowable clock skew and can no longer be
successfully replayed [11].
Implementation note: If a client generates multiple requests to
the KDC with the same timestamp, including the microsecond field,
all but the first of the requests received will be rejected as
replays. This might happen, for example, if the resolution of the
client's clock is too coarse. Client implementations SHOULD
ensure that the timestamps are not reused, possibly by
incrementing the microseconds field in the time stamp when the
clock returns the same time for multiple requests.
If multiple servers (for example, different services on one
machine, or a single service implemented on multiple machines)
share a service principal (a practice we do not recommend in
general, but acknowledge will be used in some cases), they MUST
either share this replay cache, or the application protocol MUST
be designed so as to eliminate the need for it. Note that this
applies to all of the services, if any of the application
protocols does not have replay protection built in; an
authenticator used with such a service could later be replayed to
a different service with the same service principal but no replay
protection, if the former doesn't record the authenticator
information in the common replay cache.
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If a sequence number is provided in the authenticator, the server
saves it for later use in processing KRB_SAFE and/or KRB_PRIV
messages. If a subkey is present, the server either saves it for
later use or uses it to help generate its own choice for a subkey
to be returned in a KRB_AP_REP message.
The server computes the age of the ticket: local (server) time
minus the start time inside the Ticket. If the start time is later
than the current time by more than the allowable clock skew or if
the INVALID flag is set in the ticket, the KRB_AP_ERR_TKT_NYV
error is returned. Otherwise, if the current time is later than
end time by more than the allowable clock skew, the
KRB_AP_ERR_TKT_EXPIRED error is returned.
If all these checks succeed without an error, the server is
assured that the client possesses the credentials of the principal
named in the ticket and thus, the client has been authenticated to
the server.
Passing these checks provides only authentication of the named
principal; it does not imply authorization to use the named
service. Applications MUST make a separate authorization decision
based upon the authenticated name of the user, the requested
operation, local access control information such as that contained
in a .k5login or .k5users file, and possibly a separate
distributed authorization service.
3.2.4. Generation of a KRB_AP_REP message
Typically, a client's request will include both the authentication
information and its initial request in the same message, and the
server need not explicitly reply to the KRB_AP_REQ. However, if
mutual authentication (not only authenticating the client to the
server, but also the server to the client) is being performed, the
KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
field, and a KRB_AP_REP message is required in response. As with
the error message, this message MAY be encapsulated in the
application protocol if its "raw" form is not acceptable to the
application's protocol. The timestamp and microsecond field used
in the reply MUST be the client's timestamp and microsecond field
(as provided in the authenticator) [12]. If a sequence number is
to be included, it SHOULD be randomly chosen as described above
for the authenticator. A subkey MAY be included if the server
desires to negotiate a different subkey. The KRB_AP_REP message is
encrypted in the session key extracted from the ticket.
3.2.5. Receipt of KRB_AP_REP message
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If a KRB_AP_REP message is returned, the client uses the session
key from the credentials obtained for the server [13] to decrypt
the message, and verifies that the timestamp and microsecond
fields match those in the Authenticator it sent to the server. If
they match, then the client is assured that the server is genuine.
The sequence number and subkey (if present) are retained for later
use.
3.2.6. Using the encryption key
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client
and server share an encryption key which can be used by the
application. In some cases, the use of this session key will be
implicit in the protocol; in others the method of use must be
chosen from several alternatives. The actual encryption key to be
used for KRB_PRIV, KRB_SAFE, or other application-specific uses
MAY be chosen by the application based on the session key from the
ticket and subkeys in the KRB_AP_REP message and the authenticator
[14]. To mitigate the effect of failures in random number
generation on the client it is strongly encouraged that any key
derived by an application for subsequent use include the full key
entropy derived from the KDC generated session key carried in the
ticket. We leave the protocol negotiations of how to use the key
(e.g. selecting an encryption or checksum type) to the application
programmer; the Kerberos protocol does not constrain the
implementation options, but an example of how this might be done
follows.
One way that an application may choose to negotiate a key to be
used for subsequent integrity and privacy protection is for the
client to propose a key in the subkey field of the authenticator.
The server can then choose a key using the proposed key from the
client as input, returning the new subkey in the subkey field of
the application reply. This key could then be used for subsequent
communication.
With both the one-way and mutual authentication exchanges, the
peers should take care not to send sensitive information to each
other without proper assurances. In particular, applications that
require privacy or integrity SHOULD use the KRB_AP_REP response
from the server to client to assure both client and server of
their peer's identity. If an application protocol requires privacy
of its messages, it can use the KRB_PRIV message (section 3.5).
The KRB_SAFE message (section 3.4) can be used to assure
integrity.
3.3. The Ticket-Granting Service (TGS) Exchange
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Summary
Message direction Message type Section
1. Client to Kerberos KRB_TGS_REQ 5.4.1
2. Kerberos to client KRB_TGS_REP or 5.4.2
KRB_ERROR 5.9.1
The TGS exchange between a client and the Kerberos Ticket-Granting
Server is initiated by a client when it wishes to obtain
authentication credentials for a given server (which might be
registered in a remote realm), when it wishes to renew or validate
an existing ticket, or when it wishes to obtain a proxy ticket. In
the first case, the client must already have acquired a ticket for
the Ticket-Granting Service using the AS exchange (the ticket-
granting ticket is usually obtained when a client initially
authenticates to the system, such as when a user logs in). The
message format for the TGS exchange is almost identical to that
for the AS exchange. The primary difference is that encryption
and decryption in the TGS exchange does not take place under the
client's key. Instead, the session key from the ticket-granting
ticket or renewable ticket, or sub-session key from an
Authenticator is used. As is the case for all application servers,
expired tickets are not accepted by the TGS, so once a renewable
or ticket-granting ticket expires, the client must use a separate
exchange to obtain valid tickets.
The TGS exchange consists of two messages: A request (KRB_TGS_REQ)
from the client to the Kerberos Ticket-Granting Server, and a
reply (KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes
information authenticating the client plus a request for
credentials. The authentication information consists of the
authentication header (KRB_AP_REQ) which includes the client's
previously obtained ticket-granting, renewable, or invalid ticket.
In the ticket-granting ticket and proxy cases, the request MAY
include one or more of: a list of network addresses, a collection
of typed authorization data to be sealed in the ticket for
authorization use by the application server, or additional tickets
(the use of which are described later). The TGS reply
(KRB_TGS_REP) contains the requested credentials, encrypted in the
session key from the ticket-granting ticket or renewable ticket,
or if present, in the sub-session key from the Authenticator (part
of the authentication header). The KRB_ERROR message contains an
error code and text explaining what went wrong. The KRB_ERROR
message is not encrypted. The KRB_TGS_REP message contains
information which can be used to detect replays, and to associate
it with the message to which it replies. The KRB_ERROR message
also contains information which can be used to associate it with
the message to which it replies. The same comments about integrity
protection of KRB_ERROR messages mentioned in section 3.1 apply to
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the TGS exchange.
3.3.1. Generation of KRB_TGS_REQ message
Before sending a request to the ticket-granting service, the
client MUST determine in which realm the application server is
believed to be registered [15]. If the client knows the service
principal name and realm and it does not already possess a ticket-
granting ticket for the appropriate realm, then one must be
obtained. This is first attempted by requesting a ticket-granting
ticket for the destination realm from a Kerberos server for which
the client possesses a ticket-granting ticket (using the
KRB_TGS_REQ message recursively). The Kerberos server MAY return a
TGT for the desired realm in which case one can proceed.
Alternatively, the Kerberos server MAY return a TGT for a realm
which is 'closer' to the desired realm (further along the standard
hierarchical path between the client's realm and the requested
realm server's realm). It should be noted in this case that
misconfiguration of the Kerberos servers may cause loops in the
resulting authentication path, which the client should be careful
to detect and avoid.
If the Kerberos server returns a TGT for a 'closer' realm other
than the desired realm, the client MAY use local policy
configuration to verify that the authentication path used is an
acceptable one. Alternatively, a client MAY choose its own
authentication path, rather than relying on the Kerberos server to
select one. In either case, any policy or configuration
information used to choose or validate authentication paths,
whether by the Kerberos server or client, MUST be obtained from a
trusted source.
When a client obtains a ticket-granting ticket that is 'closer' to
the destination realm, the client MAY cache this ticket and reuse
it in future KRB-TGS exchanges with services in the 'closer'
realm. However, if the client were to obtain a ticket-granting
ticket for the 'closer' realm by starting at the initial KDC
rather than as part of obtaining another ticket, then a shorter
path to the 'closer' realm might be used. This shorter path may be
desirable because fewer intermediate KDCs would know the session
key of the ticket involved. For this reason, clients SHOULD
evaluate whether they trust the realms transited in obtaining the
'closer' ticket when making a decision to use the ticket in
future.
Once the client obtains a ticket-granting ticket for the
appropriate realm, it determines which Kerberos servers serve that
realm, and contacts one. The list might be obtained through a
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configuration file or network service or it MAY be generated from
the name of the realm; as long as the secret keys exchanged by
realms are kept secret, only denial of service results from using
a false Kerberos server.
As in the AS exchange, the client MAY specify a number of options
in the KRB_TGS_REQ message. One of these options is the ENC-TKT-
IN-SKEY option used for user-to-user authentication. An overview
of user-to-user authentication can be found in section 3.7. When
generating the KRB_TGS_REQ message, this option indicates that the
client is including a ticket-granting ticket obtained from the
application server in the additional tickets field of the request
and that the KDC SHOULD encrypt the ticket for the application
server using the session key from this additional ticket, instead
of using a server key from the principal database.
The client prepares the KRB_TGS_REQ message, providing an
authentication header as an element of the padata field, and
including the same fields as used in the KRB_AS_REQ message along
with several optional fields: the enc-authorizatfion-data field
for application server use and additional tickets required by some
options.
In preparing the authentication header, the client can select a
sub-session key under which the response from the Kerberos server
will be encrypted [16]. If the sub-session key is not specified,
the session key from the ticket-granting ticket will be used. If
the enc-authorization-data is present, it MUST be encrypted in the
sub-session key, if present, from the authenticator portion of the
authentication header, or if not present, using the session key
from the ticket-granting ticket.
Once prepared, the message is sent to a Kerberos server for the
destination realm.
3.3.2. Receipt of KRB_TGS_REQ message
The KRB_TGS_REQ message is processed in a manner similar to the
KRB_AS_REQ message, but there are many additional checks to be
performed. First, the Kerberos server MUST determine which server
the accompanying ticket is for and it MUST select the appropriate
key to decrypt it. For a normal KRB_TGS_REQ message, it will be
for the ticket granting service, and the TGS's key will be used.
If the TGT was issued by another realm, then the appropriate
inter-realm key MUST be used. If the accompanying ticket is not a
ticket-granting ticket for the current realm, but is for an
application server in the current realm, the RENEW, VALIDATE, or
PROXY options are specified in the request, and the server for
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which a ticket is requested is the server named in the
accompanying ticket, then the KDC will decrypt the ticket in the
authentication header using the key of the server for which it was
issued. If no ticket can be found in the padata field, the
KDC_ERR_PADATA_TYPE_NOSUPP error is returned.
Once the accompanying ticket has been decrypted, the user-supplied
checksum in the Authenticator MUST be verified against the
contents of the request, and the message rejected if the checksums
do not match (with an error code of KRB_AP_ERR_MODIFIED) or if the
checksum is not collision-proof (with an error code of
KRB_AP_ERR_INAPP_CKSUM). If the checksum type is not supported,
the KDC_ERR_SUMTYPE_NOSUPP error is returned. If the
authorization-data are present, they are decrypted using the sub-
session key from the Authenticator.
If any of the decryptions indicate failed integrity checks, the
KRB_AP_ERR_BAD_INTEGRITY error is returned.
As discussed in section 3.1.2, the KDC MUST send a valid
KRB_TGS_REP message if it receives a KRB_TGS_REQ message identical
to one it has recently processed. However, if the authenticator is
a replay, but the rest of the request is not identical, then the
KDC SHOULD return KRB_AP_ERR_REPEAT.
3.3.3. Generation of KRB_TGS_REP message
The KRB_TGS_REP message shares its format with the KRB_AS_REP
(KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The
detailed specification is in section 5.4.2.
The response will include a ticket for the requested server or for
a ticket granting server of an intermediate KDC to be contacted to
obtain the requested ticket. The Kerberos database is queried to
retrieve the record for the appropriate server (including the key
with which the ticket will be encrypted). If the request is for a
ticket-granting ticket for a remote realm, and if no key is shared
with the requested realm, then the Kerberos server will select the
realm 'closest' to the requested realm with which it does share a
key, and use that realm instead. This is the only case where the
response for the KDC will be for a different server than that
requested by the client.
By default, the address field, the client's name and realm, the
list of transited realms, the time of initial authentication, the
expiration time, and the authorization data of the newly-issued
ticket will be copied from the ticket-granting ticket (TGT) or
renewable ticket. If the transited field needs to be updated, but
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the transited type is not supported, the KDC_ERR_TRTYPE_NOSUPP
error is returned.
If the request specifies an endtime, then the endtime of the new
ticket is set to the minimum of (a) that request, (b) the endtime
from the TGT, and (c) the starttime of the TGT plus the minimum of
the maximum life for the application server and the maximum life
for the local realm (the maximum life for the requesting principal
was already applied when the TGT was issued). If the new ticket is
to be a renewal, then the endtime above is replaced by the minimum
of (a) the value of the renew_till field of the ticket and (b) the
starttime for the new ticket plus the life (endtime-starttime) of
the old ticket.
If the FORWARDED option has been requested, then the resulting
ticket will contain the addresses specified by the client. This
option will only be honored if the FORWARDABLE flag is set in the
TGT. The PROXY option is similar; the resulting ticket will
contain the addresses specified by the client. It will be honored
only if the PROXIABLE flag in the TGT is set. The PROXY option
will not be honored on requests for additional ticket-granting
tickets.
If the requested start time is absent, indicates a time in the
past, or is within the window of acceptable clock skew for the KDC
and the POSTDATE option has not been specified, then the start
time of the ticket is set to the authentication server's current
time. If it indicates a time in the future beyond the acceptable
clock skew, but the POSTDATED option has not been specified or the
MAY-POSTDATE flag is not set in the TGT, then the error
KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the ticket-
granting ticket has the MAY-POSTDATE flag set, then the resulting
ticket will be postdated and the requested starttime is checked
against the policy of the local realm. If acceptable, the ticket's
start time is set as requested, and the INVALID flag is set. The
postdated ticket MUST be validated before use by presenting it to
the KDC after the starttime has been reached. However, in no case
may the starttime, endtime, or renew-till time of a newly-issued
postdated ticket extend beyond the renew-till time of the ticket-
granting ticket.
If the ENC-TKT-IN-SKEY option has been specified and an additional
ticket has been included in the request, it indicates that the
client is using user- to-user authentication to prove its identity
to a server that does not have access to a persistent key. Section
3.7 describes the affect of this option on the entire Kerberos
protocol. When generating the KRB_TGS_REP message, this option in
the KRB_TGS_REQ message tells the KDC to decrypt the additional
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ticket using the key for the server to which the additional ticket
was issued and verify that it is a ticket-granting ticket. If the
name of the requested server is missing from the request, the name
of the client in the additional ticket will be used. Otherwise the
name of the requested server will be compared to the name of the
client in the additional ticket and if different, the request will
be rejected. If the request succeeds, the session key from the
additional ticket will be used to encrypt the new ticket that is
issued instead of using the key of the server for which the new
ticket will be used.
If the name of the server in the ticket that is presented to the
KDC as part of the authentication header is not that of the
ticket-granting server itself, the server is registered in the
realm of the KDC, and the RENEW option is requested, then the KDC
will verify that the RENEWABLE flag is set in the ticket, that the
INVALID flag is not set in the ticket, and that the renew_till
time is still in the future. If the VALIDATE option is requested,
the KDC will check that the starttime has passed and the INVALID
flag is set. If the PROXY option is requested, then the KDC will
check that the PROXIABLE flag is set in the ticket. If the tests
succeed, and the ticket passes the hotlist check described in the
next section, the KDC will issue the appropriate new ticket.
The ciphertext part of the response in the KRB_TGS_REP message is
encrypted in the sub-session key from the Authenticator, if
present, or the session key from the ticket-granting ticket. It is
not encrypted using the client's secret key. Furthermore, the
client's key's expiration date and the key version number fields
are left out since these values are stored along with the client's
database record, and that record is not needed to satisfy a
request based on a ticket-granting ticket.
3.3.3.1. Checking for revoked tickets
Whenever a request is made to the ticket-granting server, the
presented ticket(s) is(are) checked against a hot-list of tickets
which have been canceled. This hot-list might be implemented by
storing a range of issue timestamps for 'suspect tickets'; if a
presented ticket had an authtime in that range, it would be
rejected. In this way, a stolen ticket-granting ticket or
renewable ticket cannot be used to gain additional tickets
(renewals or otherwise) once the theft has been reported to the
KDC for the realm in which the server resides. Any normal ticket
obtained before it was reported stolen will still be valid
(because they require no interaction with the KDC), but only until
their normal expiration time. If TGT's have been issued for cross-
realm authentication, use of the cross-realm TGT will not be
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affected unless the hot-list is propagated to the KDCs for the
realms for which such cross-realm tickets were issued.
3.3.3.2. Encoding the transited field
If the identity of the server in the TGT that is presented to the
KDC as part of the authentication header is that of the ticket-
granting service, but the TGT was issued from another realm, the
KDC will look up the inter-realm key shared with that realm and
use that key to decrypt the ticket. If the ticket is valid, then
the KDC will honor the request, subject to the constraints
outlined above in the section describing the AS exchange. The
realm part of the client's identity will be taken from the ticket-
granting ticket. The name of the realm that issued the ticket-
granting ticket, if it is not the realm of the client principal,
will be added to the transited field of the ticket to be issued.
This is accomplished by reading the transited field from the
ticket-granting ticket (which is treated as an unordered set of
realm names), adding the new realm to the set, then constructing
and writing out its encoded (shorthand) form (this may involve a
rearrangement of the existing encoding).
Note that the ticket-granting service does not add the name of its
own realm. Instead, its responsibility is to add the name of the
previous realm. This prevents a malicious Kerberos server from
intentionally leaving out its own name (it could, however, omit
other realms' names).
The names of neither the local realm nor the principal's realm are
to be included in the transited field. They appear elsewhere in
the ticket and both are known to have taken part in authenticating
the principal. Since the endpoints are not included, both local
and single-hop inter-realm authentication result in a transited
field that is empty.
Because the name of each realm transited is added to this field,
it might potentially be very long. To decrease the length of this
field, its contents are encoded. The initially supported encoding
is optimized for the normal case of inter-realm communication: a
hierarchical arrangement of realms using either domain or X.500
style realm names. This encoding (called DOMAIN-X500-COMPRESS) is
now described.
Realm names in the transited field are separated by a ",". The
",", "\", trailing "."s, and leading spaces (" ") are special
characters, and if they are part of a realm name, they MUST be
quoted in the transited field by preceding them with a "\".
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A realm name ending with a "." is interpreted as being prepended
to the previous realm. For example, we can encode traversal of
EDU, MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and
CS.WASHINGTON.EDU as:
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points,
that they would not be included in this field, and we would have:
"EDU,MIT.,WASHINGTON.EDU"
A realm name beginning with a "/" is interpreted as being appended
to the previous realm. For the purpose of appending, the realm
preceding the first listed realm is considered to be the null
realm (""). If a realm name beginning with a "/" is to stand by
itself, then it SHOULD be preceded by a space (" "). For example,
we can encode traversal of /COM/HP/APOLLO, /COM/HP, /COM, and
/COM/DEC as:
"/COM,/HP,/APOLLO, /COM/DEC".
Like the example above, if /COM/HP/APOLLO and /COM/DEC are
endpoints, they would not be included in this field, and we would
have:
"/COM,/HP"
A null subfield preceding or following a "," indicates that all
realms between the previous realm and the next realm have been
traversed. For the purpose of interpreting null subfields, the
client's realm is considered to precede those in the transited
field, and the server's realm is considered to follow them. Thus,
"," means that all realms along the path between the client and
the server have been traversed. ",EDU, /COM," means that all
realms from the client's realm up to EDU (in a domain style
hierarchy) have been traversed, and that everything from /COM down
to the server's realm in an X.500 style has also been traversed.
This could occur if the EDU realm in one hierarchy shares an
inter-realm key directly with the /COM realm in another hierarchy.
3.3.4. Receipt of KRB_TGS_REP message
When the KRB_TGS_REP is received by the client, it is processed in
the same manner as the KRB_AS_REP processing described above. The
primary difference is that the ciphertext part of the response
must be decrypted using the sub-session key from the
Authenticator, if it was specified in the request, or the session
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key from the ticket-granting ticket, rather than the client's
secret key. The server name returned in the reply is the true
principal name of the service.
3.4. The KRB_SAFE Exchange
The KRB_SAFE message MAY be used by clients requiring the ability
to detect modifications of messages they exchange. It achieves
this by including a keyed collision-proof checksum of the user
data and some control information. The checksum is keyed with an
encryption key (usually the last key negotiated via subkeys, or
the session key if no negotiation has occurred).
3.4.1. Generation of a KRB_SAFE message
When an application wishes to send a KRB_SAFE message, it collects
its data and the appropriate control information and computes a
checksum over them. The checksum algorithm should be the keyed
checksum mandated to be implemented along with the crypto system
used for the sub-session or session key. The checksum is generated
using the sub-session key if present or the session key. Some
implementations use a different checksum algorithm for the
KRB_SAFE messages but doing so in a interoperable manner is not
always possible.
The control information for the KRB_SAFE message includes both a
timestamp and a sequence number. The designer of an application
using the KRB_SAFE message MUST choose at least one of the two
mechanisms. This choice SHOULD be based on the needs of the
application protocol.
Sequence numbers are useful when all messages sent will be
received by one's peer. Connection state is presently required to
maintain the session key, so maintaining the next sequence number
should not present an additional problem.
If the application protocol is expected to tolerate lost messages
without them being resent, the use of the timestamp is the
appropriate replay detection mechanism. Using timestamps is also
the appropriate mechanism for multi-cast protocols where all of
one's peers share a common sub-session key, but some messages will
be sent to a subset of one's peers.
After computing the checksum, the client then transmits the
information and checksum to the recipient in the message format
specified in section 5.6.1.
3.4.2. Receipt of KRB_SAFE message
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When an application receives a KRB_SAFE message, it verifies it as
follows. If any error occurs, an error code is reported for use
by the application.
The message is first checked by verifying that the protocol
version and type fields match the current version and KRB_SAFE,
respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
KRB_AP_ERR_MSG_TYPE error. The application verifies that the
checksum used is a collision-proof keyed checksum that uses keys
compatible with the sub-session or session key as appropriate (or
with the application key derived from the session or sub-session
keys), and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is
generated. The sender's address MUST be included in the control
information; the recipient verifies that the operating system's
report of the sender's address matches the sender's address in the
message, and (if a recipient address is specified or the recipient
requires an address) that one of the recipient's addresses appears
as the recipient's address in the message. To work with network
address translation, senders MAY use the directional address type
specified in section 8.1 for the sender address and not include
recipient addresses. A failed match for either case generates a
KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the
sequence number fields are checked. If timestamp and usec are
expected and not present, or they are present but not current, the
KRB_AP_ERR_SKEW error is generated. Timestamps are not required to
be strictly ordered; they are only required to be in the skew
window. If the server name, along with the client name, time and
microsecond fields from the Authenticator match any recently-seen
(sent or received) such tuples, the KRB_AP_ERR_REPEAT error is
generated. If an incorrect sequence number is included, or a
sequence number is expected but not present, the
KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp
and usec or a sequence number is present, a KRB_AP_ERR_MODIFIED
error is generated. Finally, the checksum is computed over the
data and control information, and if it doesn't match the received
checksum, a KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application is assured that the
message was generated by its peer and was not modified in transit.
Implementations SHOULD accept any checksum algorithm they
implement that both have adequate security and that have keys
compatible with the sub-session or session key. Unkeyed or non-
collision-proof checksums are not suitable for this use.
3.5. The KRB_PRIV Exchange
The KRB_PRIV message MAY be used by clients requiring
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confidentiality and the ability to detect modifications of
exchanged messages. It achieves this by encrypting the messages
and adding control information.
3.5.1. Generation of a KRB_PRIV message
When an application wishes to send a KRB_PRIV message, it collects
its data and the appropriate control information (specified in
section 5.7.1) and encrypts them under an encryption key (usually
the last key negotiated via subkeys, or the session key if no
negotiation has occurred). As part of the control information, the
client MUST choose to use either a timestamp or a sequence number
(or both); see the discussion in section 3.4.1 for guidelines on
which to use. After the user data and control information are
encrypted, the client transmits the ciphertext and some 'envelope'
information to the recipient.
3.5.2. Receipt of KRB_PRIV message
When an application receives a KRB_PRIV message, it verifies it as
follows. If any error occurs, an error code is reported for use
by the application.
The message is first checked by verifying that the protocol
version and type fields match the current version and KRB_PRIV,
respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
KRB_AP_ERR_MSG_TYPE error. The application then decrypts the
ciphertext and processes the resultant plaintext. If decryption
shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
error is generated.
The sender's address MUST be included in the control information;
the recipient verifies that the operating system's report of the
sender's address matches the sender's address in the message. If
a recipient address is specified or the recipient requires an
address then one of the recipient's addresses MUST also appear as
the recipient's address in the message. Where a sender's or
receiver's address might not otherwise match the address in a
message because of network address translation, an application MAY
be written to use addresses of the directional address type in
place of the actual network address.
A failed match for either case generates a KRB_AP_ERR_BADADDR
error. To work with network address translation, implementations
MAY use the directional address type defined in section 7.1 for
the sender address and include no recipient address.
Then the timestamp and usec and/or the sequence number fields are
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checked. If timestamp and usec are expected and not present, or
they are present but not current, the KRB_AP_ERR_SKEW error is
generated. If the server name, along with the client name, time
and microsecond fields from the Authenticator match any recently-
seen such tuples, the KRB_AP_ERR_REPEAT error is generated. If an
incorrect sequence number is included, or a sequence number is
expected but not present, the KRB_AP_ERR_BADORDER error is
generated. If neither a time-stamp and usec or a sequence number
is present, a KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application can assume the message
was generated by its peer, and was securely transmitted (without
intruders able to see the unencrypted contents).
3.6. The KRB_CRED Exchange
The KRB_CRED message MAY be used by clients requiring the ability
to send Kerberos credentials from one host to another. It achieves
this by sending the tickets together with encrypted data
containing the session keys and other information associated with
the tickets.
3.6.1. Generation of a KRB_CRED message
When an application wishes to send a KRB_CRED message it first
(using the KRB_TGS exchange) obtains credentials to be sent to the
remote host. It then constructs a KRB_CRED message using the
ticket or tickets so obtained, placing the session key needed to
use each ticket in the key field of the corresponding KrbCredInfo
sequence of the encrypted part of the KRB_CRED message.
Other information associated with each ticket and obtained during
the KRB_TGS exchange is also placed in the corresponding
KrbCredInfo sequence in the encrypted part of the KRB_CRED
message. The current time and, if specifically required by the
application the nonce, s-address, and r-address fields, are placed
in the encrypted part of the KRB_CRED message which is then
encrypted under an encryption key previously exchanged in the
KRB_AP exchange (usually the last key negotiated via subkeys, or
the session key if no negotiation has occurred).
Implementation note: When constructing a KRB_CRED message for
inclusion in a GSSAPI initial context token, the MIT
implementation of Kerberos will not encrypt the KRB_CRED message
if the session key is a DES or triple DES key. For
interoperability with MIT, the Microsoft implementation will not
encrypt the KRB_CRED in a GSSAPI token if it is using a DES
session key. Starting at version 1.2.5, MIT Kerberos can receive
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and decode either encrypted or unencrypted KRB_CRED tokens in the
GSSAPI exchange. The Heimdal implementation of Kerberos can also
accept either encrypted or unencrypted KRB_CRED messages. Since
the KRB_CRED message in a GSSAPI token is encrypted in the
authenticator, the MIT behavior does not present a security
problem, although it is a violation of the Kerberos specification.
3.6.2. Receipt of KRB_CRED message
When an application receives a KRB_CRED message, it verifies it.
If any error occurs, an error code is reported for use by the
application. The message is verified by checking that the protocol
version and type fields match the current version and KRB_CRED,
respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
KRB_AP_ERR_MSG_TYPE error. The application then decrypts the
ciphertext and processes the resultant plaintext. If decryption
shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
error is generated.
If present or required, the recipient MAY verify that the
operating system's report of the sender's address matches the
sender's address in the message, and that one of the recipient's
addresses appears as the recipient's address in the message. The
address check does not provide any added security, since the
address if present has already been checked in the KRB_AP_REQ
message and there is not any benefit to be gained by an attacker
in reflecting a KRB_CRED message back to its originator. Thus, the
recipient MAY ignore the address even if present in order to work
better in NAT environments. A failed match for either case
generates a KRB_AP_ERR_BADADDR error. Recipients MAY skip the
address check as the KRB_CRED message cannot generally be
reflected back to the originator. The timestamp and usec fields
(and the nonce field if required) are checked next. If the
timestamp and usec are not present, or they are present but not
current, the KRB_AP_ERR_SKEW error is generated.
If all the checks succeed, the application stores each of the new
tickets in its credentials cache together with the session key and
other information in the corresponding KrbCredInfo sequence from
the encrypted part of the KRB_CRED message.
3.7. User-to-User Authentication Exchanges
User-to-User authentication provides a method to perform
authentication when the verifier does not have a access to long
term service key. This might be the case when running a server
(for example a window server) as a user on a workstation. In such
cases, the server may have access to the ticket-granting ticket
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obtained when the user logged in to the workstation, but because
the server is running as an unprivileged user it might not have
access to system keys. Similar situations may arise when running
peer-to-peer applications.
Summary
Message direction Message type Sections
0. Message from application server Not Specified
1. Client to Kerberos KRB_TGS_REQ 3.3 + 5.4.1
2. Kerberos to client KRB_TGS_REP or 3.3 + 5.4.2
KRB_ERROR 5.9.1
3. Client to Application server KRB_AP_REQ 3.2 + 5.5.1
To address this problem, the Kerberos protocol allows the client
to request that the ticket issued by the KDC be encrypted using a
session key from a ticket-granting ticket issued to the party that
will verify the authentication. This ticket-granting ticket must
be obtained from the verifier by means of an exchange external to
the Kerberos protocol, usually as part of the application
protocol. This message is shown in the summary above as message 0.
Note that because the ticket-granting ticket is encrypted in the
KDC's secret key, it can not be used for authentication without
possession of the corresponding secret key. Furthermore, because
the verifier does not reveal the corresponding secret key,
providing a copy of the verifier's ticket-granting ticket does not
allow impersonation of the verifier.
Message 0 in the table above represents an application specific
negotiation between the client and server, at the end of which
both have determined that they will use user-to-user
authentication and the client has obtained the server's TGT.
Next, the client includes the server's TGT as an additional ticket
in its KRB_TGS_REQ request to the KDC (message 1 in the table
above) and specifies the ENC-TKT-IN-SKEY option in its request.
If validated according to the instructions in 3.3.3, the
application ticket returned to the client (message 2 in the table
above) will be encrypted using the session key from the additional
ticket and the client will note this when it uses or stores the
application ticket.
When contacting the server using a ticket obtained for user-to-
user authentication (message 3 in the table above), the client
MUST specify the USE-SESSION-KEY flag in the ap-options field.
This tells the application server to use the session key
associated with its ticket-granting ticket to decrypt the server
ticket provided in the application request.
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4. Encryption and Checksum Specifications
The Kerberos protocols described in this document are designed to
encrypt messages of arbitrary sizes, using stream or block
encryption ciphers. Encryption is used to prove the identities of
the network entities participating in message exchanges. The Key
Distribution Center for each realm is trusted by all principals
registered in that realm to store a secret key in confidence.
Proof of knowledge of this secret key is used to verify the
authenticity of a principal.
The KDC uses the principal's secret key (in the AS exchange) or a
shared session key (in the TGS exchange) to encrypt responses to
ticket requests; the ability to obtain the secret key or session
key implies the knowledge of the appropriate keys and the identity
of the KDC. The ability of a principal to decrypt the KDC response
and present a Ticket and a properly formed Authenticator
(generated with the session key from the KDC response) to a
service verifies the identity of the principal; likewise the
ability of the service to extract the session key from the Ticket
and prove its knowledge thereof in a response verifies the
identity of the service.
[@KCRYPTO] defines a framework for defining encryption and
checksum mechanisms for use with Kerberos. It also defines several
such mechanisms, and more may be added in future updates to that
document.
The string-to-key operation provided by [@KCRYPTO] is used to
produce a long-term key for a principal (generally for a user).
The default salt string, if none is provided via pre-
authentication data, is the concatenation of the principal's realm
and name components, in order, with no separators. Unless
otherwise indicated, the default string-to-key opaque parameter
set as defined in [@KCRYPTO] is used.
Encrypted data, keys and checksums are transmitted using the
EncryptedData, EncryptionKey and Checksum data objects defined in
section 5.2.9. The encryption, decryption, and checksum operations
described in this document use the corresponding encryption,
decryption, and get_mic operations described in [@KCRYPTO], with
implicit "specific key" generation using the "key usage" values
specified in the description of each EncryptedData or Checksum
object to vary the key for each operation. Note that in some
cases, the value to be used is dependent on the method of choosing
the key or the context of the message.
Key usages are unsigned 32 bit integers; zero is not permitted.
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The key usage values for encrypting or checksumming Kerberos
messages are indicated in section 5 along with the message
definitions. Key usage values 512-1023 are reserved for uses
internal to a Kerberos implementation. (For example, seeding a
pseudo-random number generator with a value produced by encrypting
something with a session key and a key usage value not used for
any other purpose.) Key usage values between 1024 and 2047
(inclusive) are reserved for application use; applications SHOULD
use even values for encryption and odd values for checksums within
this range. Key usage values are also summarized in a table in
section 7.5.1.
There might exist other documents which define protocols in terms
of the RFC1510 encryption types or checksum types. Such documents
would not know about key usages. In order that these
specifications continue to be meaningful until they are updated,
if no key usage values are specified then key usages 1024 and 1025
must be used to derive keys for encryption and checksums,
respectively (this does not apply to protocols that do their own
encryption independent of this framework, directly using the key
resulting from the Kerberos authentication exchange.) New
protocols defined in terms of the Kerberos encryption and checksum
types SHOULD use their own key usage values.
Unless otherwise indicated, no cipher state chaining is done from
one encryption operation to another.
Implementation note: While not recommended, some application
protocols will continue to use the key data directly, even if only
in currently existing protocol specifications. An implementation
intended to support general Kerberos applications may therefore
need to make key data available, as well as the attributes and
operations described in [@KCRYPTO]. One of the more common
reasons for directly performing encryption is direct control over
negotiation and selection of a "sufficiently strong" encryption
algorithm (in the context of a given application). While Kerberos
does not directly provide a facility for negotiating encryption
types between the application client and server, there are
approaches for using Kerberos to facilitate this negotiation - for
example, a client may request only "sufficiently strong" session
key types from the KDC and expect that any type returned by the
KDC will be understood and supported by the application server.
5. Message Specifications
NOTE: The ASN.1 collected here should be identical to the contents
of Appendix A. In case of conflict, the contents of Appendix A
shall take precedence.
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The Kerberos protocol is defined here in terms of Abstract Syntax
Notation One (ASN.1) [X680], which provides a syntax for
specifying both the abstract layout of protocol messages as well
as their encodings. Implementors not utilizing an existing ASN.1
compiler or support library are cautioned to thoroughly understand
the actual ASN.1 specification to ensure correct implementation
behavior, as there is more complexity in the notation than is
immediately obvious, and some tutorials and guides to ASN.1 are
misleading or erroneous.
Note that in several places, there have been changes here from RFC
1510 that change the abstract types. This is in part to address
widespread assumptions that various implementors have made, in
some cases resulting in unintentional violations of the ASN.1
standard. These are clearly flagged where they occur. The
differences between the abstract types in RFC 1510 and abstract
types in this document can cause incompatible encodings to be
emitted when certain encoding rules, e.g. the Packed Encoding
Rules (PER), are used. This theoretical incompatibility should not
be relevant for Kerberos, since Kerberos explicitly specifies the
use of the Distinguished Encoding Rules (DER). It might be an
issue for protocols wishing to use Kerberos types with other
encoding rules. (This practice is not recommended.) With very few
exceptions (most notably the usages of BIT STRING), the encodings
resulting from using the DER remain identical between the types
defined in RFC 1510 and the types defined in this document.
The type definitions in this section assume an ASN.1 module
definition of the following form:
KerberosV5Spec2 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) krb5spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
-- rest of definitions here
END
This specifies that the tagging context for the module will be
explicit and non-automatic.
Note that in some other publications [RFC1510] [RFC1964], the
"dod" portion of the object identifier is erroneously specified as
having the value "5". In the case of RFC 1964, use of the
"correct" OID value would result in a change in the wire protocol;
therefore, it remains unchanged for now.
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Note that elsewhere in this document, nomenclature for various
message types is inconsistent, but largely follows C language
conventions, including use of underscore (_) characters and all-
caps spelling of names intended to be numeric constants. Also, in
some places, identifiers (especially ones referring to constants)
are written in all-caps in order to distinguish them from
surrounding explanatory text.
The ASN.1 notation does not permit underscores in identifiers, so
in actual ASN.1 definitions, underscores are replaced with hyphens
(-). Additionally, structure member names and defined values in
ASN.1 MUST begin with a lowercase letter, while type names MUST
begin with an uppercase letter.
5.1. Specific Compatibility Notes on ASN.1
For compatibility purposes, implementors should heed the following
specific notes regarding the use of ASN.1 in Kerberos. These notes
do not describe deviations from standard usage of ASN.1. The
purpose of these notes is to instead describe some historical
quirks and non-compliance of various implementations, as well as
historical ambiguities, which, while being valid ASN.1, can lead
to confusion during implementation.
5.1.1. ASN.1 Distinguished Encoding Rules
The encoding of Kerberos protocol messages shall obey the
Distinguished Encoding Rules (DER) of ASN.1 as described in
[X690]. Some implementations (believed to be primarily ones
derived from DCE 1.1 and earlier) are known to use the more
general Basic Encoding Rules (BER); in particular, these
implementations send indefinite encodings of lengths.
Implementations MAY accept such encodings in the interests of
backwards compatibility, though implementors are warned that
decoding fully-general BER is fraught with peril.
5.1.2. Optional Integer Fields
Some implementations do not internally distinguish between an
omitted optional integer value and a transmitted value of zero.
The places in the protocol where this is relevant include various
microseconds fields, nonces, and sequence numbers. Implementations
SHOULD treat omitted optional integer values as having been
transmitted with a value of zero, if the application is expecting
this.
5.1.3. Empty SEQUENCE OF Types
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There are places in the protocol where a message contains a
SEQUENCE OF type as an optional member. This can result in an
encoding that contains an empty SEQUENCE OF encoding. The Kerberos
protocol does not semantically distinguish between an absent
optional SEQUENCE OF type and a present optional but empty
SEQUENCE OF type. Implementations SHOULD NOT send empty SEQUENCE
OF encodings that are marked OPTIONAL, but SHOULD accept them as
being equivalent to an omitted OPTIONAL type. In the ASN.1 syntax
describing Kerberos messages, instances of these problematic
optional SEQUENCE OF types are indicated with a comment.
5.1.4. Unrecognized Tag Numbers
Future revisions to this protocol may include new message types
with different APPLICATION class tag numbers. Such revisions
should protect older implementations by only sending the message
types to parties that are known to understand them, e.g. by means
of a flag bit set by the receiver in a preceding request. In the
interest of robust error handling, implementations SHOULD
gracefully handle receiving a message with an unrecognized tag
anyway, and return an error message if appropriate.
In particular, KDCs SHOULD return KRB_AP_ERR_MSG_TYPE if the
incorrect tag is sent over a TCP transport. The KDCs SHOULD NOT
respond to messages received with an unknown tag over UDP
transport in order to avoid denial of service attacks. For non-
KDC applications, the Kerberos implementation typically indicates
an error to the application which takes appropriate steps based on
the application protocol.
5.1.5. Tag Numbers Greater Than 30
A naive implementation of a DER ASN.1 decoder may experience
problems with ASN.1 tag numbers greater than 30, due to such tag
numbers being encoded using more than one byte. Future revisions
of this protocol may utilize tag numbers greater than 30, and
implementations SHOULD be prepared to gracefully return an error,
if appropriate, if they do not recognize the tag.
5.2. Basic Kerberos Types
This section defines a number of basic types that are potentially
used in multiple Kerberos protocol messages.
5.2.1. KerberosString
The original specification of the Kerberos protocol in RFC 1510
uses GeneralString in numerous places for human-readable string
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data. Historical implementations of Kerberos cannot utilize the
full power of GeneralString. This ASN.1 type requires the use of
designation and invocation escape sequences as specified in
ISO-2022/ECMA-35 [ISO-2022/ECMA-35] to switch character sets, and
the default character set that is designated as G0 is the
ISO-646/ECMA-6 [ISO-646,ECMA-6] International Reference Version
(IRV) (aka U.S. ASCII), which mostly works.
ISO-2022/ECMA-35 defines four character-set code elements (G0..G3)
and two Control-function code elements (C0..C1). DER prohibits the
designation of character sets as any but the G0 and C0 sets.
Unfortunately, this seems to have the side effect of prohibiting
the use of ISO-8859 (ISO Latin) [ISO-8859] character-sets or any
other character-sets that utilize a 96-character set, since it is
prohibited by ISO-2022/ECMA-35 to designate them as the G0 code
element. This side effect is being investigated in the ASN.1
standards community.
In practice, many implementations treat GeneralStrings as if they
were 8-bit strings of whichever character set the implementation
defaults to, without regard for correct usage of character-set
designation escape sequences. The default character set is often
determined by the current user's operating system dependent
locale. At least one major implementation places unescaped UTF-8
encoded Unicode characters in the GeneralString. This failure to
adhere to the GeneralString specifications results in
interoperability issues when conflicting character encodings are
utilized by the Kerberos clients, services, and KDC.
This unfortunate situation is the result of improper documentation
of the restrictions of the ASN.1 GeneralString type in prior
Kerberos specifications.
The new (post-RFC 1510) type KerberosString, defined below, is a
GeneralString that is constrained to only contain characters in
IA5String
KerberosString ::= GeneralString (IA5String)
In general, US-ASCII control characters should not be used in
KerberosString. Control characters SHOULD NOT be used in principal
names or realm names.
For compatibility, implementations MAY choose to accept
GeneralString values that contain characters other than those
permitted by IA5String, but they should be aware that character
set designation codes will likely be absent, and that the encoding
should probably be treated as locale-specific in almost every way.
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Implementations MAY also choose to emit GeneralString values that
are beyond those permitted by IA5String, but should be aware that
doing so is extraordinarily risky from an interoperability
perspective.
Some existing implementations use GeneralString to encode
unescaped locale-specific characters. This is a violation of the
ASN.1 standard. Most of these implementations encode US-ASCII in
the left-hand half, so as long the implementation transmits only
US-ASCII, the ASN.1 standard is not violated in this regard. As
soon as such an implementation encodes unescaped locale-specific
characters with the high bit set, it violates the ASN.1 standard.
Other implementations have been known to use GeneralString to
contain a UTF-8 encoding. This also violates the ASN.1 standard,
since UTF-8 is a different encoding, not a 94 or 96 character "G"
set as defined by ISO 2022. It is believed that these
implementations do not even use the ISO 2022 escape sequence to
change the character encoding. Even if implementations were to
announce the change of encoding by using that escape sequence, the
ASN.1 standard prohibits the use of any escape sequences other
than those used to designate/invoke "G" or "C" sets allowed by
GeneralString.
Future revisions to this protocol will almost certainly allow for
a more interoperable representation of principal names, probably
including UTF8String.
Note that applying a new constraint to a previously unconstrained
type constitutes creation of a new ASN.1 type. In this particular
case, the change does not result in a changed encoding under DER.
5.2.2. Realm and PrincipalName
Realm ::= KerberosString
PrincipalName ::= SEQUENCE {
name-type [0] Int32,
name-string [1] SEQUENCE OF KerberosString
}
Kerberos realm names are encoded as KerberosStrings. Realms shall
not contain a character with the code 0 (the US-ASCII NUL). Most
realms will usually consist of several components separated by
periods (.), in the style of Internet Domain Names, or separated
by slashes (/) in the style of X.500 names. Acceptable forms for
realm names are specified in section 6.1.. A PrincipalName is a
typed sequence of components consisting of the following sub-
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fields:
name-type
This field specifies the type of name that follows. Pre-defined
values for this field are specified in section 6.2. The name-type
SHOULD be treated as a hint. Ignoring the name type, no two names
can be the same (i.e. at least one of the components, or the
realm, must be different).
name-string
This field encodes a sequence of components that form a name, each
component encoded as a KerberosString. Taken together, a
PrincipalName and a Realm form a principal identifier. Most
PrincipalNames will have only a few components (typically one or
two).
5.2.3. KerberosTime
KerberosTime ::= GeneralizedTime -- with no fractional seconds
The timestamps used in Kerberos are encoded as GeneralizedTimes. A
KerberosTime value shall not include any fractional portions of
the seconds. As required by the DER, it further shall not include
any separators, and it shall specify the UTC time zone (Z).
Example: The only valid format for UTC time 6 minutes, 27 seconds
after 9 pm on 6 November 1985 is 19851106210627Z.
5.2.4. Constrained Integer types
Some integer members of types SHOULD be constrained to values
representable in 32 bits, for compatibility with reasonable
implementation limits.
Int32 ::= INTEGER (-2147483648..2147483647)
-- signed values representable in 32 bits
UInt32 ::= INTEGER (0..4294967295)
-- unsigned 32 bit values
Microseconds ::= INTEGER (0..999999)
-- microseconds
While this results in changes to the abstract types from the RFC
1510 version, the encoding in DER should be unaltered. Historical
implementations were typically limited to 32-bit integer values
anyway, and assigned numbers SHOULD fall in the space of integer
values representable in 32 bits in order to promote
interoperability anyway.
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There are several integer fields in messages that are constrained
to fixed values.
pvno
also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always
the constant integer 5. There is no easy way to make this field
into a useful protocol version number, so its value is fixed.
msg-type
this integer field is usually identical to the application tag
number of the containing message type.
5.2.5. HostAddress and HostAddresses
HostAddress ::= SEQUENCE {
addr-type [0] Int32,
address [1] OCTET STRING
}
-- NOTE: HostAddresses is always used as an OPTIONAL field and
-- should not be empty.
HostAddresses -- NOTE: subtly different from rfc1510,
-- but has a value mapping and encodes the same
::= SEQUENCE OF HostAddress
The host address encodings consists of two fields:
addr-type
This field specifies the type of address that follows. Pre-defined
values for this field are specified in section 7.5.3.
address
This field encodes a single address of type addr-type.
5.2.6. AuthorizationData
-- NOTE: AuthorizationData is always used as an OPTIONAL field and
-- should not be empty.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
ad-type [0] Int32,
ad-data [1] OCTET STRING
}
ad-data
This field contains authorization data to be interpreted according
to the value of the corresponding ad-type field.
ad-type
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This field specifies the format for the ad-data subfield. All
negative values are reserved for local use. Non-negative values
are reserved for registered use.
Each sequence of type and data is referred to as an authorization
element. Elements MAY be application specific, however, there is a
common set of recursive elements that should be understood by all
implementations. These elements contain other elements embedded
within them, and the interpretation of the encapsulating element
determines which of the embedded elements must be interpreted, and
which may be ignored.
These common authorization data elements are recursively defined,
meaning the ad-data for these types will itself contain a sequence of
authorization data whose interpretation is affected by the
encapsulating element. Depending on the meaning of the encapsulating
element, the encapsulated elements may be ignored, might be
interpreted as issued directly by the KDC, or they might be stored in
a separate plaintext part of the ticket. The types of the
encapsulating elements are specified as part of the Kerberos
specification because the behavior based on these values should be
understood across implementations whereas other elements need only be
understood by the applications which they affect.
Authorization data elements are considered critical if present in a
ticket or authenticator. Unless encapsulated in a known authorization
data element amending the criticality of the elements it contains, if
an unknown authorization data element type is received by a server
either in an AP-REQ or in a ticket contained in an AP-REQ, then
authentication MUST fail. Authorization data is intended to restrict
the use of a ticket. If the service cannot determine whether the
restriction applies to that service then a security weakness may
result if the ticket can be used for that service. Authorization
elements that are optional can be enclosed in AD-IF-RELEVANT element.
In the definitions that follow, the value of the ad-type for the
element will be specified as the least significant part of the
subsection number, and the value of the ad-data will be as shown in
the ASN.1 structure that follows the subsection heading.
contents of ad-data ad-type
DER encoding of AD-IF-RELEVANT 1
DER encoding of AD-KDCIssued 4
DER encoding of AD-AND-OR 5
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DER encoding of AD-MANDATORY-FOR-KDC 8
5.2.6.1. IF-RELEVANT
AD-IF-RELEVANT ::= AuthorizationData
AD elements encapsulated within the if-relevant element are
intended for interpretation only by application servers that
understand the particular ad-type of the embedded element.
Application servers that do not understand the type of an element
embedded within the if-relevant element MAY ignore the
uninterpretable element. This element promotes interoperability
across implementations which may have local extensions for
authorization. The ad-type for AD-IF-RELEVANT is (1).
5.2.6.2. KDCIssued
AD-KDCIssued ::= SEQUENCE {
ad-checksum [0] Checksum,
i-realm [1] Realm OPTIONAL,
i-sname [2] PrincipalName OPTIONAL,
elements [3] AuthorizationData
}
ad-checksum
A cryptographic checksum computed over the DER encoding of the
AuthorizationData in the "elements" field, keyed with the session
key. Its checksumtype is the mandatory checksum type for the
encryption type of the session key, and its key usage value is 19.
i-realm, i-sname
The name of the issuing principal if different from the KDC
itself. This field would be used when the KDC can verify the
authenticity of elements signed by the issuing principal and it
allows this KDC to notify the application server of the validity
of those elements.
elements
A sequence of authorization data elements issued by the KDC.
The KDC-issued ad-data field is intended to provide a means for
Kerberos principal credentials to embed within themselves privilege
attributes and other mechanisms for positive authorization,
amplifying the privileges of the principal beyond what can be done
using a credentials without such an a-data element.
This can not be provided without this element because the definition
of the authorization-data field allows elements to be added at will
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by the bearer of a TGT at the time that they request service tickets
and elements may also be added to a delegated ticket by inclusion in
the authenticator.
For KDC-issued elements this is prevented because the elements are
signed by the KDC by including a checksum encrypted using the
server's key (the same key used to encrypt the ticket - or a key
derived from that key). Elements encapsulated with in the KDC-issued
element MUST be ignored by the application server if this
"signature" is not present. Further, elements encapsulated within
this element from a ticket-granting ticket MAY be interpreted by the
KDC, and used as a basis according to policy for including new signed
elements within derivative tickets, but they will not be copied to a
derivative ticket directly. If they are copied directly to a
derivative ticket by a KDC that is not aware of this element, the
signature will not be correct for the application ticket elements,
and the field will be ignored by the application server.
This element and the elements it encapsulates MAY be safely ignored
by applications, application servers, and KDCs that do not implement
this element.
The ad-type for AD-KDC-ISSUED is (4).
5.2.6.3. AND-OR
AD-AND-OR ::= SEQUENCE {
condition-count [0] INTEGER,
elements [1] AuthorizationData
}
When restrictive AD elements are encapsulated within the and-or
element, the and-or element is considered satisfied if and only if
at least the number of encapsulated elements specified in
condition-count are satisfied. Therefore, this element MAY be
used to implement an "or" operation by setting the condition-count
field to 1, and it MAY specify an "and" operation by setting the
condition count to the number of embedded elements. Application
servers that do not implement this element MUST reject tickets
that contain authorization data elements of this type.
The ad-type for AD-AND-OR is (5).
5.2.6.4. MANDATORY-FOR-KDC
AD-MANDATORY-FOR-KDC ::= AuthorizationData
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AD elements encapsulated within the mandatory-for-kdc element are
to be interpreted by the KDC. KDCs that do not understand the type
of an element embedded within the mandatory-for-kdc element MUST
reject the request.
The ad-type for AD-MANDATORY-FOR-KDC is (8).
5.2.7. PA-DATA
Historically, PA-DATA have been known as "pre-authentication
data", meaning that they were used to augment the initial
authentication with the KDC. Since that time, they have also been
used as a typed hole with which to extend protocol exchanges with
the KDC.
PA-DATA ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
padata-type [1] Int32,
padata-value [2] OCTET STRING -- might be encoded AP-REQ
}
padata-type
indicates the way that the padata-value element is to be
interpreted. Negative values of padata-type are reserved for
unregistered use; non-negative values are used for a registered
interpretation of the element type.
padata-value
Usually contains the DER encoding of another type; the padata-type
field identifies which type is encoded here.
padata-type name contents of padata-value
1 pa-tgs-req DER encoding of AP-REQ
2 pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP
3 pa-pw-salt salt (not ASN.1 encoded)
11 pa-etype-info DER encoding of ETYPE-INFO
19 pa-etype-info2 DER encoding of ETYPE-INFO2
This field MAY also contain information needed by certain
extensions to the Kerberos protocol. For example, it might be used
to initially verify the identity of a client before any response
is returned.
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The padata field can also contain information needed to help the
KDC or the client select the key needed for generating or
decrypting the response. This form of the padata is useful for
supporting the use of certain token cards with Kerberos. The
details of such extensions are specified in separate documents.
See [Pat92] for additional uses of this field.
5.2.7.1. PA-TGS-REQ
In the case of requests for additional tickets (KRB_TGS_REQ),
padata-value will contain an encoded AP-REQ. The checksum in the
authenticator (which MUST be collision-proof) is to be computed
over the KDC-REQ-BODY encoding.
5.2.7.2. Encrypted Timestamp Pre-authentication
There are pre-authentication types that may be used to pre-
authenticate a client by means of an encrypted timestamp.
PA-ENC-TIMESTAMP ::= EncryptedData -- PA-ENC-TS-ENC
PA-ENC-TS-ENC ::= SEQUENCE {
patimestamp [0] KerberosTime -- client's time --,
pausec [1] Microseconds OPTIONAL
}
Patimestamp contains the client's time, and pausec contains the
microseconds, which MAY be omitted if a client will not generate
more than one request per second. The ciphertext (padata-value)
consists of the PA-ENC-TS-ENC encoding, encrypted using the
client's secret key and a key usage value of 1.
This pre-authentication type was not present in RFC 1510, but many
implementations support it.
5.2.7.3. PA-PW-SALT
The padata-value for this pre-authentication type contains the
salt for the string-to-key to be used by the client to obtain the
key for decrypting the encrypted part of an AS-REP message.
Unfortunately, for historical reasons, the character set to be
used is unspecified and probably locale-specific.
This pre-authentication type was not present in RFC 1510, but many
implementations support it. It is necessary in any case where the
salt for the string-to-key algorithm is not the default.
In the trivial example, a zero-length salt string is very
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commonplace for realms that have converted their principal
databases from Kerberos 4.
A KDC SHOULD NOT send PA-PW-SALT when issuing a KRB-ERROR message
that requests additional pre-authentication. Implementation note:
some KDC implementations issue an erroneous PA-PW-SALT when
issuing a KRB-ERROR message that requests additional pre-
authentication. Therefore, clients SHOULD ignore a PA-PW-SALT
accompanying a KRB-ERROR message that requests additional pre-
authentication. As noted in section 3.1.3, a KDC MUST NOT send
PA-PW-SALT when the client's AS-REQ includes at least one "newer"
etype.
5.2.7.4. PA-ETYPE-INFO
The ETYPE-INFO pre-authentication type is sent by the KDC in a
KRB-ERROR indicating a requirement for additional pre-
authentication. It is usually used to notify a client of which key
to use for the encryption of an encrypted timestamp for the
purposes of sending a PA-ENC-TIMESTAMP pre-authentication value.
It MAY also be sent in an AS-REP to provide information to the
client about which key salt to use for the string-to-key to be
used by the client to obtain the key for decrypting the encrypted
part the AS-REP.
ETYPE-INFO-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] OCTET STRING OPTIONAL
}
ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY
The salt, like that of PA-PW-SALT, is also completely unspecified
with respect to character set and is probably locale-specific.
If ETYPE-INFO is sent in an AS-REP, there shall be exactly one
ETYPE-INFO-ENTRY, and its etype shall match that of the enc-part
in the AS-REP.
This pre-authentication type was not present in RFC 1510, but many
implementations that support encrypted timestamps for pre-
authentication need to support ETYPE-INFO as well. As noted in
section 3.1.3, a KDC MUST NOT send PA-ETYPE-INFO when the client's
AS-REQ includes at least one "newer" etype.
5.2.7.5. PA-ETYPE-INFO2
The ETYPE-INFO2 pre-authentication type is sent by the KDC in a
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KRB-ERROR indicating a requirement for additional pre-
authentication. It is usually used to notify a client of which key
to use for the encryption of an encrypted timestamp for the
purposes of sending a PA-ENC-TIMESTAMP pre-authentication value.
It MAY also be sent in an AS-REP to provide information to the
client about which key salt to use for the string-to-key to be
used by the client to obtain the key for decrypting the encrypted
part the AS-REP.
ETYPE-INFO2-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] KerberosString OPTIONAL,
s2kparams [2] OCTET STRING OPTIONAL
}
ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY
The type of the salt is KerberosString, but existing installations
might have locale-specific characters stored in salt strings, and
implementors MAY choose to handle them.
The interpretation of s2kparams is specified in the cryptosystem
description associated with the etype. Each cryptosystem has a
default interpretation of s2kparams that will hold if that element
is omitted from the encoding of ETYPE-INFO2-ENTRY.
If ETYPE-INFO2 is sent in an AS-REP, there shall be exactly one
ETYPE-INFO2-ENTRY, and its etype shall match that of the enc-part
in the AS-REP.
The preferred ordering of the "hint" pre-authentication data that
affect client key selection is: ETYPE-INFO2, followed by ETYPE-
INFO, followed by PW-SALT. As noted in section 3.1.3, a KDC MUST
NOT send ETYPE-INFO or PW-SALT when the client's AS-REQ includes
at least one "newer" etype.
The ETYPE-INFO2 pre-authentication type was not present in RFC
1510.
5.2.8. KerberosFlags
For several message types, a specific constrained bit string type,
KerberosFlags, is used.
KerberosFlags ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits
-- shall be sent, but no fewer than 32
Compatibility note: the following paragraphs describe a change
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from the RFC1510 description of bit strings that would result in
incompatility in the case of an implementation that strictly
conformed to ASN.1 DER and RFC1510.
ASN.1 bit strings have multiple uses. The simplest use of a bit
string is to contain a vector of bits, with no particular meaning
attached to individual bits. This vector of bits is not
necessarily a multiple of eight bits long. The use in Kerberos of
a bit string as a compact boolean vector wherein each element has
a distinct meaning poses some problems. The natural notation for a
compact boolean vector is the ASN.1 "NamedBit" notation, and the
DER require that encodings of a bit string using "NamedBit"
notation exclude any trailing zero bits. This truncation is easy
to neglect, especially given C language implementations that
naturally choose to store boolean vectors as 32 bit integers.
For example, if the notation for KDCOptions were to include the
"NamedBit" notation, as in RFC 1510, and a KDCOptions value to be
encoded had only the "forwardable" (bit number one) bit set, the
DER encoding MUST include only two bits: the first reserved bit
("reserved", bit number zero, value zero) and the one-valued bit
(bit number one) for "forwardable".
Most existing implementations of Kerberos unconditionally send 32
bits on the wire when encoding bit strings used as boolean
vectors. This behavior violates the ASN.1 syntax used for flag
values in RFC 1510, but occurs on such a widely installed base
that the protocol description is being modified to accommodate it.
Consequently, this document removes the "NamedBit" notations for
individual bits, relegating them to comments. The size constraint
on the KerberosFlags type requires that at least 32 bits be
encoded at all times, though a lenient implementation MAY choose
to accept fewer than 32 bits and to treat the missing bits as set
to zero.
Currently, no uses of KerberosFlags specify more than 32 bits
worth of flags, although future revisions of this document may do
so. When more than 32 bits are to be transmitted in a
KerberosFlags value, future revisions to this document will likely
specify that the smallest number of bits needed to encode the
highest-numbered one-valued bit should be sent. This is somewhat
similar to the DER encoding of a bit string that is declared with
the "NamedBit" notation.
5.2.9. Cryptosystem-related Types
Many Kerberos protocol messages contain an EncryptedData as a
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container for arbitrary encrypted data, which is often the
encrypted encoding of another data type. Fields within
EncryptedData assist the recipient in selecting a key with which
to decrypt the enclosed data.
EncryptedData ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
kvno [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING -- ciphertext
}
etype
This field identifies which encryption algorithm was used to
encipher the cipher.
kvno
This field contains the version number of the key under which data
is encrypted. It is only present in messages encrypted under long
lasting keys, such as principals' secret keys.
cipher
This field contains the enciphered text, encoded as an OCTET
STRING. (Note that the encryption mechanisms defined in
[@KCRYPTO] MUST incorporate integrity protection as well, so no
additional checksum is required.)
The EncryptionKey type is the means by which cryptographic keys used
for encryption are transferred.
EncryptionKey ::= SEQUENCE {
keytype [0] Int32 -- actually encryption type --,
keyvalue [1] OCTET STRING
}
keytype
This field specifies the encryption type of the encryption key
that follows in the keyvalue field. While its name is "keytype",
it actually specifies an encryption type. Previously, multiple
cryptosystems that performed encryption differently but were
capable of using keys with the same characteristics were permitted
to share an assigned number to designate the type of key; this
usage is now deprecated.
keyvalue
This field contains the key itself, encoded as an octet string.
Messages containing cleartext data to be authenticated will usually
do so by using a member of type Checksum. Most instances of Checksum
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use a keyed hash, though exceptions will be noted.
Checksum ::= SEQUENCE {
cksumtype [0] Int32,
checksum [1] OCTET STRING
}
cksumtype
This field indicates the algorithm used to generate the
accompanying checksum.
checksum
This field contains the checksum itself, encoded as an octet
string.
See section 4 for a brief description of the use of encryption and
checksums in Kerberos.
5.3. Tickets
This section describes the format and encryption parameters for
tickets and authenticators. When a ticket or authenticator is
included in a protocol message it is treated as an opaque object.
A ticket is a record that helps a client authenticate to a
service. A Ticket contains the following information:
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData -- EncTicketPart
}
-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags [0] TicketFlags,
key [1] EncryptionKey,
crealm [2] Realm,
cname [3] PrincipalName,
transited [4] TransitedEncoding,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
caddr [9] HostAddresses OPTIONAL,
authorization-data [10] AuthorizationData OPTIONAL
}
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-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type [0] Int32 -- must be registered --,
contents [1] OCTET STRING
}
TicketFlags ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- may-postdate(5),
-- postdated(6),
-- invalid(7),
-- renewable(8),
-- initial(9),
-- pre-authent(10),
-- hw-authent(11),
-- the following are new since 1510
-- transited-policy-checked(12),
-- ok-as-delegate(13)
tkt-vno
This field specifies the version number for the ticket format.
This document describes version number 5.
realm
This field specifies the realm that issued a ticket. It also
serves to identify the realm part of the server's principal
identifier. Since a Kerberos server can only issue tickets for
servers within its realm, the two will always be identical.
sname
This field specifies all components of the name part of the
server's identity, including those parts that identify a specific
instance of a service.
enc-part
This field holds the encrypted encoding of the EncTicketPart
sequence. It is encrypted in the key shared by Kerberos and the
end server (the server's secret key), using a key usage value of
2.
flags
This field indicates which of various options were used or
requested when the ticket was issued. The meanings of the flags
are:
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Bit(s) Name Description
0 reserved Reserved for future expansion of this
field.
The FORWARDABLE flag is normally only
interpreted by the TGS, and can be
ignored by end servers. When set, this
1 forwardable flag tells the ticket-granting server
that it is OK to issue a new
ticket-granting ticket with a
different network address based on the
presented ticket.
When set, this flag indicates that the
ticket has either been forwarded or
2 forwarded was issued based on authentication
involving a forwarded ticket-granting
ticket.
The PROXIABLE flag is normally only
interpreted by the TGS, and can be
ignored by end servers. The PROXIABLE
flag has an interpretation identical
3 proxiable to that of the FORWARDABLE flag,
except that the PROXIABLE flag tells
the ticket-granting server that only
non-ticket-granting tickets may be
issued with different network
addresses.
4 proxy When set, this flag indicates that a
ticket is a proxy.
The MAY-POSTDATE flag is normally only
interpreted by the TGS, and can be
5 may-postdate ignored by end servers. This flag
tells the ticket-granting server that
a post-dated ticket MAY be issued
based on this ticket-granting ticket.
This flag indicates that this ticket
has been postdated. The end-service
6 postdated can check the authtime field to see
when the original authentication
occurred.
This flag indicates that a ticket is
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invalid, and it must be validated by
7 invalid the KDC before use. Application
servers must reject tickets which have
this flag set.
The RENEWABLE flag is normally only
interpreted by the TGS, and can
usually be ignored by end servers
8 renewable (some particularly careful servers MAY
disallow renewable tickets). A
renewable ticket can be used to obtain
a replacement ticket that expires at a
later date.
This flag indicates that this ticket
9 initial was issued using the AS protocol, and
not issued based on a ticket-granting
ticket.
This flag indicates that during
initial authentication, the client was
authenticated by the KDC before a
10 pre-authent ticket was issued. The strength of the
pre-authentication method is not
indicated, but is acceptable to the
KDC.
This flag indicates that the protocol
employed for initial authentication
required the use of hardware expected
11 hw-authent to be possessed solely by the named
client. The hardware authentication
method is selected by the KDC and the
strength of the method is not
indicated.
This flag indicates that the KDC for
the realm has checked the transited
field against a realm defined policy
for trusted certifiers. If this flag
is reset (0), then the application
server must check the transited field
itself, and if unable to do so it must
reject the authentication. If the flag
12 transited- is set (1) then the application server
policy-checked MAY skip its own validation of the
transited field, relying on the
validation performed by the KDC. At
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its option the application server MAY
still apply its own validation based
on a separate policy for acceptance.
This flag is new since RFC 1510.
This flag indicates that the server
(not the client) specified in the
ticket has been determined by policy
of the realm to be a suitable
recipient of delegation. A client can
use the presence of this flag to help
it make a decision whether to delegate
credentials (either grant a proxy or a
forwarded ticket-granting ticket) to
13 ok-as-delegate this server. The client is free to
ignore the value of this flag. When
setting this flag, an administrator
should consider the Security and
placement of the server on which the
service will run, as well as whether
the service requires the use of
delegated credentials.
This flag is new since RFC 1510.
14-31 reserved Reserved for future use.
key
This field exists in the ticket and the KDC response and is used
to pass the session key from Kerberos to the application server
and the client.
crealm
This field contains the name of the realm in which the client is
registered and in which initial authentication took place.
cname
This field contains the name part of the client's principal
identifier.
transited
This field lists the names of the Kerberos realms that took part
in authenticating the user to whom this ticket was issued. It does
not specify the order in which the realms were transited. See
section 3.3.3.2 for details on how this field encodes the
traversed realms. When the names of CA's are to be embedded in
the transited field (as specified for some extensions to the
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protocol), the X.500 names of the CA's SHOULD be mapped into items
in the transited field using the mapping defined by RFC2253.
authtime
This field indicates the time of initial authentication for the
named principal. It is the time of issue for the original ticket
on which this ticket is based. It is included in the ticket to
provide additional information to the end service, and to provide
the necessary information for implementation of a `hot list'
service at the KDC. An end service that is particularly paranoid
could refuse to accept tickets for which the initial
authentication occurred "too far" in the past. This field is also
returned as part of the response from the KDC. When returned as
part of the response to initial authentication (KRB_AS_REP), this
is the current time on the Kerberos server. It is NOT recommended
that this time value be used to adjust the workstation's clock
since the workstation cannot reliably determine that such a
KRB_AS_REP actually came from the proper KDC in a timely manner.
starttime
This field in the ticket specifies the time after which the ticket
is valid. Together with endtime, this field specifies the life of
the ticket. If the starttime field is absent from the ticket, then
the authtime field SHOULD be used in its place to determine the
life of the ticket.
endtime
This field contains the time after which the ticket will not be
honored (its expiration time). Note that individual services MAY
place their own limits on the life of a ticket and MAY reject
tickets which have not yet expired. As such, this is really an
upper bound on the expiration time for the ticket.
renew-till
This field is only present in tickets that have the RENEWABLE flag
set in the flags field. It indicates the maximum endtime that may
be included in a renewal. It can be thought of as the absolute
expiration time for the ticket, including all renewals.
caddr
This field in a ticket contains zero (if omitted) or more (if
present) host addresses. These are the addresses from which the
ticket can be used. If there are no addresses, the ticket can be
used from any location. The decision by the KDC to issue or by the
end server to accept addressless tickets is a policy decision and
is left to the Kerberos and end-service administrators; they MAY
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refuse to issue or accept such tickets. Because of the wide
deployment of network address translation, it is recommended that
policy allow the issue and acceptance of such tickets.
Network addresses are included in the ticket to make it harder for
an attacker to use stolen credentials. Because the session key is
not sent over the network in cleartext, credentials can't be
stolen simply by listening to the network; an attacker has to gain
access to the session key (perhaps through operating system
security breaches or a careless user's unattended session) to make
use of stolen tickets.
It is important to note that the network address from which a
connection is received cannot be reliably determined. Even if it
could be, an attacker who has compromised the client's workstation
could use the credentials from there. Including the network
addresses only makes it more difficult, not impossible, for an
attacker to walk off with stolen credentials and then use them
from a "safe" location.
authorization-data
The authorization-data field is used to pass authorization data
from the principal on whose behalf a ticket was issued to the
application service. If no authorization data is included, this
field will be left out. Experience has shown that the name of this
field is confusing, and that a better name for this field would be
restrictions. Unfortunately, it is not possible to change the name
of this field at this time.
This field contains restrictions on any authority obtained on the
basis of authentication using the ticket. It is possible for any
principal in possession of credentials to add entries to the
authorization data field since these entries further restrict what
can be done with the ticket. Such additions can be made by
specifying the additional entries when a new ticket is obtained
during the TGS exchange, or they MAY be added during chained
delegation using the authorization data field of the
authenticator.
Because entries may be added to this field by the holder of
credentials, except when an entry is separately authenticated by
encapsulation in the KDC-issued element, it is not allowable for
the presence of an entry in the authorization data field of a
ticket to amplify the privileges one would obtain from using a
ticket.
The data in this field may be specific to the end service; the
field will contain the names of service specific objects, and the
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rights to those objects. The format for this field is described in
section 5.2.6. Although Kerberos is not concerned with the format
of the contents of the sub-fields, it does carry type information
(ad-type).
By using the authorization_data field, a principal is able to
issue a proxy that is valid for a specific purpose. For example, a
client wishing to print a file can obtain a file server proxy to
be passed to the print server. By specifying the name of the file
in the authorization_data field, the file server knows that the
print server can only use the client's rights when accessing the
particular file to be printed.
A separate service providing authorization or certifying group
membership may be built using the authorization-data field. In
this case, the entity granting authorization (not the authorized
entity), may obtain a ticket in its own name (e.g. the ticket is
issued in the name of a privilege server), and this entity adds
restrictions on its own authority and delegates the restricted
authority through a proxy to the client. The client would then
present this authorization credential to the application server
separately from the authentication exchange. Alternatively, such
authorization credentials MAY be embedded in the ticket
authenticating the authorized entity, when the authorization is
separately authenticated using the KDC-issued authorization data
element (see 5.2.6.2).
Similarly, if one specifies the authorization-data field of a
proxy and leaves the host addresses blank, the resulting ticket
and session key can be treated as a capability. See [Neu93] for
some suggested uses of this field.
The authorization-data field is optional and does not have to be
included in a ticket.
5.4. Specifications for the AS and TGS exchanges
This section specifies the format of the messages used in the
exchange between the client and the Kerberos server. The format of
possible error messages appears in section 5.9.1.
5.4.1. KRB_KDC_REQ definition
The KRB_KDC_REQ message has no application tag number of its own.
Instead, it is incorporated into one of KRB_AS_REQ or KRB_TGS_REQ,
which each have an application tag, depending on whether the
request is for an initial ticket or an additional ticket. In
either case, the message is sent from the client to the KDC to
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request credentials for a service.
The message fields are:
AS-REQ ::= [APPLICATION 10] KDC-REQ
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
pvno [1] INTEGER (5) ,
msg-type [2] INTEGER (10 -- AS -- | 12 -- TGS --),
padata [3] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
req-body [4] KDC-REQ-BODY
}
KDC-REQ-BODY ::= SEQUENCE {
kdc-options [0] KDCOptions,
cname [1] PrincipalName OPTIONAL
-- Used only in AS-REQ --,
realm [2] Realm
-- Server's realm
-- Also client's in AS-REQ --,
sname [3] PrincipalName OPTIONAL,
from [4] KerberosTime OPTIONAL,
till [5] KerberosTime,
rtime [6] KerberosTime OPTIONAL,
nonce [7] UInt32,
etype [8] SEQUENCE OF Int32 -- EncryptionType
-- in preference order --,
addresses [9] HostAddresses OPTIONAL,
enc-authorization-data [10] EncryptedData -- AuthorizationData --,
additional-tickets [11] SEQUENCE OF Ticket OPTIONAL
-- NOTE: not empty
}
KDCOptions ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- allow-postdate(5),
-- postdated(6),
-- unused7(7),
-- renewable(8),
-- unused9(9),
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-- unused10(10),
-- opt-hardware-auth(11),
-- unused12(12),
-- unused13(13),
-- 15 is reserved for canonicalize
-- unused15(15),
-- 26 was unused in 1510
-- disable-transited-check(26),
--
-- renewable-ok(27),
-- enc-tkt-in-skey(28),
-- renew(30),
-- validate(31)
The fields in this message are:
pvno
This field is included in each message, and specifies the protocol
version number. This document specifies protocol version 5.
msg-type
This field indicates the type of a protocol message. It will
almost always be the same as the application identifier associated
with a message. It is included to make the identifier more readily
accessible to the application. For the KDC-REQ message, this type
will be KRB_AS_REQ or KRB_TGS_REQ.
padata
Contains pre-authentication data. Requests for additional tickets
(KRB_TGS_REQ) MUST contain a padata of PA-TGS-REQ.
The padata (pre-authentication data) field contains a sequence of
authentication information which may be needed before credentials
can be issued or decrypted.
req-body
This field is a placeholder delimiting the extent of the remaining
fields. If a checksum is to be calculated over the request, it is
calculated over an encoding of the KDC-REQ-BODY sequence which is
enclosed within the req-body field.
kdc-options
This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to
the KDC and indicates the flags that the client wants set on the
tickets as well as other information that is to modify the
behavior of the KDC. Where appropriate, the name of an option may
be the same as the flag that is set by that option. Although in
most case, the bit in the options field will be the same as that
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in the flags field, this is not guaranteed, so it is not
acceptable to simply copy the options field to the flags field.
There are various checks that must be made before honoring an
option anyway.
The kdc_options field is a bit-field, where the selected options
are indicated by the bit being set (1), and the unselected options
and reserved fields being reset (0). The encoding of the bits is
specified in section 5.2. The options are described in more detail
above in section 2. The meanings of the options are:
Bits Name Description
0 RESERVED Reserved for future expansion of
this field.
The FORWARDABLE option indicates
that the ticket to be issued is to
have its forwardable flag set. It
1 FORWARDABLE may only be set on the initial
request, or in a subsequent request
if the ticket-granting ticket on
which it is based is also
forwardable.
The FORWARDED option is only
specified in a request to the
ticket-granting server and will only
be honored if the ticket-granting
ticket in the request has its
2 FORWARDED FORWARDABLE bit set. This option
indicates that this is a request for
forwarding. The address(es) of the
host from which the resulting ticket
is to be valid are included in the
addresses field of the request.
The PROXIABLE option indicates that
the ticket to be issued is to have
its proxiable flag set. It may only
3 PROXIABLE be set on the initial request, or in
a subsequent request if the
ticket-granting ticket on which it
is based is also proxiable.
The PROXY option indicates that this
is a request for a proxy. This
option will only be honored if the
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ticket-granting ticket in the
4 PROXY request has its PROXIABLE bit set.
The address(es) of the host from
which the resulting ticket is to be
valid are included in the addresses
field of the request.
The ALLOW-POSTDATE option indicates
that the ticket to be issued is to
have its MAY-POSTDATE flag set. It
5 ALLOW-POSTDATE may only be set on the initial
request, or in a subsequent request
if the ticket-granting ticket on
which it is based also has its
MAY-POSTDATE flag set.
The POSTDATED option indicates that
this is a request for a postdated
ticket. This option will only be
honored if the ticket-granting
ticket on which it is based has its
6 POSTDATED MAY-POSTDATE flag set. The resulting
ticket will also have its INVALID
flag set, and that flag may be reset
by a subsequent request to the KDC
after the starttime in the ticket
has been reached.
7 RESERVED This option is presently unused.
The RENEWABLE option indicates that
the ticket to be issued is to have
its RENEWABLE flag set. It may only
be set on the initial request, or
when the ticket-granting ticket on
8 RENEWABLE which the request is based is also
renewable. If this option is
requested, then the rtime field in
the request contains the desired
absolute expiration time for the
ticket.
9 RESERVED Reserved for PK-Cross
10 RESERVED Reserved for future use.
11 RESERVED Reserved for opt-hardware-auth.
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12-25 RESERVED Reserved for future use.
By default the KDC will check the
transited field of a
ticket-granting-ticket against the
policy of the local realm before it
will issue derivative tickets based
on the ticket-granting ticket. If
this flag is set in the request,
checking of the transited field is
disabled. Tickets issued without the
26 DISABLE-TRANSITED-CHECK performance of this check will be
noted by the reset (0) value of the
TRANSITED-POLICY-CHECKED flag,
indicating to the application server
that the tranisted field must be
checked locally. KDCs are
encouraged but not required to honor
the DISABLE-TRANSITED-CHECK option.
This flag is new since RFC 1510
The RENEWABLE-OK option indicates
that a renewable ticket will be
acceptable if a ticket with the
requested life cannot otherwise be
provided. If a ticket with the
requested life cannot be provided,
27 RENEWABLE-OK then a renewable ticket may be
issued with a renew-till equal to
the requested endtime. The value
of the renew-till field may still be
limited by local limits, or limits
selected by the individual principal
or server.
This option is used only by the
ticket-granting service. The
ENC-TKT-IN-SKEY option indicates
28 ENC-TKT-IN-SKEY that the ticket for the end server
is to be encrypted in the session
key from the additional
ticket-granting ticket provided.
29 RESERVED Reserved for future use.
This option is used only by the
ticket-granting service. The RENEW
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option indicates that the present
request is for a renewal. The ticket
provided is encrypted in the secret
key for the server on which it is
30 RENEW valid. This option will only be
honored if the ticket to be renewed
has its RENEWABLE flag set and if
the time in its renew-till field has
not passed. The ticket to be renewed
is passed in the padata field as
part of the authentication header.
This option is used only by the
ticket-granting service. The
VALIDATE option indicates that the
request is to validate a postdated
ticket. It will only be honored if
the ticket presented is postdated,
presently has its INVALID flag set,
31 VALIDATE and would be otherwise usable at
this time. A ticket cannot be
validated before its starttime. The
ticket presented for validation is
encrypted in the key of the server
for which it is valid and is passed
in the padata field as part of the
authentication header.
cname and sname
These fields are the same as those described for the ticket in
section 5.3. The sname may only be absent when the ENC-TKT-IN-SKEY
option is specified. If absent, the name of the server is taken
from the name of the client in the ticket passed as additional-
tickets.
enc-authorization-data
The enc-authorization-data, if present (and it can only be present
in the TGS_REQ form), is an encoding of the desired authorization-
data encrypted under the sub-session key if present in the
Authenticator, or alternatively from the session key in the
ticket-granting ticket (both the Authenticator and ticket-granting
ticket come from the padata field in the KRB_TGS_REQ). The key
usage value used when encrypting is 5 if a sub-session key is
used, or 4 if the session key is used.
realm
This field specifies the realm part of the server's principal
identifier. In the AS exchange, this is also the realm part of the
client's principal identifier.
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from
This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
requests when the requested ticket is to be postdated. It
specifies the desired start time for the requested ticket. If this
field is omitted then the KDC SHOULD use the current time instead.
till
This field contains the expiration date requested by the client in
a ticket request. It is not optional, but if the requested endtime
is "19700101000000Z", the requested ticket is to have the maximum
endtime permitted according to KDC policy. Implementation note:
This special timestamp corresponds to a UNIX time_t value of zero
on most systems.
rtime
This field is the requested renew-till time sent from a client to
the KDC in a ticket request. It is optional.
nonce
This field is part of the KDC request and response. It is intended
to hold a random number generated by the client. If the same
number is included in the encrypted response from the KDC, it
provides evidence that the response is fresh and has not been
replayed by an attacker. Nonces MUST NEVER be reused.
etype
This field specifies the desired encryption algorithm to be used
in the response.
addresses
This field is included in the initial request for tickets, and
optionally included in requests for additional tickets from the
ticket-granting server. It specifies the addresses from which the
requested ticket is to be valid. Normally it includes the
addresses for the client's host. If a proxy is requested, this
field will contain other addresses. The contents of this field are
usually copied by the KDC into the caddr field of the resulting
ticket.
additional-tickets
Additional tickets MAY be optionally included in a request to the
ticket-granting server. If the ENC-TKT-IN-SKEY option has been
specified, then the session key from the additional ticket will be
used in place of the server's key to encrypt the new ticket. When
the ENC-TKT-IN-SKEY option is used for user-to-user
authentication, this additional ticket MAY be a TGT issued by the
local realm or an inter-realm TGT issued for the current KDC's
realm by a remote KDC. If more than one option which requires
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additional tickets has been specified, then the additional tickets
are used in the order specified by the ordering of the options
bits (see kdc-options, above).
The application tag number will be either ten (10) or twelve (12)
depending on whether the request is for an initial ticket (AS-REQ) or
for an additional ticket (TGS-REQ).
The optional fields (addresses, authorization-data and additional-
tickets) are only included if necessary to perform the operation
specified in the kdc-options field.
It should be noted that in KRB_TGS_REQ, the protocol version number
appears twice and two different message types appear: the KRB_TGS_REQ
message contains these fields as does the authentication header
(KRB_AP_REQ) that is passed in the padata field.
5.4.2. KRB_KDC_REP definition
The KRB_KDC_REP message format is used for the reply from the KDC
for either an initial (AS) request or a subsequent (TGS) request.
There is no message type for KRB_KDC_REP. Instead, the type will
be either KRB_AS_REP or KRB_TGS_REP. The key used to encrypt the
ciphertext part of the reply depends on the message type. For
KRB_AS_REP, the ciphertext is encrypted in the client's secret
key, and the client's key version number is included in the key
version number for the encrypted data. For KRB_TGS_REP, the
ciphertext is encrypted in the sub-session key from the
Authenticator, or if absent, the session key from the ticket-
granting ticket used in the request. In that case, no version
number will be present in the EncryptedData sequence.
The KRB_KDC_REP message contains the following fields:
AS-REP ::= [APPLICATION 11] KDC-REP
TGS-REP ::= [APPLICATION 13] KDC-REP
KDC-REP ::= SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (11 -- AS -- | 13 -- TGS --),
padata [2] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
crealm [3] Realm,
cname [4] PrincipalName,
ticket [5] Ticket,
enc-part [6] EncryptedData
-- EncASRepPart or EncTGSRepPart,
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-- as appropriate
}
EncASRepPart ::= [APPLICATION 25] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE {
key [0] EncryptionKey,
last-req [1] LastReq,
nonce [2] UInt32,
key-expiration [3] KerberosTime OPTIONAL,
flags [4] TicketFlags,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
srealm [9] Realm,
sname [10] PrincipalName,
caddr [11] HostAddresses OPTIONAL
}
LastReq ::= SEQUENCE OF SEQUENCE {
lr-type [0] Int32,
lr-value [1] KerberosTime
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
either KRB_AS_REP or KRB_TGS_REP.
padata
This field is described in detail in section 5.4.1. One possible
use for this field is to encode an alternate "salt" string to be
used with a string-to-key algorithm. This ability is useful to
ease transitions if a realm name needs to change (e.g. when a
company is acquired); in such a case all existing password-derived
entries in the KDC database would be flagged as needing a special
salt string until the next password change.
crealm, cname, srealm and sname
These fields are the same as those described for the ticket in
section 5.3.
ticket
The newly-issued ticket, from section 5.3.
enc-part
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This field is a place holder for the ciphertext and related
information that forms the encrypted part of a message. The
description of the encrypted part of the message follows each
appearance of this field.
The key usage value for encrypting this field is 3 in an AS-REP
message, using the client's long-term key or another key selected
via pre-authentication mechanisms. In a TGS-REP message, the key
usage value is 8 if the TGS session key is used, or 9 if a TGS
authenticator subkey is used.
Compatibility note: Some implementations unconditionally send an
encrypted EncTGSRepPart (application tag number 26) in this field
regardless of whether the reply is a AS-REP or a TGS-REP. In the
interests of compatibility, implementors MAY relax the check on
the tag number of the decrypted ENC-PART.
key
This field is the same as described for the ticket in section 5.3.
last-req
This field is returned by the KDC and specifies the time(s) of the
last request by a principal. Depending on what information is
available, this might be the last time that a request for a
ticket-granting ticket was made, or the last time that a request
based on a ticket-granting ticket was successful. It also might
cover all servers for a realm, or just the particular server. Some
implementations MAY display this information to the user to aid in
discovering unauthorized use of one's identity. It is similar in
spirit to the last login time displayed when logging into
timesharing systems.
lr-type
This field indicates how the following lr-value field is to be
interpreted. Negative values indicate that the information
pertains only to the responding server. Non-negative values
pertain to all servers for the realm.
If the lr-type field is zero (0), then no information is
conveyed by the lr-value subfield. If the absolute value of the
lr-type field is one (1), then the lr-value subfield is the
time of last initial request for a TGT. If it is two (2), then
the lr-value subfield is the time of last initial request. If
it is three (3), then the lr-value subfield is the time of
issue for the newest ticket-granting ticket used. If it is four
(4), then the lr-value subfield is the time of the last
renewal. If it is five (5), then the lr-value subfield is the
time of last request (of any type). If it is (6), then the lr-
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value subfield is the time when the password will expire. If
it is (7), then the lr-value subfield is the time when the
account will expire.
lr-value
This field contains the time of the last request. The time MUST
be interpreted according to the contents of the accompanying
lr-type subfield.
nonce
This field is described above in section 5.4.1.
key-expiration
The key-expiration field is part of the response from the KDC and
specifies the time that the client's secret key is due to expire.
The expiration might be the result of password aging or an account
expiration. If present, it SHOULD be set to the earliest of the
user's key expiration and account expiration. The use of this
field is deprecated and the last-req field SHOULD be used to
convey this information instead. This field will usually be left
out of the TGS reply since the response to the TGS request is
encrypted in a session key and no client information need be
retrieved from the KDC database. It is up to the application
client (usually the login program) to take appropriate action
(such as notifying the user) if the expiration time is imminent.
flags, authtime, starttime, endtime, renew-till and caddr
These fields are duplicates of those found in the encrypted
portion of the attached ticket (see section 5.3), provided so the
client MAY verify they match the intended request and to assist in
proper ticket caching. If the message is of type KRB_TGS_REP, the
caddr field will only be filled in if the request was for a proxy
or forwarded ticket, or if the user is substituting a subset of
the addresses from the ticket-granting ticket. If the client-
requested addresses are not present or not used, then the
addresses contained in the ticket will be the same as those
included in the ticket-granting ticket.
5.5. Client/Server (CS) message specifications
This section specifies the format of the messages used for the
authentication of the client to the application server.
5.5.1. KRB_AP_REQ definition
The KRB_AP_REQ message contains the Kerberos protocol version
number, the message type KRB_AP_REQ, an options field to indicate
any options in use, and the ticket and authenticator themselves.
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The KRB_AP_REQ message is often referred to as the 'authentication
header'.
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (14),
ap-options [2] APOptions,
ticket [3] Ticket,
authenticator [4] EncryptedData -- Authenticator
}
APOptions ::= KerberosFlags
-- reserved(0),
-- use-session-key(1),
-- mutual-required(2)
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_AP_REQ.
ap-options
This field appears in the application request (KRB_AP_REQ) and
affects the way the request is processed. It is a bit-field, where
the selected options are indicated by the bit being set (1), and
the unselected options and reserved fields being reset (0). The
encoding of the bits is specified in section 5.2. The meanings of
the options are:
Bit(s) Name Description
0 reserved Reserved for future expansion of this field.
The USE-SESSION-KEY option indicates that the
ticket the client is presenting to a server
1 use-session-key is encrypted in the session key from the
server's ticket-granting ticket. When this
option is not specified, the ticket is
encrypted in the server's secret key.
The MUTUAL-REQUIRED option tells the server
2 mutual-required that the client requires mutual
authentication, and that it must respond with
a KRB_AP_REP message.
3-31 reserved Reserved for future use.
ticket
This field is a ticket authenticating the client to the server.
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authenticator
This contains the encrypted authenticator, which includes the
client's choice of a subkey.
The encrypted authenticator is included in the AP-REQ; it certifies
to a server that the sender has recent knowledge of the encryption
key in the accompanying ticket, to help the server detect replays. It
also assists in the selection of a "true session key" to use with the
particular session. The DER encoding of the following is encrypted
in the ticket's session key, with a key usage value of 11 in normal
application exchanges, or 7 when used as the PA-TGS-REQ PA-DATA field
of a TGS-REQ exchange (see section 5.4.1):
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno [0] INTEGER (5),
crealm [1] Realm,
cname [2] PrincipalName,
cksum [3] Checksum OPTIONAL,
cusec [4] Microseconds,
ctime [5] KerberosTime,
subkey [6] EncryptionKey OPTIONAL,
seq-number [7] UInt32 OPTIONAL,
authorization-data [8] AuthorizationData OPTIONAL
}
authenticator-vno
This field specifies the version number for the format of the
authenticator. This document specifies version 5.
crealm and cname
These fields are the same as those described for the ticket in
section 5.3.
cksum
This field contains a checksum of the application data that
accompanies the KRB_AP_REQ, computed using a key usage value of 10
in normal application exchanges, or 6 when used in the TGS-REQ PA-
TGS-REQ AP-DATA field.
cusec
This field contains the microsecond part of the client's
timestamp. Its value (before encryption) ranges from 0 to 999999.
It often appears along with ctime. The two fields are used
together to specify a reasonably accurate timestamp.
ctime
This field contains the current time on the client's host.
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subkey
This field contains the client's choice for an encryption key
which is to be used to protect this specific application session.
Unless an application specifies otherwise, if this field is left
out the session key from the ticket will be used.
seq-number
This optional field includes the initial sequence number to be
used by the KRB_PRIV or KRB_SAFE messages when sequence numbers
are used to detect replays (It may also be used by application
specific messages). When included in the authenticator this field
specifies the initial sequence number for messages from the client
to the server. When included in the AP-REP message, the initial
sequence number is that for messages from the server to the
client. When used in KRB_PRIV or KRB_SAFE messages, it is
incremented by one after each message is sent. Sequence numbers
fall in the range of 0 through 2^32 - 1 and wrap to zero following
the value 2^32 - 1.
For sequence numbers to adequately support the detection of
replays they SHOULD be non-repeating, even across connection
boundaries. The initial sequence number SHOULD be random and
uniformly distributed across the full space of possible sequence
numbers, so that it cannot be guessed by an attacker and so that
it and the successive sequence numbers do not repeat other
sequences. In the event that more than 2^32 messages are to be
generated in a series of KRB_PRIV or KRB_SAFE messages, rekeying
SHOULD be performed before sequence numbers are reused with the
same encryption key.
Implmentation note: historically, some implementations transmit
signed twos-complement numbers for sequence numbers. In the
interests of compatibility, implementations MAY accept the
equivalent negative number where a positive number greater than
2^31 - 1 is expected.
Implementation note: as noted before, some implementations omit
the optional sequence number when its value would be zero.
Implementations MAY accept an omitted sequence number when
expecting a value of zero, and SHOULD NOT transmit an
Authenticator with a initial sequence number of zero.
authorization-data
This field is the same as described for the ticket in section 5.3.
It is optional and will only appear when additional restrictions
are to be placed on the use of a ticket, beyond those carried in
the ticket itself.
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5.5.2. KRB_AP_REP definition
The KRB_AP_REP message contains the Kerberos protocol version
number, the message type, and an encrypted time-stamp. The message
is sent in response to an application request (KRB_AP_REQ) where
the mutual authentication option has been selected in the ap-
options field.
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (15),
enc-part [2] EncryptedData -- EncAPRepPart
}
EncAPRepPart ::= [APPLICATION 27] SEQUENCE {
ctime [0] KerberosTime,
cusec [1] Microseconds,
subkey [2] EncryptionKey OPTIONAL,
seq-number [3] UInt32 OPTIONAL
}
The encoded EncAPRepPart is encrypted in the shared session key of
the ticket. The optional subkey field can be used in an
application-arranged negotiation to choose a per association
session key.
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_AP_REP.
enc-part
This field is described above in section 5.4.2. It is computed
with a key usage value of 12.
ctime
This field contains the current time on the client's host.
cusec
This field contains the microsecond part of the client's
timestamp.
subkey
This field contains an encryption key which is to be used to
protect this specific application session. See section 3.2.6 for
specifics on how this field is used to negotiate a key. Unless an
application specifies otherwise, if this field is left out, the
sub-session key from the authenticator, or if also left out, the
session key from the ticket will be used.
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seq-number
This field is described above in section 5.3.2.
5.5.3. Error message reply
If an error occurs while processing the application request, the
KRB_ERROR message will be sent in response. See section 5.9.1 for
the format of the error message. The cname and crealm fields MAY
be left out if the server cannot determine their appropriate
values from the corresponding KRB_AP_REQ message. If the
authenticator was decipherable, the ctime and cusec fields will
contain the values from it.
5.6. KRB_SAFE message specification
This section specifies the format of a message that can be used by
either side (client or server) of an application to send a tamper-
proof message to its peer. It presumes that a session key has
previously been exchanged (for example, by using the
KRB_AP_REQ/KRB_AP_REP messages).
5.6.1. KRB_SAFE definition
The KRB_SAFE message contains user data along with a collision-
proof checksum keyed with the last encryption key negotiated via
subkeys, or the session key if no negotiation has occurred. The
message fields are:
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (20),
safe-body [2] KRB-SAFE-BODY,
cksum [3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress,
r-address [5] HostAddress OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_SAFE.
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safe-body
This field is a placeholder for the body of the KRB-SAFE message.
cksum
This field contains the checksum of the application data, computed
with a key usage value of 15.
The checksum is computed over the encoding of the KRB-SAFE
sequence. First, the cksum is set to a type zero, zero-length
value and the checksum is computed over the encoding of the KRB-
SAFE sequence, then the checksum is set to the result of that
computation, and finally the KRB-SAFE sequence is encoded again.
This method, while different than the one specified in RFC 1510,
corresponds to existing practice.
user-data
This field is part of the KRB_SAFE and KRB_PRIV messages and
contain the application specific data that is being passed from
the sender to the recipient.
timestamp
This field is part of the KRB_SAFE and KRB_PRIV messages. Its
contents are the current time as known by the sender of the
message. By checking the timestamp, the recipient of the message
is able to make sure that it was recently generated, and is not a
replay.
usec
This field is part of the KRB_SAFE and KRB_PRIV headers. It
contains the microsecond part of the timestamp.
seq-number
This field is described above in section 5.3.2.
s-address
Sender's address.
This field specifies the address in use by the sender of the
message.
r-address
This field specifies the address in use by the recipient of the
message. It MAY be omitted for some uses (such as broadcast
protocols), but the recipient MAY arbitrarily reject such
messages. This field, along with s-address, can be used to help
detect messages which have been incorrectly or maliciously
delivered to the wrong recipient.
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5.7. KRB_PRIV message specification
This section specifies the format of a message that can be used by
either side (client or server) of an application to securely and
privately send a message to its peer. It presumes that a session
key has previously been exchanged (for example, by using the
KRB_AP_REQ/KRB_AP_REP messages).
5.7.1. KRB_PRIV definition
The KRB_PRIV message contains user data encrypted in the Session
Key. The message fields are:
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (21),
-- NOTE: there is no [2] tag
enc-part [3] EncryptedData -- EncKrbPrivPart
}
EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress -- sender's addr --,
r-address [5] HostAddress OPTIONAL -- recip's addr
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_PRIV.
enc-part
This field holds an encoding of the EncKrbPrivPart sequence
encrypted under the session key, with a key usage value of 13.
This encrypted encoding is used for the enc-part field of the KRB-
PRIV message.
user-data, timestamp, usec, s-address and r-address
These fields are described above in section 5.6.1.
seq-number
This field is described above in section 5.3.2.
5.8. KRB_CRED message specification
This section specifies the format of a message that can be used to
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send Kerberos credentials from one principal to another. It is
presented here to encourage a common mechanism to be used by
applications when forwarding tickets or providing proxies to
subordinate servers. It presumes that a session key has already
been exchanged perhaps by using the KRB_AP_REQ/KRB_AP_REP
messages.
5.8.1. KRB_CRED definition
The KRB_CRED message contains a sequence of tickets to be sent and
information needed to use the tickets, including the session key
from each. The information needed to use the tickets is encrypted
under an encryption key previously exchanged or transferred
alongside the KRB_CRED message. The message fields are:
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (22),
tickets [2] SEQUENCE OF Ticket,
enc-part [3] EncryptedData -- EncKrbCredPart
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info [0] SEQUENCE OF KrbCredInfo,
nonce [1] UInt32 OPTIONAL,
timestamp [2] KerberosTime OPTIONAL,
usec [3] Microseconds OPTIONAL,
s-address [4] HostAddress OPTIONAL,
r-address [5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key [0] EncryptionKey,
prealm [1] Realm OPTIONAL,
pname [2] PrincipalName OPTIONAL,
flags [3] TicketFlags OPTIONAL,
authtime [4] KerberosTime OPTIONAL,
starttime [5] KerberosTime OPTIONAL,
endtime [6] KerberosTime OPTIONAL,
renew-till [7] KerberosTime OPTIONAL,
srealm [8] Realm OPTIONAL,
sname [9] PrincipalName OPTIONAL,
caddr [10] HostAddresses OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_CRED.
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tickets
These are the tickets obtained from the KDC specifically for use
by the intended recipient. Successive tickets are paired with the
corresponding KrbCredInfo sequence from the enc-part of the KRB-
CRED message.
enc-part
This field holds an encoding of the EncKrbCredPart sequence
encrypted under the session key shared between the sender and the
intended recipient, with a key usage value of 14. This encrypted
encoding is used for the enc-part field of the KRB-CRED message.
Implementation note: implementations of certain applications, most
notably certain implementations of the Kerberos GSS-API mechanism,
do not separately encrypt the contents of the EncKrbCredPart of
the KRB-CRED message when sending it. In the case of those GSS-
API mechanisms, this is not a security vulnerability, as the
entire KRB-CRED message is itself embedded in an encrypted
message.
nonce
If practical, an application MAY require the inclusion of a nonce
generated by the recipient of the message. If the same value is
included as the nonce in the message, it provides evidence that
the message is fresh and has not been replayed by an attacker. A
nonce MUST NEVER be reused.
timestamp and usec
These fields specify the time that the KRB-CRED message was
generated. The time is used to provide assurance that the message
is fresh.
s-address and r-address
These fields are described above in section 5.6.1. They are used
optionally to provide additional assurance of the integrity of the
KRB-CRED message.
key
This field exists in the corresponding ticket passed by the KRB-
CRED message and is used to pass the session key from the sender
to the intended recipient. The field's encoding is described in
section 5.2.9.
The following fields are optional. If present, they can be associated
with the credentials in the remote ticket file. If left out, then it
is assumed that the recipient of the credentials already knows their
value.
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prealm and pname
The name and realm of the delegated principal identity.
flags, authtime, starttime, endtime, renew-till, srealm, sname, and
caddr
These fields contain the values of the corresponding fields from
the ticket found in the ticket field. Descriptions of the fields
are identical to the descriptions in the KDC-REP message.
5.9. Error message specification
This section specifies the format for the KRB_ERROR message. The
fields included in the message are intended to return as much
information as possible about an error. It is not expected that
all the information required by the fields will be available for
all types of errors. If the appropriate information is not
available when the message is composed, the corresponding field
will be left out of the message.
Note that since the KRB_ERROR message is not integrity protected,
it is quite possible for an intruder to synthesize or modify such
a message. In particular, this means that the client SHOULD NOT
use any fields in this message for security-critical purposes,
such as setting a system clock or generating a fresh
authenticator. The message can be useful, however, for advising a
user on the reason for some failure.
5.9.1. KRB_ERROR definition
The KRB_ERROR message consists of the following fields:
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (30),
ctime [2] KerberosTime OPTIONAL,
cusec [3] Microseconds OPTIONAL,
stime [4] KerberosTime,
susec [5] Microseconds,
error-code [6] Int32,
crealm [7] Realm OPTIONAL,
cname [8] PrincipalName OPTIONAL,
realm [9] Realm -- service realm --,
sname [10] PrincipalName -- service name --,
e-text [11] KerberosString OPTIONAL,
e-data [12] OCTET STRING OPTIONAL
}
pvno and msg-type
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These fields are described above in section 5.4.1. msg-type is
KRB_ERROR.
ctime
This field is described above in section 5.5.2.
cusec
This field is described above in section 5.5.2.
stime
This field contains the current time on the server. It is of type
KerberosTime.
susec
This field contains the microsecond part of the server's
timestamp. Its value ranges from 0 to 999999. It appears along
with stime. The two fields are used in conjunction to specify a
reasonably accurate timestamp.
error-code
This field contains the error code returned by Kerberos or the
server when a request fails. To interpret the value of this field
see the list of error codes in section 7.5.9. Implementations are
encouraged to provide for national language support in the display
of error messages.
crealm, cname, realm and sname
These fields are described above in section 5.3.
e-text
This field contains additional text to help explain the error code
associated with the failed request (for example, it might include
a principal name which was unknown).
e-data
This field contains additional data about the error for use by the
application to help it recover from or handle the error. If the
errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
contain an encoding of a sequence of padata fields, each
corresponding to an acceptable pre-authentication method and
optionally containing data for the method:
METHOD-DATA ::= SEQUENCE OF PA-DATA
For error codes defined in this document other than
KDC_ERR_PREAUTH_REQUIRED, the format and contents of the e-data field
are implementation-defined. Similarly, for future error codes, the
format and contents of the e-data field are implementation-defined
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unless specified. Whether defined by the implementation or in a
future document, the e-data field MAY take the form of TYPED-DATA:
TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
data-type [0] INTEGER,
data-value [1] OCTET STRING OPTIONAL
}
5.10. Application Tag Numbers
The following table lists the application class tag numbers used
by various data types defined in this section.
Tag Number(s) Type Name Comments
0 unused
1 Ticket PDU
2 Authenticator non-PDU
3 EncTicketPart non-PDU
4-9 unused
10 AS-REQ PDU
11 AS-REP PDU
12 TGS-REQ PDU
13 TGS-REP PDU
14 AP-REQ PDU
15 AP-REP PDU
16 RESERVED16 TGT-REQ (for user-to-user)
17 RESERVED17 TGT-REP (for user-to-user)
18-19 unused
20 KRB-SAFE PDU
21 KRB-PRIV PDU
22 KRB-CRED PDU
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23-24 unused
25 EncASRepPart non-PDU
26 EncTGSRepPart non-PDU
27 EncApRepPart non-PDU
28 EncKrbPrivPart non-PDU
29 EncKrbCredPart non-PDU
30 KRB-ERROR PDU
The ASN.1 types marked as "PDU" (Protocol Data Unit) in the above
are the only ASN.1 types intended as top-level types of the
Kerberos protocol, and are the only types that may be used as
elements in another protocol that makes use of Kerberos.
6. Naming Constraints
6.1. Realm Names
Although realm names are encoded as GeneralStrings and although a
realm can technically select any name it chooses, interoperability
across realm boundaries requires agreement on how realm names are
to be assigned, and what information they imply.
To enforce these conventions, each realm MUST conform to the
conventions itself, and it MUST require that any realms with which
inter-realm keys are shared also conform to the conventions and
require the same from its neighbors.
Kerberos realm names are case sensitive. Realm names that differ
only in the case of the characters are not equivalent. There are
presently three styles of realm names: domain, X500, and other.
Examples of each style follow:
domain: ATHENA.MIT.EDU
X500: C=US/O=OSF
other: NAMETYPE:rest/of.name=without-restrictions
Domain style realm names MUST look like domain names: they consist
of components separated by periods (.) and they contain neither
colons (:) nor slashes (/). Though domain names themselves are
case insensitive, in order for realms to match, the case must
match as well. When establishing a new realm name based on an
internet domain name it is recommended by convention that the
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characters be converted to upper case.
X.500 names contain an equal (=) and cannot contain a colon (:)
before the equal. The realm names for X.500 names will be string
representations of the names with components separated by slashes.
Leading and trailing slashes will not be included. Note that the
slash separator is consistent with Kerberos implementations based
on RFC1510, but it is different from the separator recommended in
RFC2253.
Names that fall into the other category MUST begin with a prefix
that contains no equal (=) or period (.) and the prefix MUST be
followed by a colon (:) and the rest of the name. All prefixes
expect those beginning with used. Presently none are assigned.
The reserved category includes strings which do not fall into the
first three categories. All names in this category are reserved.
It is unlikely that names will be assigned to this category unless
there is a very strong argument for not using the 'other'
category.
These rules guarantee that there will be no conflicts between the
various name styles. The following additional constraints apply to
the assignment of realm names in the domain and X.500 categories:
the name of a realm for the domain or X.500 formats must either be
used by the organization owning (to whom it was assigned) an
Internet domain name or X.500 name, or in the case that no such
names are registered, authority to use a realm name MAY be derived
from the authority of the parent realm. For example, if there is
no domain name for E40.MIT.EDU, then the administrator of the
MIT.EDU realm can authorize the creation of a realm with that
name.
This is acceptable because the organization to which the parent is
assigned is presumably the organization authorized to assign names
to its children in the X.500 and domain name systems as well. If
the parent assigns a realm name without also registering it in the
domain name or X.500 hierarchy, it is the parent's responsibility
to make sure that there will not in the future exist a name
identical to the realm name of the child unless it is assigned to
the same entity as the realm name.
6.2. Principal Names
As was the case for realm names, conventions are needed to ensure
that all agree on what information is implied by a principal name.
The name-type field that is part of the principal name indicates
the kind of information implied by the name. The name-type SHOULD
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be treated only as a hint to interpreting the meaning of a name.
It is not significant when checking for equivalence. Principal
names that differ only in the name-type identify the same
principal. The name type does not partition the name space.
Ignoring the name type, no two names can be the same (i.e. at
least one of the components, or the realm, MUST be different). The
following name types are defined:
name-type value meaning
name types
NT-UNKNOWN 0 Name type not known
NT-PRINCIPAL 1 Just the name of the principal as in DCE, or for users
NT-SRV-INST 2 Service and other unique instance (krbtgt)
NT-SRV-HST 3 Service with host name as instance (telnet, rcommands)
NT-SRV-XHST 4 Service with host as remaining components
NT-UID 5 Unique ID
NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253]
NT-SMTP-NAME 7 Name in form of SMTP email name (e.g. user@foo.com)
NT-ENTERPRISE 10 Enterprise name - may be mapped to principal name
When a name implies no information other than its uniqueness at a
particular time the name type PRINCIPAL SHOULD be used. The
principal name type SHOULD be used for users, and it might also be
used for a unique server. If the name is a unique machine
generated ID that is guaranteed never to be reassigned then the
name type of UID SHOULD be used (note that it is generally a bad
idea to reassign names of any type since stale entries might
remain in access control lists).
If the first component of a name identifies a service and the
remaining components identify an instance of the service in a
server specified manner, then the name type of SRV-INST SHOULD be
used. An example of this name type is the Kerberos ticket-granting
service whose name has a first component of krbtgt and a second
component identifying the realm for which the ticket is valid.
If the first component of a name identifies a service and there is
a single component following the service name identifying the
instance as the host on which the server is running, then the name
type SRV-HST SHOULD be used. This type is typically used for
Internet services such as telnet and the Berkeley R commands. If
the separate components of the host name appear as successive
components following the name of the service, then the name type
SRV-XHST SHOULD be used. This type might be used to identify
servers on hosts with X.500 names where the slash (/) might
otherwise be ambiguous.
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A name type of NT-X500-PRINCIPAL SHOULD be used when a name from
an X.509 certificate is translated into a Kerberos name. The
encoding of the X.509 name as a Kerberos principal shall conform
to the encoding rules specified in RFC 2253.
A name type of SMTP allows a name to be of a form that resembles a
SMTP email name. This name, including an "@" and a domain name, is
used as the one component of the principal name.
A name type of UNKNOWN SHOULD be used when the form of the name is
not known. When comparing names, a name of type UNKNOWN will match
principals authenticated with names of any type. A principal
authenticated with a name of type UNKNOWN, however, will only
match other names of type UNKNOWN.
Names of any type with an initial component of 'krbtgt' are
reserved for the Kerberos ticket granting service. See section 7.3
for the form of such names.
6.2.1. Name of server principals
The principal identifier for a server on a host will generally be
composed of two parts: (1) the realm of the KDC with which the
server is registered, and (2) a two-component name of type NT-SRV-
HST if the host name is an Internet domain name or a multi-
component name of type NT-SRV-XHST if the name of the host is of a
form such as X.500 that allows slash (/) separators. The first
component of the two- or multi-component name will identify the
service and the latter components will identify the host. Where
the name of the host is not case sensitive (for example, with
Internet domain names) the name of the host MUST be lower case. If
specified by the application protocol for services such as telnet
and the Berkeley R commands which run with system privileges, the
first component MAY be the string 'host' instead of a service
specific identifier.
7. Constants and other defined values
7.1. Host address types
All negative values for the host address type are reserved for
local use. All non-negative values are reserved for officially
assigned type fields and interpretations.
Internet (IPv4) Addresses
Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded
in MSB order. The IPv4 loopback address SHOULD NOT appear in a
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Kerberos packet. The type of IPv4 addresses is two (2).
Internet (IPv6) Addresses
IPv6 addresses [RFC2373] are 128-bit (16-octet) quantities,
encoded in MSB order. The type of IPv6 addresses is twenty-four
(24). The following addresses MUST NOT appear in any Kerberos
packet:
* the Unspecified Address
* the Loopback Address
* Link-Local addresses
IPv4-mapped IPv6 addresses MUST be represented as addresses of
type 2.
DECnet Phase IV addresses
DECnet Phase IV addresses are 16-bit addresses, encoded in LSB
order. The type of DECnet Phase IV addresses is twelve (12).
Netbios addresses
Netbios addresses are 16-octet addresses typically composed of 1
to 15 alphanumeric characters and padded with the US-ASCII SPC
character (code 32). The 16th octet MUST be the US-ASCII NUL
character (code 0). The type of Netbios addresses is twenty (20).
Directional Addresses
In many environments, including the sender address in KRB_SAFE and
KRB_PRIV messages is undesirable because the addresses may be
changed in transport by network address translators. However, if
these addresses are removed, the messages may be subject to a
reflection attack in which a message is reflected back to its
originator. The directional address type provides a way to avoid
transport addresses and reflection attacks. Directional addresses
are encoded as four byte unsigned integers in network byte order.
If the message is originated by the party sending the original
KRB_AP_REQ message, then an address of 0 SHOULD be used. If the
message is originated by the party to whom that KRB_AP_REQ was
sent, then the address 1 SHOULD be used. Applications involving
multiple parties can specify the use of other addresses.
Directional addresses MUST only be used for the sender address
field in the KRB_SAFE or KRB_PRIV messages. They MUST NOT be used
as a ticket address or in a KRB_AP_REQ message. This address type
SHOULD only be used in situations where the sending party knows
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that the receiving party supports the address type. This generally
means that directional addresses may only be used when the
application protocol requires their support. Directional addresses
are type (3).
7.2. KDC messaging - IP Transports
Kerberos defines two IP transport mechanisms for communication
between clients and servers: UDP/IP and TCP/IP.
7.2.1. UDP/IP transport
Kerberos servers (KDCs) supporting IP transports MUST accept UDP
requests and SHOULD listen for such requests on port 88 (decimal)
unless specifically configured to listen on an alternative UDP
port. Alternate ports MAY be used when running multiple KDCs for
multiple realms on the same host.
Kerberos clients supporting IP transports SHOULD support the
sending of UDP requests. Clients SHOULD use KDC discovery [7.2.3]
to identify the IP address and port to which they will send their
request.
When contacting a KDC for a KRB_KDC_REQ request using UDP/IP
transport, the client shall send a UDP datagram containing only an
encoding of the request to the KDC. The KDC will respond with a
reply datagram containing only an encoding of the reply message
(either a KRB_ERROR or a KRB_KDC_REP) to the sending port at the
sender's IP address. The response to a request made through UDP/IP
transport MUST also use UDP/IP transport. If the response can not
be handled using UDP (for example because it is too large), the
KDC MUST return KRB_ERR_RESPONSE_TOO_BIG, forcing the client to
retry the request using the TCP transport.
7.2.2. TCP/IP transport
Kerberos servers (KDCs) supporting IP transports MUST accept TCP
requests and SHOULD listen for such requests on port 88 (decimal)
unless specifically configured to listen on an alternate TCP port.
Alternate ports MAY be used when running multiple KDCs for
multiple realms on the same host.
Clients MUST support the sending of TCP requests, but MAY choose
to initially try a request using the UDP transport. Clients SHOULD
use KDC discovery [7.2.3] to identify the IP address and port to
which they will send their request.
Implementation note: Some extensions to the Kerberos protocol will
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not succeed if any client or KDC not supporting the TCP transport
is involved. Implementations of RFC 1510 were not required to
support TCP/IP transports.
When the KRB_KDC_REQ message is sent to the KDC over a TCP stream,
the response (KRB_KDC_REP or KRB_ERROR message) MUST be returned
to the client on the same TCP stream that was established for the
request. The KDC MAY close the TCP stream after sending a
response, but MAY leave the stream open for a reasonable period of
time if it expects a followup. Care must be taken in managing
TCP/IP connections on the KDC to prevent denial of service attacks
based on the number of open TCP/IP connections.
The client MUST be prepared to have the stream closed by the KDC
at anytime after the receipt of a response. A stream closure
SHOULD NOT be treated as a fatal error. Instead, if multiple
exchanges are required (e.g., certain forms of pre-authentication)
the client may need to establish a new connection when it is ready
to send subsequent messages. A client MAY close the stream after
receiving a response, and SHOULD close the stream if it does not
expect to send followup messages.
A client MAY send multiple requests before receiving responses,
though it must be prepared to handle the connection being closed
after the first response.
Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or KRB_ERROR)
sent over the TCP stream is preceded by the length of the request
as 4 octets in network byte order. The high bit of the length is
reserved for future expansion and MUST currently be set to zero.
If a KDC that does not understand how to interpret a set high bit
of the length encoding receives a request with the high order bit
of the length set, it MUST return a KRB-ERROR message with the
error KRB_ERR_FIELD_TOOLONG and MUST close the TCP stream.
If multiple requests are sent over a single TCP connection, and
the KDC sends multiple responses, the KDC is not required to send
the responses in the order of the corresponding requests. This may
permit some implementations to send each response as soon as it is
ready even if earlier requests are still being processed (for
example, waiting for a response from an external device or
database).
7.2.3. KDC Discovery on IP Networks
Kerberos client implementations MUST provide a means for the
client to determine the location of the Kerberos Key Distribution
Centers (KDCs). Traditionally, Kerberos implementations have
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stored such configuration information in a file on each client
machine. Experience has shown this method of storing configuration
information presents problems with out-of-date information and
scaling problems, especially when using cross-realm
authentication. This section describes a method for using the
Domain Name System [RFC 1035] for storing KDC location
information.
7.2.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names
In Kerberos, realm names are case sensitive. While it is strongly
encouraged that all realm names be all upper case this
recommendation has not been adopted by all sites. Some sites use
all lower case names and other use mixed case. DNS on the other
hand is case insensitive for queries. Since the realm names
"MYREALM", "myrealm", and "MyRealm" are all different, but resolve
the same in the domain name system, it is necessary that only one
of the possible combinations of upper and lower case characters be
used in realm names.
7.2.3.2. Specifying KDC Location information with DNS SRV records
KDC location information is to be stored using the DNS SRV RR [RFC
2782]. The format of this RR is as follows:
_Service._Proto.Realm TTL Class SRV Priority Weight Port Target
The Service name for Kerberos is always "kerberos".
The Proto can be one of "udp", "tcp". If these SRV records are to
be used, both "udp" and "tcp" records MUST be specified for all
KDC deployments.
The Realm is the Kerberos realm that this record corresponds to.
The realm MUST be a domain style realm name.
TTL, Class, SRV, Priority, Weight, and Target have the standard
meaning as defined in RFC 2782.
As per RFC 2782 the Port number used for "_udp" and "_tcp" SRV
records SHOULD be the value assigned to "kerberos" by the Internet
Assigned Number Authority: 88 (decimal) unless the KDC is
configured to listen on an alternate TCP port.
Implementation note: Many existing client implementations do not
support KDC Discovery and are configured to send requests to the
IANA assigned port (88 decimal), so it is strongly recommended
that KDCs be configured to listen on that port.
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7.2.3.3. KDC Discovery for Domain Style Realm Names on IP Networks
These are DNS records for a Kerberos realm EXAMPLE.COM. It has two
Kerberos servers, kdc1.example.com and kdc2.example.com. Queries
should be directed to kdc1.example.com first as per the specified
priority. Weights are not used in these sample records.
_kerberos._udp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
_kerberos._udp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
_kerberos._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
_kerberos._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
7.3. Name of the TGS
The principal identifier of the ticket-granting service shall be
composed of three parts: (1) the realm of the KDC issuing the TGS
ticket (2) a two-part name of type NT-SRV-INST, with the first
part "krbtgt" and the second part the name of the realm which will
accept the ticket-granting ticket. For example, a ticket-granting
ticket issued by the ATHENA.MIT.EDU realm to be used to get
tickets from the ATHENA.MIT.EDU KDC has a principal identifier of
"ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A
ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be
used to get tickets from the MIT.EDU realm has a principal
identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU")
(name).
7.4. OID arc for KerberosV5
This OID MAY be used to identify Kerberos protocol messages
encapsulated in other protocols. It also designates the OID arc
for KerberosV5-related OIDs assigned by future IETF action.
Implementation note:: RFC 1510 had an incorrect value (5) for
"dod" in its OID.
id-krb5 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2)
}
Assignment of OIDs beneath the id-krb5 arc must be obtained by
contacting the registrar for the id-krb5 arc, or its designee. At
the time of the issuance of this RFC, such registrations can be
obtained by contacting krb5-oid-registrar@mit.edu.
7.5. Protocol constants and associated values
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The following tables list constants used in the protocol and
define their meanings. Ranges are specified in the "specification"
section that limit the values of constants for which values are
defined here. This allows implementations to make assumptions
about the maximum values that will be received for these
constants. Implementation receiving values outside the range
specified in the "specification" section MAY reject the request,
but they MUST recover cleanly.
7.5.1. Key usage numbers
The encryption and checksum specifications in [@KCRYPTO] require
as input a "key usage number", to alter the encryption key used in
any specific message, to make certain types of cryptographic
attack more difficult. These are the key usage values assigned in
this document:
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted
with the client key (section 5.2.7.2)
2. AS-REP Ticket and TGS-REP Ticket (includes TGS session
key or application session key), encrypted with the
service key (section 5.3)
3. AS-REP encrypted part (includes TGS session key or
application session key), encrypted with the client key
(section 5.4.2)
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
the TGS session key (section 5.4.1)
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
the TGS authenticator subkey (section 5.4.1)
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum,
keyed with the TGS session key (sections 5.5.1)
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator
(includes TGS authenticator subkey), encrypted with the
TGS session key (section 5.5.1)
8. TGS-REP encrypted part (includes application session
key), encrypted with the TGS session key (section
5.4.2)
9. TGS-REP encrypted part (includes application session
key), encrypted with the TGS authenticator subkey
(section 5.4.2)
10. AP-REQ Authenticator cksum, keyed with the application
session key (section 5.5.1)
11. AP-REQ Authenticator (includes application
authenticator subkey), encrypted with the application
session key (section 5.5.1)
12. AP-REP encrypted part (includes application session
subkey), encrypted with the application session key
(section 5.5.2)
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13. KRB-PRIV encrypted part, encrypted with a key chosen by
the application (section 5.7.1)
14. KRB-CRED encrypted part, encrypted with a key chosen by
the application (section 5.8.1)
15. KRB-SAFE cksum, keyed with a key chosen by the
application (section 5.6.1)
19. AD-KDC-ISSUED checksum (ad-checksum in 5.2.6.4)
22-25. Reserved for use in GSSAPI mechanisms derived from RFC
1964. (raeburn/MIT)
16-18,20-21,26-511. Reserved for future use in Kerberos and related
protocols.
512-1023. Reserved for uses internal to a Kerberos
implementation.
1024. Encryption for application use in protocols that
do not specify key usage values
1025. Checksums for application use in protocols that
do not specify key usage values
1026-2047. Reserved for application use.
7.5.2. PreAuthentication Data Types
padata and data types padata-type value comment
PA-TGS-REQ 1
PA-ENC-TIMESTAMP 2
PA-PW-SALT 3
[reserved] 4
PA-ENC-UNIX-TIME 5 (deprecated)
PA-SANDIA-SECUREID 6
PA-SESAME 7
PA-OSF-DCE 8
PA-CYBERSAFE-SECUREID 9
PA-AFS3-SALT 10
PA-ETYPE-INFO 11
PA-SAM-CHALLENGE 12 (sam/otp)
PA-SAM-RESPONSE 13 (sam/otp)
PA-PK-AS-REQ 14 (pkinit)
PA-PK-AS-REP 15 (pkinit)
PA-ETYPE-INFO2 19 (replaces pa-etype-info)
PA-USE-SPECIFIED-KVNO 20
PA-SAM-REDIRECT 21 (sam/otp)
PA-GET-FROM-TYPED-DATA 22 (embedded in typed data)
TD-PADATA 22 (embeds padata)
PA-SAM-ETYPE-INFO 23 (sam/otp)
PA-ALT-PRINC 24 (crawdad@fnal.gov)
PA-SAM-CHALLENGE2 30 (kenh@pobox.com)
PA-SAM-RESPONSE2 31 (kenh@pobox.com)
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PA-EXTRA-TGT 41 Reserved extra TGT
TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS
TD-KRB-PRINCIPAL 102 PrincipalName
TD-KRB-REALM 103 Realm
TD-TRUSTED-CERTIFIERS 104 from PKINIT
TD-CERTIFICATE-INDEX 105 from PKINIT
TD-APP-DEFINED-ERROR 106 application specific
TD-REQ-NONCE 107 INTEGER
TD-REQ-SEQ 108 INTEGER
PA-PAC-REQUEST 128 (jbrezak@exchange.microsoft.com)
7.5.3. Address Types
Address type value
IPv4 2
Directional 3
ChaosNet 5
XNS 6
ISO 7
DECNET Phase IV 12
AppleTalk DDP 16
NetBios 20
IPv6 24
7.5.4. Authorization Data Types
authorization data type ad-type value
AD-IF-RELEVANT 1
AD-INTENDED-FOR-SERVER 2
AD-INTENDED-FOR-APPLICATION-CLASS 3
AD-KDC-ISSUED 4
AD-AND-OR 5
AD-MANDATORY-TICKET-EXTENSIONS 6
AD-IN-TICKET-EXTENSIONS 7
AD-MANDATORY-FOR-KDC 8
reserved values 9-63
OSF-DCE 64
SESAME 65
AD-OSF-DCE-PKI-CERTID 66 (hemsath@us.ibm.com)
AD-WIN2K-PAC 128 (jbrezak@exchange.microsoft.com)
7.5.5. Transited Encoding Types
transited encoding type tr-type value
DOMAIN-X500-COMPRESS 1
reserved values all others
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7.5.6. Protocol Version Number
Label Value Meaning or MIT code
pvno 5 current Kerberos protocol version number
7.5.7. Kerberos Message Types
message types
KRB_AS_REQ 10 Request for initial authentication
KRB_AS_REP 11 Response to KRB_AS_REQ request
KRB_TGS_REQ 12 Request for authentication based on TGT
KRB_TGS_REP 13 Response to KRB_TGS_REQ request
KRB_AP_REQ 14 application request to server
KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL
KRB_RESERVED16 16 Reserved for user-to-user krb_tgt_request
KRB_RESERVED17 17 Reserved for user-to-user krb_tgt_reply
KRB_SAFE 20 Safe (checksummed) application message
KRB_PRIV 21 Private (encrypted) application message
KRB_CRED 22 Private (encrypted) message to forward credentials
KRB_ERROR 30 Error response
7.5.8. Name Types
name types
KRB_NT_UNKNOWN 0 Name type not known
KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or for users
KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt)
KRB_NT_SRV_HST 3 Service with host name as instance (telnet, rcommands)
KRB_NT_SRV_XHST 4 Service with host as remaining components
KRB_NT_UID 5 Unique ID
KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253]
KRB_NT_SMTP_NAME 7 Name in form of SMTP email name (e.g. user@foo.com)
KRB_NT_ENTERPRISE 10 Enterprise name - may be mapped to principal name
7.5.9. Error Codes
error codes
KDC_ERR_NONE 0 No error
KDC_ERR_NAME_EXP 1 Client's entry in database has expired
KDC_ERR_SERVICE_EXP 2 Server's entry in database has expired
KDC_ERR_BAD_PVNO 3 Requested protocol version number
not supported
KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old master key
KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old master key
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KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos database
KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos database
KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in database
KDC_ERR_NULL_KEY 9 The client or server has a null key
KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating
KDC_ERR_NEVER_VALID 11 Requested start time is later than end time
KDC_ERR_POLICY 12 KDC policy rejects request
KDC_ERR_BADOPTION 13 KDC cannot accommodate requested option
KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption type
KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum type
KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type
KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited type
KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been revoked
KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been revoked
KDC_ERR_TGT_REVOKED 20 TGT has been revoked
KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again later
KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again later
KDC_ERR_KEY_EXPIRED 23 Password has expired
- change password to reset
KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was invalid
KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-authenticationrequired
KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket don't match
KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for user2user only
KDC_ERR_PATH_NOT_ACCPETED 28 KDC Policy rejects transited path
KDC_ERR_SVC_UNAVAILABLE 29 A service is not available
KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field failed
KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired
KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid
KRB_AP_ERR_REPEAT 34 Request is a replay
KRB_AP_ERR_NOT_US 35 The ticket isn't for us
KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't match
KRB_AP_ERR_SKEW 37 Clock skew too great
KRB_AP_ERR_BADADDR 38 Incorrect net address
KRB_AP_ERR_BADVERSION 39 Protocol version mismatch
KRB_AP_ERR_MSG_TYPE 40 Invalid msg type
KRB_AP_ERR_MODIFIED 41 Message stream modified
KRB_AP_ERR_BADORDER 42 Message out of order
KRB_AP_ERR_BADKEYVER 44 Specified version of key is not available
KRB_AP_ERR_NOKEY 45 Service key not available
KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed
KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction
KRB_AP_ERR_METHOD 48 Alternative authentication method required
KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in message
KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in message
KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited path
KRB_ERR_RESPONSE_TOO_BIG 52 Response too big for UDP, retry with TCP
KRB_ERR_GENERIC 60 Generic error (description in e-text)
KRB_ERR_FIELD_TOOLONG 61 Field is too long for this implementation
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KDC_ERROR_CLIENT_NOT_TRUSTED 62 Reserved for PKINIT
KDC_ERROR_KDC_NOT_TRUSTED 63 Reserved for PKINIT
KDC_ERROR_INVALID_SIG 64 Reserved for PKINIT
KDC_ERR_KEY_TOO_WEAK 65 Reserved for PKINIT
KDC_ERR_CERTIFICATE_MISMATCH 66 Reserved for PKINIT
KRB_AP_ERR_NO_TGT 67 No TGT available to validate USER-TO-USER
KDC_ERR_WRONG_REALM 68 USER-TO-USER TGT issued different KDC
KRB_AP_ERR_USER_TO_USER_REQUIRED 69 Ticket must be for USER-TO-USER
KDC_ERR_CANT_VERIFY_CERTIFICATE 70 Reserved for PKINIT
KDC_ERR_INVALID_CERTIFICATE 71 Reserved for PKINIT
KDC_ERR_REVOKED_CERTIFICATE 72 Reserved for PKINIT
KDC_ERR_REVOCATION_STATUS_UNKNOWN 73 Reserved for PKINIT
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74 Reserved for PKINIT
KDC_ERR_CLIENT_NAME_MISMATCH 75 Reserved for PKINIT
KDC_ERR_KDC_NAME_MISMATCH 76 Reserved for PKINIT
8. Interoperability requirements
Version 5 of the Kerberos protocol supports a myriad of options.
Among these are multiple encryption and checksum types,
alternative encoding schemes for the transited field, optional
mechanisms for pre-authentication, the handling of tickets with no
addresses, options for mutual authentication, user-to-user
authentication, support for proxies, forwarding, postdating, and
renewing tickets, the format of realm names, and the handling of
authorization data.
In order to ensure the interoperability of realms, it is necessary
to define a minimal configuration which must be supported by all
implementations. This minimal configuration is subject to change
as technology does. For example, if at some later date it is
discovered that one of the required encryption or checksum
algorithms is not secure, it will be replaced.
8.1. Specification 2
This section defines the second specification of these options.
Implementations which are configured in this way can be said to
support Kerberos Version 5 Specification 2 (5.2). Specification 1
(deprecated) may be found in RFC1510.
Transport
TCP/IP and UDP/IP transport MUST be supported by clients and KDCs
claiming conformance to specification 2.
Encryption and checksum methods
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The following encryption and checksum mechanisms MUST be
supported.
Encryption: AES256-CTS-HMAC-SHA1-96
Checksums: HMAC-SHA1-96-AES256
Implementations SHOULD support other mechanisms as well, but the
additional mechanisms may only be used when communicating with
principals known to also support them. The mechanisms that SHOULD
be supported are:
Encryption: DES-CBC-MD5, DES3-CBC-SHA1-KD
Checksums: DES-MD5, HMAC-SHA1-DES3-KD
Implementations MAY support other mechanisms as well, but the
additional mechanisms may only be used when communicating with
principals known to also support them.
Implementation note: earlier implementations of Kerberos generate
messages using the CRC-32, RSA-MD5 checksum methods. For
interoperability with these earlier releases implementors MAY
consider supporting these checksum methods but should carefully
analyze the security impplications to limit the situations within
which these methods are accepted.
Realm Names
All implementations MUST understand hierarchical realms in both
the Internet Domain and the X.500 style. When a ticket-granting
ticket for an unknown realm is requested, the KDC MUST be able to
determine the names of the intermediate realms between the KDCs
realm and the requested realm.
Transited field encoding
DOMAIN-X500-COMPRESS (described in section 3.3.3.2) MUST be
supported. Alternative encodings MAY be supported, but they may
be used only when that encoding is supported by ALL intermediate
realms.
Pre-authentication methods
The TGS-REQ method MUST be supported. The TGS-REQ method is not
used on the initial request. The PA-ENC-TIMESTAMP method MUST be
supported by clients but whether it is enabled by default MAY be
determined on a realm by realm basis. If not used in the initial
request and the error KDC_ERR_PREAUTH_REQUIRED is returned
specifying PA-ENC-TIMESTAMP as an acceptable method, the client
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SHOULD retry the initial request using the PA-ENC-TIMESTAMP pre-
authentication method. Servers need not support the PA-ENC-
TIMESTAMP method, but if not supported the server SHOULD ignore
the presence of PA-ENC-TIMESTAMP pre-authentication in a request.
The ETYPE-INFO2 method MUST be supported; this method is used to
communicate the set of supported encryption types, and
corresponding salt and string to key paramters. The ETYPE-INFO
method SHOULD be supported for interoperability with older
implementation.
Mutual authentication
Mutual authentication (via the KRB_AP_REP message) MUST be
supported.
Ticket addresses and flags
All KDCs MUST pass through tickets that carry no addresses (i.e.
if a TGT contains no addresses, the KDC will return derivative
tickets). Implementations SHOULD default to requesting
addressless tickets as this significantly increases
interoperability with network address translation. In some cases
realms or application servers MAY require that tickets have an
address.
Implementations SHOULD accept directional address type for the
KRB_SAFE and KRB_PRIV message and SHOULD include directional
addresses in these messages when other address types are not
available.
Proxies and forwarded tickets MUST be supported. Individual realms
and application servers can set their own policy on when such
tickets will be accepted.
All implementations MUST recognize renewable and postdated
tickets, but need not actually implement them. If these options
are not supported, the starttime and endtime in the ticket shall
specify a ticket's entire useful life. When a postdated ticket is
decoded by a server, all implementations shall make the presence
of the postdated flag visible to the calling server.
User-to-user authentication
Support for user-to-user authentication (via the ENC-TKT-IN-SKEY
KDC option) MUST be provided by implementations, but individual
realms MAY decide as a matter of policy to reject such requests on
a per-principal or realm-wide basis.
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Authorization data
Implementations MUST pass all authorization data subfields from
ticket-granting tickets to any derivative tickets unless directed
to suppress a subfield as part of the definition of that
registered subfield type (it is never incorrect to pass on a
subfield, and no registered subfield types presently specify
suppression at the KDC).
Implementations MUST make the contents of any authorization data
subfields available to the server when a ticket is used.
Implementations are not required to allow clients to specify the
contents of the authorization data fields.
Constant ranges
All protocol constants are constrained to 32 bit (signed) values
unless further constrained by the protocol definition. This limit
is provided to allow implementations to make assumptions about the
maximum values that will be received for these constants.
Implementation receiving values outside this range MAY reject the
request, but they MUST recover cleanly.
8.2. Recommended KDC values
Following is a list of recommended values for a KDC configuration.
minimum lifetime 5 minutes
maximum renewable lifetime 1 week
maximum ticket lifetime 1 day
acceptable clock skew 5 minutes
empty addresses Allowed.
proxiable, etc. Allowed.
9. IANA considerations
Section 7 of this document specifies protocol constants and other
defined values required for the interoperability of multiple
implementations. Until otherwise specified in a subsequent RFC, or
upon disbanding of the Kerberos working group, allocations of
additional protocol constants and other defined values required
for extensions to the Kerberos protocol will be administered by
the kerberos working group. Following the recomendations outlined
in [RFC 2434], guidance is provided to the IANA as follows:
"reserved" realm name types in section 6.1 and "other" realm types
except those beginning with "X-" or "x-" will not be registered
without IETF standards action, at which point guidlines for
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further assignment will be specified. Realm name types beginning
with "X-" or "x-" are for private use.
For host address types described in section 7.1, negative values
are for private use. Assignment of additional positive numbers is
subject to review by the kerberos working group or other expert
review.
Additional key usage numbers as defined in section 7.5.1 will be
assigned subject to review by the kerberos working group or other
expert review.
Additional preauthentciation data type values as defined in
section 7.5.2 will be assigned subject to review by the kerberos
working group or other expert review.
Additional Authorization Data Types as defined in section 7.5.4
will be assigned subject to review by the kerberos working group
or other expert review. Although it is anticipated that there may
be significant demand for private use types, provision is
intentionaly not made for a private use portion of the namespace
because conficts between privately assigned values coule have
detrimental security implications.
Additional Transited Encoding Types as defined in section 7.5.5
present special concerns for interoperability with existing
implementations. As such, such assignments will only be made by
standards action, except that the Kerberos working group or
another other working group with competent jurisdiction may make
preliminary assignments for documents which are moving through the
standards process.
Additional Kerberos Message Types as described in section 7.5.7
will be assigned subject to review by the kerberos working group
or other expert review.
Additional Name Types as described in section 7.5.8 will be
assigned subject to review by the kerberos working group or other
expert review.
Additional error codes described in section 7.5.9 will be assigned
subject to review by the kerberos working group or other expert
review.
10. Security Considerations
As an authentication service, Kerberos provides a means of
verifying the identity of principals on a network. Kerberos does
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not, by itself, provide authorization. Applications should not
accept the issuance of a service ticket by the Kerberos server as
granting authority to use the service, since such applications may
become vulnerable to the bypass of this authorization check in an
environment if they inter-operate with other KDCs or where other
options for application authentication are provided.
Denial of service attacks are not solved with Kerberos. There are
places in the protocols where an intruder can prevent an
application from participating in the proper authentication steps.
Because authentication is a required step for the use of many
services, successful denial of service attacks on a Kerberos
server might result in the denial of other network services that
rely on Kerberos for authentication. Kerberos is vulnerable to
many kinds of denial of service attacks: denial of service attacks
on the network which would prevent clients from contacting the
KDC; denial of service attacks on the domain name system which
could prevent a client from finding the IP address of the Kerberos
server; and denial of service attack by overloading the Kerberos
KDC itself with repeated requests.
Interoperability conflicts caused by incompatible character-set
usage (see 5.2.1) can result in denial of service for clients that
utilize character-sets in Kerberos strings other than those stored
in the KDC database.
Authentication servers maintain a database of principals (i.e.,
users and servers) and their secret keys. The security of the
authentication server machines is critical. The breach of security
of an authentication server will compromise the security of all
servers that rely upon the compromised KDC, and will compromise
the authentication of any principals registered in the realm of
the compromised KDC.
Principals must keep their secret keys secret. If an intruder
somehow steals a principal's key, it will be able to masquerade as
that principal or impersonate any server to the legitimate
principal.
Password guessing attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to
successfully mount an off-line dictionary attack by repeatedly
attempting to decrypt, with successive entries from a dictionary,
messages obtained which are encrypted under a key derived from the
user's password.
Unless pre-authentication options are required by the policy of a
realm, the KDC will not know whether a request for authentication
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succeeds. An attacker can request a reply with credentials for any
principal. These credentials will likely not be of much use to the
attacker unless it knows the client's secret key, but the
availability of the response encrypted in the client's secret key
provides the attacker with ciphertext that may be used to mount
brute force or dictionary attacks to decrypt the credentials, by
guessing the user's password. For this reason it is strongly
encouraged that Kerberos realms require the use of pre-
authentication. Even with pre-authentication, attackers may try
brute force or dictionary attacks against credentials that are
observed by eavesdropping on the network.
Because a client can request a ticket for any server principal and
can attempt a brute force or dictionary attack against the server
principal's key using that ticket, it is strongly encouraged that
keys be randomly generated (rather than generated from passwords)
for any principals that are usable as the target principal for a
KRB_TGS_REQ or KRB_AS_REQ messages. [RFC1750]
Although the DES-CBC-MD5 encryption method and DES-MD5 checksum
methods are listed as SHOULD be implemented for backward
compatibility, the single DES encryption algorithm on which these
are based is weak and stronger algorithms should be used whenever
possible.
Each host on the network must have a clock which is loosely
synchronized to the time of the other hosts; this synchronization
is used to reduce the bookkeeping needs of application servers
when they do replay detection. The degree of "looseness" can be
configured on a per-server basis, but is typically on the order of
5 minutes. If the clocks are synchronized over the network, the
clock synchronization protocol MUST itself be secured from network
attackers.
Principal identifiers must not recycled on a short-term basis. A
typical mode of access control will use access control lists
(ACLs) to grant permissions to particular principals. If a stale
ACL entry remains for a deleted principal and the principal
identifier is reused, the new principal will inherit rights
specified in the stale ACL entry. By not reusing principal
identifiers, the danger of inadvertent access is removed.
Proper decryption of an KRB_AS_REP message from the KDC is not
sufficient for the host to verify the identity of the user; the
user and an attacker could cooperate to generate a KRB_AS_REP
format message which decrypts properly but is not from the proper
KDC. To authenticate a user logging on to a local system, the
credentials obtained in the AS exchange may first be used in a TGS
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exchange to obtain credentials for a local server. Those
credentials must then be verified by a local server through
successful completion of the Client/Server exchange.
Many RFC 1510 compliant implementations ignore unknown
authorization data elements. Depending on these implementations to
honor authorization data restrictions may create a security
weakness.
Kerberos credentials contain clear-text information identifying
the principals to which they apply. If privacy of this information
is needed, this exchange should itself be encapsulated in a
protocol providing for confidentiality on the exchange of these
credentials.
Applications must take care to protect communications subsequent
to authentication either by using the KRB_PRIV or KRB_SAFE
messages as appropriate, or by applying their own confidentiality
or integrity mechanisms on such communications. Completion of the
KRB_AP_REQ and KRB_AP_REP exchange without subsequent use of
confidentiality and integrity mechanisms provides only for
authentication of the parties to the communication and not
confidentiality and integrity of the subsequent communication.
Application applying confidentiality and integrity protection
mechanisms other than KRB_PRIV and KRB_SAFE must make sure that
the authentication step is appropriately linked with the protected
communication channel that is established by the application.
Unless the application server provides its own suitable means to
protect against replay (for example, a challenge-response sequence
initiated by the server after authentication, or use of a server-
generated encryption subkey), the server must utilize a replay
cache to remember any authenticator presented within the allowable
clock skew. All services sharing a key need to use the same replay
cache. If separate replay caches are used, then and authenticator
used with one such service could later be replayed to a different
service with the same service principal.
If a server loses track of authenticators presented within the
allowable clock skew, it must reject all requests until the clock
skew interval has passed, providing assurance that any lost or
replayed authenticators will fall outside the allowable clock skew
and can no longer be successfully replayed.
Implementations of Kerberos should not use untrusted directory
servers to determine the realm of a host. To allow such would
allow the compromise of the directory server to enable an attacker
to direct the client to accept authentication with the wrong
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principal (i.e. one with a similar name, but in a realm with which
the legitimate host was not registered).
Implementations of Kerberos must not use DNS to map one name to
another (canonicalize) to determine the host part of the principal
name with which one is to communicate. To allow such
canonicalization would allow a compromise of the DNS to result in
a client obtaining credentials and correctly authenticating to the
wrong principal. Though the client will know who it is
communicating with, it will not be the principal with which it
intended to communicate.
If the Kerberos server returns a TGT for a 'closer' realm other
than the desired realm, the client may use local policy
configuration to verify that the authentication path used is an
acceptable one. Alternatively, a client may choose its own
authentication path, rather than relying on the Kerberos server to
select one. In either case, any policy or configuration
information used to choose or validate authentication paths,
whether by the Kerberos server or client, must be obtained from a
trusted source.
The Kerberos protocol in its basic form does not provide perfect
forward secrecy for communications. If traffic has been recorded
by an eavesdropper, then messages encrypted using the KRB_PRIV
message, or messages encrypted using application specific
encryption under keys exchanged using Kerberos can be decrypted if
any of the user's, application server's, or KDC's key is
subsequently discovered. This is because the session key use to
encrypt such messages is transmitted over the network encrypted in
the key of the application server, and also encrypted under the
session key from the user's ticket-granting ticket when returned
to the user in the KRB_TGS_REP message. The session key from the
ticket-granting ticket was sent to the user in the KRB_AS_REP
message encrypted in the user's secret key, and embedded in the
ticket-granting ticket, which was encrypted in the key of the KDC.
Application requiring perfect forward secrecy must exchange keys
through mechanisms that provide such assurance, but may use
Kerberos for authentication of the encrypted channel established
through such other means.
11. Author's Addresses
Clifford Neuman
Information Sciences Institute
University of Southern California
4676 Admiralty Way
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Marina del Rey, CA 90292, USA
Email: bcn@isi.edu
Tom Yu
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139, USA
Email: tlyu@mit.edu
Sam Hartman
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139, USA
Email: hartmans@mit.edu
Kenneth Raeburn
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139, USA
Email: raeburn@MIT.EDU
12. Acknowledgements
This document is a revision to RFC1510 which was co-authored with
John Kohl. The specification of the Kerberos protocol described
in this document is the result of many years of effort. Over this
period many individuals have contributed to the definition of the
protocol and to the writing of the specification. Unfortunately it
is not possible to list all contributors as authors of this
document, though there are many not listed who are authors in
spirit, because they contributed text for parts of some sections,
because they contributed to the design of parts of the protocol,
or because they contributed significantly to the discussion of the
protocol in the IETF common authentication technology (CAT) and
Kerberos working groups.
Among those contributing to the development and specification of
Kerberos were Jeffrey Altman, John Brezak, Marc Colan, Johan
Danielsson, Don Davis, Doug Engert, Dan Geer, Paul Hill, John
Kohl, Marc Horowitz, Matt Hur, Jeffrey Hutzelman, Paul Leach, John
Linn, Ari Medvinsky, Sasha Medvinsky, Steve Miller, Jon Rochlis,
Jerome Saltzer, Jeffrey Schiller, Jennifer Steiner, Ralph Swick,
Mike Swift, Jonathan Trostle, Theodore Ts'o, Brian Tung, Jacques
Vidrine, Assar Westerlund, and Nicolas Williams. Many other
members of MIT Project Athena, the MIT networking group, and the
Kerberos and CAT working groups of the IETF contributed but are
not listed.
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Funding for the RFC Editor function is currently provided by the
Internet Society.
13. REFERENCES
13.1 NORMATIVE REFERENCES
[@KCRYPTO]
RFC-Editor: To be replaced by RFC number for draft-ietf-krb-wg-
crypto.
[@AES]
RFC-Editor: To be replaced by RFC number for draft-raeburn0krb-
rijndael-krb.
[ISO-646/ECMA-6]
7-bit Coded Character Set
[ISO-2022/ECMA-35]
Character Code Structure and Extension Techniques
[ISO-4873/ECMA-43]
8-bit Coded Character Set Structure and Rules
[RFC1035]
P.V. Mockapetris, RFC1035: "Domain Names - Implementations and
Specification," November 1, 1987, Obsoletes - RFC973, RFC882,
RFC883. Updated by RFC1101, RFC1183, RFC1348, RFCRFC1876, RFC1982,
RFC1995, RFC1996, RFC2065, RFC2136, RFC2137, RFC2181, RFC2308,
RFC2535, RFC2845, and RFC3425. Status: Standard.
[RFC2119]
S. Bradner, RFC2119: "Key words for use in RFC's to Indicate
Requirement Levels", March 1997.
[RFC2434]
T. Narten, H. Alvestrand, RFC2434: "Guidelines for writing IANA
Consideration Secionts in RFCs" October, 1998.
[RFC2782]
A. Gulbrandsen, P. Vixie and L. Esibov., RFC2782: "A DNS RR for
Specifying the Location of Services (DNS SRV)," February 2000.
[RFC2253]
M. Wahl, S. Killie, and T. Howes, RFC2253: "Lightweight Directory
Access Protocol (v3): UTF-8 String Representation or Distinguished
Names," December 1997, Obsoletes - RFC1779, Updated by RFC3377,
February 2004 [Page 122]
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Status: Proposed Standard.
[RFC2373]
R. Hinden, S. Deering, RFC2373: "IP Version 6 Addressing
Architecture," July 1998, Status: Proposed Standard.
[X680]
Abstract Syntax Notation One (ASN.1): Specification of Basic
Notation, ITU-T Recommendation X.680 (1997) | ISO/IEC
International Standard 8824-1:1998.
[X690]
ASN.1 encoding rules: Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished Encoding Rules
(DER), ITU-T Recommendation X.690 (1997)| ISO/IEC International
Standard 8825-1:1998.
13.2 INFORMATIVE REFERENCES
[DGT96]
Don Davis, Daniel Geer, and Theodore Ts'o, "Kerberos With Clocks
Adrift: History, Protocols, and Implementation", USENIX Computing
Systems 9:1 (January 1996).
[DS81]
Dorothy E. Denning and Giovanni Maria Sacco, "Time-stamps in Key
Distribution Protocols," Communications of the ACM, Vol. 24(8),
pp. 533-536 (August 1981).
[KNT94]
John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, "The
Evolution of the Kerberos Authentication System". In Distributed
Open Systems, pages 78-94. IEEE Computer Society Press, 1994.
[MNSS87]
S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer,
Section E.2.1: Kerberos Authentication and Authorization System,
M.I.T. Project Athena, Cambridge, Massachusetts (December 21,
1987).
[NS78]
Roger M. Needham and Michael D. Schroeder, "Using Encryption for
Authentication in Large Networks of Computers," Communications of
the ACM, Vol. 21(12), pp. 993-999 (December, 1978).
[Neu93]
B. Clifford Neuman, "Proxy-Based Authorization and Accounting for
February 2004 [Page 123]
Neuman, et al. draft-ietf-krb-wg-kerberos-clarifications-05.txt DRAFT
Distributed Systems," in Proceedings of the 13th International
Conference on Distributed Computing Systems, Pittsburgh, PA (May,
1993).
[NT94]
B. Clifford Neuman and Theodore Y. Ts'o, "An Authentication
Service for Computer Networks," IEEE Communications Magazine, Vol.
32(9), pp. 33-38 (September 1994).
[Pat92].
J. Pato, Using Pre-Authentication to Avoid Password Guessing
Attacks, Open Software Foundation DCE Request for Comments 26
(December 1992).
[RFC1510]
J. Kohl and B. C. Neuman, RFC1510: "The Kerberos Network
Authentication Service (v5)," September 1993, Status: Proposed
Standard.
[RFC1750]
D. Eastlake, S. Crocker, and J. Schiller "Randomness
Recommendation for Security" December 1994, Status: Informational.
[RFC2026]
S. Bradner, RFC2026: "The Internet Standard Process - Revision
3," October 1996, Obsoletes - RFC 1602, Status: Best Current
Practice.
[SNS88]
J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Kerberos: An
Authentication Service for Open Network Systems," pp. 191-202 in
Usenix Conference Proceedings, Dallas, Texas (February, 1988).
14. Copyright Statement
Copyright (C) The Internet Society (2004). This document is
subject to the rights, licenses and restrictions contained in BCP
78 and except as set forth therein, the authors retain all their
rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
February 2004 [Page 124]
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PARTICULAR PURPOSE.
15. Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights. Information on the procedures with respect to rights
in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to the
IETF at ietf-ipr@ietf.org.
A. ASN.1 module
KerberosV5Spec2 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) krb5spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
-- OID arc for KerberosV5
--
-- This OID may be used to identify Kerberos protocol messages
-- encapsulated in other protocols.
--
-- This OID also designates the OID arc for KerberosV5-related OIDs.
--
-- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its OID.
id-krb5 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2)
}
Int32 ::= INTEGER (-2147483648..2147483647)
-- signed values representable in 32 bits
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UInt32 ::= INTEGER (0..4294967295)
-- unsigned 32 bit values
Microseconds ::= INTEGER (0..999999)
-- microseconds
KerberosString ::= GeneralString (IA5String)
Realm ::= KerberosString
PrincipalName ::= SEQUENCE {
name-type [0] Int32,
name-string [1] SEQUENCE OF KerberosString
}
KerberosTime ::= GeneralizedTime -- with no fractional seconds
HostAddress ::= SEQUENCE {
addr-type [0] Int32,
address [1] OCTET STRING
}
-- NOTE: HostAddresses is always used as an OPTIONAL field and
-- should not be empty.
HostAddresses -- NOTE: subtly different from rfc1510,
-- but has a value mapping and encodes the same
::= SEQUENCE OF HostAddress
-- NOTE: AuthorizationData is always used as an OPTIONAL field and
-- should not be empty.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
ad-type [0] Int32,
ad-data [1] OCTET STRING
}
PA-DATA ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
padata-type [1] Int32,
padata-value [2] OCTET STRING -- might be encoded AP-REQ
}
KerberosFlags ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits
-- shall be sent, but no fewer than 32
EncryptedData ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
kvno [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING -- ciphertext
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}
EncryptionKey ::= SEQUENCE {
keytype [0] Int32 -- actually encryption type --,
keyvalue [1] OCTET STRING
}
Checksum ::= SEQUENCE {
cksumtype [0] Int32,
checksum [1] OCTET STRING
}
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData -- EncTicketPart
}
-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags [0] TicketFlags,
key [1] EncryptionKey,
crealm [2] Realm,
cname [3] PrincipalName,
transited [4] TransitedEncoding,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
caddr [9] HostAddresses OPTIONAL,
authorization-data [10] AuthorizationData OPTIONAL
}
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type [0] Int32 -- must be registered --,
contents [1] OCTET STRING
}
TicketFlags ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- may-postdate(5),
-- postdated(6),
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-- invalid(7),
-- renewable(8),
-- initial(9),
-- pre-authent(10),
-- hw-authent(11),
-- the following are new since 1510
-- transited-policy-checked(12),
-- ok-as-delegate(13)
AS-REQ ::= [APPLICATION 10] KDC-REQ
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
pvno [1] INTEGER (5) ,
msg-type [2] INTEGER (10 -- AS -- | 12 -- TGS --),
padata [3] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
req-body [4] KDC-REQ-BODY
}
KDC-REQ-BODY ::= SEQUENCE {
kdc-options [0] KDCOptions,
cname [1] PrincipalName OPTIONAL
-- Used only in AS-REQ --,
realm [2] Realm
-- Server's realm
-- Also client's in AS-REQ --,
sname [3] PrincipalName OPTIONAL,
from [4] KerberosTime OPTIONAL,
till [5] KerberosTime,
rtime [6] KerberosTime OPTIONAL,
nonce [7] UInt32,
etype [8] SEQUENCE OF Int32 -- EncryptionType
-- in preference order --,
addresses [9] HostAddresses OPTIONAL,
enc-authorization-data [10] EncryptedData -- AuthorizationData --,
additional-tickets [11] SEQUENCE OF Ticket OPTIONAL
-- NOTE: not empty
}
KDCOptions ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
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-- allow-postdate(5),
-- postdated(6),
-- unused7(7),
-- renewable(8),
-- unused9(9),
-- unused10(10),
-- opt-hardware-auth(11),
-- unused12(12),
-- unused13(13),
-- 15 is reserved for canonicalize
-- unused15(15),
-- 26 was unused in 1510
-- disable-transited-check(26),
--
-- renewable-ok(27),
-- enc-tkt-in-skey(28),
-- renew(30),
-- validate(31)
AS-REP ::= [APPLICATION 11] KDC-REP
TGS-REP ::= [APPLICATION 13] KDC-REP
KDC-REP ::= SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (11 -- AS -- | 13 -- TGS --),
padata [2] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
crealm [3] Realm,
cname [4] PrincipalName,
ticket [5] Ticket,
enc-part [6] EncryptedData
-- EncASRepPart or EncTGSRepPart,
-- as appropriate
}
EncASRepPart ::= [APPLICATION 25] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE {
key [0] EncryptionKey,
last-req [1] LastReq,
nonce [2] UInt32,
key-expiration [3] KerberosTime OPTIONAL,
flags [4] TicketFlags,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
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endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
srealm [9] Realm,
sname [10] PrincipalName,
caddr [11] HostAddresses OPTIONAL
}
LastReq ::= SEQUENCE OF SEQUENCE {
lr-type [0] Int32,
lr-value [1] KerberosTime
}
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (14),
ap-options [2] APOptions,
ticket [3] Ticket,
authenticator [4] EncryptedData -- Authenticator
}
APOptions ::= KerberosFlags
-- reserved(0),
-- use-session-key(1),
-- mutual-required(2)
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno [0] INTEGER (5),
crealm [1] Realm,
cname [2] PrincipalName,
cksum [3] Checksum OPTIONAL,
cusec [4] Microseconds,
ctime [5] KerberosTime,
subkey [6] EncryptionKey OPTIONAL,
seq-number [7] UInt32 OPTIONAL,
authorization-data [8] AuthorizationData OPTIONAL
}
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (15),
enc-part [2] EncryptedData -- EncAPRepPart
}
EncAPRepPart ::= [APPLICATION 27] SEQUENCE {
ctime [0] KerberosTime,
cusec [1] Microseconds,
subkey [2] EncryptionKey OPTIONAL,
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seq-number [3] UInt32 OPTIONAL
}
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (20),
safe-body [2] KRB-SAFE-BODY,
cksum [3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress,
r-address [5] HostAddress OPTIONAL
}
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (21),
-- NOTE: there is no [2] tag
enc-part [3] EncryptedData -- EncKrbPrivPart
}
EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress -- sender's addr --,
r-address [5] HostAddress OPTIONAL -- recip's addr
}
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (22),
tickets [2] SEQUENCE OF Ticket,
enc-part [3] EncryptedData -- EncKrbCredPart
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info [0] SEQUENCE OF KrbCredInfo,
nonce [1] UInt32 OPTIONAL,
timestamp [2] KerberosTime OPTIONAL,
usec [3] Microseconds OPTIONAL,
s-address [4] HostAddress OPTIONAL,
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r-address [5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key [0] EncryptionKey,
prealm [1] Realm OPTIONAL,
pname [2] PrincipalName OPTIONAL,
flags [3] TicketFlags OPTIONAL,
authtime [4] KerberosTime OPTIONAL,
starttime [5] KerberosTime OPTIONAL,
endtime [6] KerberosTime OPTIONAL,
renew-till [7] KerberosTime OPTIONAL,
srealm [8] Realm OPTIONAL,
sname [9] PrincipalName OPTIONAL,
caddr [10] HostAddresses OPTIONAL
}
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (30),
ctime [2] KerberosTime OPTIONAL,
cusec [3] Microseconds OPTIONAL,
stime [4] KerberosTime,
susec [5] Microseconds,
error-code [6] Int32,
crealm [7] Realm OPTIONAL,
cname [8] PrincipalName OPTIONAL,
realm [9] Realm -- service realm --,
sname [10] PrincipalName -- service name --,
e-text [11] KerberosString OPTIONAL,
e-data [12] OCTET STRING OPTIONAL
}
METHOD-DATA ::= SEQUENCE OF PA-DATA
TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
data-type [0] INTEGER,
data-value [1] OCTET STRING OPTIONAL
}
-- preauth stuff follows
PA-ENC-TIMESTAMP ::= EncryptedData -- PA-ENC-TS-ENC
PA-ENC-TS-ENC ::= SEQUENCE {
patimestamp [0] KerberosTime -- client's time --,
pausec [1] Microseconds OPTIONAL
}
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ETYPE-INFO-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] OCTET STRING OPTIONAL
}
ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY
ETYPE-INFO2-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] KerberosString OPTIONAL,
s2kparams [2] OCTET STRING OPTIONAL
}
ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY
AD-IF-RELEVANT ::= AuthorizationData
AD-KDCIssued ::= SEQUENCE {
ad-checksum [0] Checksum,
i-realm [1] Realm OPTIONAL,
i-sname [2] PrincipalName OPTIONAL,
elements [3] AuthorizationData
}
AD-AND-OR ::= SEQUENCE {
condition-count [0] INTEGER,
elements [1] AuthorizationData
}
AD-MANDATORY-FOR-KDC ::= AuthorizationData
END
B. Changes since RFC-1510
This document replaces RFC-1510 and clarifies specification of
items that were not completely specified. Where changes to
recommended implementation choices were made, or where new options
were added, those changes are described within the document and
listed in this section. More significantly, "Specification 2" in
section 8 changes the required encryption and checksum methods to
bring them in line with the best current practices and to
deprecate methods that are no longer considered sufficiently
strong.
Discussion was added to section 1 regarding the ability to rely on
the KDC to check the transited field, and on the inclusion of a
flag in a ticket indicating that this check has occurred. This is
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a new capability not present in RFC1510. Pre-existing
implementations may ignore or not set this flag without negative
security implications.
The definition of the secret key says that in the case of a user
the key may be derived from a password. In 1510, it said that the
key was derived from the password. This change was made to
accommodate situations where the user key might be stored on a
smart-card, or otherwise obtained independent of a password.
The introduction mentions the use of public key cryptography for
initial authentication in Kerberos by reference. RFC1510 did not
include such a reference.
Section 1.2 was added to explain that while Kerberos provides
authentication of a named principal, it is still the
responsibility of the application to ensure that the authenticated
name is the entity with which the application wishes to
communicate.
Discussion of extensibility has been added to the introduction.
Discussion of how extensibility affects ticket flags and KDC
options was added to the introduction of section 2. No changes
were made to existing options and flags specified in RFC1510,
though some of the sections in the specification were renumbered,
and text was revised to make the description and intent of
existing options clearer, especially with respect to the ENC-TKT-
IN-SKEY option (now section 2.9.2) which is used for user-to-user
authentication. The new option and ticket flag transited policy
checking (section 2.7) was added.
A warning regarding generation of session keys for application use
was added to section 3, urging the inclusion of key entropy from
the KDC generated session key in the ticket. An example regarding
use of the sub-session key was added to section 3.2.6.
Descriptions of the pa-etype-info, pa-etype-info2, and pa-pw-salt
pre-authentication data items were added. The recommendation for
use of pre-authentication was changed from "may" to "should" and a
note was added regarding known plaintext attacks.
In RFC 1510, section 4 described the database in the KDC. This
discussion was not necessary for interoperability and
unnecessarily constrained implementation. The old section 4 was
removed.
The current section 4 was formerly section 6 on encryption and
checksum specifications. The major part of this section was
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brought up to date to support new encryption methods, and move to
a separate document. Those few remaining aspects of the encryption
and checksum specification specific to Kerberos are now specified
in section 4.
Significant changes were made to the layout of section 5 to
clarify the correct behavior for optional fields. Many of these
changes were made necessary because of improper ASN.1 description
in the original Kerberos specification which left the correct
behavior underspecified. Additionally, the wording in this section
was tightened wherever possible to ensure that implementations
conforming to this specification will be extensible with the
addition of new fields in future specifications.
Text was added describing time_t=0 issues in the ASN.1. Text was
also added, clarifying issues with implementations treating
omitted optional integers as zero. Text was added clarifying
behavior for optional SEQUENCE or SEQUENCE OF that may be empty.
Discussion was added regarding sequence numbers and behavior of
some implementations, including "zero" behavior and negative
numbers. A compatibility note was added regarding the
unconditional sending of EncTGSRepPart regardless of the enclosing
reply type. Minor changes were made to the description of the
HostAddresses type. Integer types were constrained. KerberosString
was defined as a (significantly) constrained GeneralString.
KerberosFlags was defined to reflect existing implementation
behavior that departs from the definition in RFC 1510. The
transited-policy-checked(12) and the ok-as-delegate(13) ticket
flags were added. The disable-transited-check(26) KDC option was
added.
Descriptions of commonly implemented PA-DATA were added to section
5. The description of KRB-SAFE has been updated to note the
existing implementation behavior of double-encoding.
There were two definitions of METHOD-DATA in RFC 1510. The second
one, intended for use with KRB_AP_ERR_METHOD was removed leaving
the SEQUENCE OF PA-DATA definition.
Section 7, naming constraints, from RFC1510 was moved to section
6.
Words were added describing the convention that domain based realm
names for newly created realms should be specified as upper case.
This recommendation does not make lower case realm names illegal.
Words were added highlighting that the slash separated components
in the X500 style of realm names is consistent with existing
RFC1510 based implementations, but that it conflicts with the
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general recommendation of X.500 name representation specified in
RFC2253.
Section 8, network transport, constants and defined values, from
RFC1510 was moved to section 7. Since RFC1510, the definition of
the TCP transport for Kerberos messages was added, and the
encryption and checksum number assignments have been moved into a
separate document.
"Specification 2" in section 8 of the current document changes the
required encryption and checksum methods to bring them in line
with the best current practices and to deprecate methods that are
no longer considered sufficiently strong.
Two new sections, on IANA considerations and security
considerations were added.
The pseudo-code has been removed from the appendix. The pseudo-
code was sometimes misinterpreted to limit implementation choices
and in RFC 1510, it was not always consistent with the words in
the specification. Effort was made to clear up any ambiguities in
the specification, rather than to rely on the pseudo-code.
An appendix was added containing the complete ASN.1 module drawn
from the discussion in section 5 of the current document.
END NOTES
[TM] Project Athena, Athena, and Kerberos are trademarks of the
Massachusetts Institute of Technology (MIT). No commercial use of
these trademarks may be made without prior written permission of
MIT.
[1] Note, however, that many applications use Kerberos' functions
only upon the initiation of a stream-based network connection.
Unless an application subsequently provides integrity protection
for the data stream, the identity verification applies only to the
initiation of the connection, and does not guarantee that
subsequent messages on the connection originate from the same
principal.
[2] Secret and private are often used interchangeably in the
literature. In our usage, it takes two (or more) to share a
secret, thus a shared DES key is a secret key. Something is only
private when no one but its owner knows it. Thus, in public key
cryptosystems, one has a public and a private key.
[3] Of course, with appropriate permission the client could
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arrange registration of a separately-named principal in a remote
realm, and engage in normal exchanges with that realm's services.
However, for even small numbers of clients this becomes
cumbersome, and more automatic methods as described here are
necessary.
[4] Though it is permissible to request or issue tickets with no
network addresses specified.
[5] The password-changing request must not be honored unless the
requester can provide the old password (the user's current secret
key). Otherwise, it would be possible for someone to walk up to an
unattended session and change another user's password.
[6] To authenticate a user logging on to a local system, the
credentials obtained in the AS exchange may first be used in a TGS
exchange to obtain credentials for a local server. Those
credentials must then be verified by a local server through
successful completion of the Client/Server exchange.
[7] "Random" means that, among other things, it should be
impossible to guess the next session key based on knowledge of
past session keys. This can only be achieved in a pseudo-random
number generator if it is based on cryptographic principles. It is
more desirable to use a truly random number generator, such as one
based on measurements of random physical phenomena. See [RFC1750]
for an in depth discussion of randomness.
[8] Tickets contain both an encrypted and unencrypted portion, so
cleartext here refers to the entire unit, which can be copied from
one message and replayed in another without any cryptographic
skill.
[9] Note that this can make applications based on unreliable
transports difficult to code correctly. If the transport might
deliver duplicated messages, either a new authenticator must be
generated for each retry, or the application server must match
requests and replies and replay the first reply in response to a
detected duplicate.
[10] Note also that the rejection here is restricted to
authenticators from the same principal to the same server. Other
client principals communicating with the same server principal
should not be have their authenticators rejected if the time and
microsecond fields happen to match some other client's
authenticator.
[11] If this is not done, an attacker could subvert the
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Neuman, et al. draft-ietf-krb-wg-kerberos-clarifications-05.txt DRAFT
authentication by recording the ticket and authenticator sent over
the network to a server and replaying them following an event that
caused the server to lose track of recently seen authenticators.
[12] In the Kerberos version 4 protocol, the timestamp in the
reply was the client's timestamp plus one. This is not necessary
in version 5 because version 5 messages are formatted in such a
way that it is not possible to create the reply by judicious
message surgery (even in encrypted form) without knowledge of the
appropriate encryption keys.
[13] Note that for encrypting the KRB_AP_REP message, the sub-
session key is not used, even if present in the Authenticator.
[14] Implementations of the protocol may provide routines to
choose subkeys based on session keys and random numbers and to
generate a negotiated key to be returned in the KRB_AP_REP
message.
[15]This can be accomplished in several ways. It might be known
beforehand (since the realm is part of the principal identifier),
it might be stored in a nameserver, or it might be obtained from a
configuration file. If the realm to be used is obtained from a
nameserver, there is a danger of being spoofed if the nameservice
providing the realm name is not authenticated. This might result
in the use of a realm which has been compromised, and would result
in an attacker's ability to compromise the authentication of the
application server to the client.
[16] If the client selects a sub-session key, care must be taken
to ensure the randomness of the selected sub-session key. One
approach would be to generate a random number and XOR it with the
session key from the ticket-granting ticket.
February 2004 [Page 138]
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