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Kerberos Working Group L. Zhu
Internet-Draft Microsoft Corporation
Updates: 4120 (if approved) S. Hartman
Intended status: Standards Track Painless Security
Expires: January 15, 2009 July 14, 2008
A Generalized Framework for Kerberos Pre-Authentication
draft-ietf-krb-wg-preauth-framework-08
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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.
This Internet-Draft will expire on January 15, 2009.
Abstract
Kerberos is a protocol for verifying the identity of principals
(e.g., a workstation user or a network server) on an open network.
The Kerberos protocol provides a mechanism called pre-authentication
for proving the identity of a principal and for better protecting the
long-term secret of the principal.
This document describes a model for Kerberos pre-authentication
mechanisms. The model describes what state in the Kerberos request a
pre-authentication mechanism is likely to change. It also describes
how multiple pre-authentication mechanisms used in the same request
will interact.
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This document also provides common tools needed by multiple pre-
authentication mechanisms. One of these tools is a secure channel
between the client and the KDC with a reply key delivery mechanism;
this secure channel can be used to protect the authentication
exchange thus eliminate offline dictionary attacks. With these
tools, it is relatively straightforward to chain multiple
authentication mechanisms, utilize a different key management system,
or support a new key agreement algorithm.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminology Used in This Document . . . . . . 5
3. Model for Pre-Authentication . . . . . . . . . . . . . . . . . 5
3.1. Information Managed by the Pre-authentication Model . . . 6
3.2. Initial Pre-authentication Required Error . . . . . . . . 8
3.3. Client to KDC . . . . . . . . . . . . . . . . . . . . . . 9
3.4. KDC to Client . . . . . . . . . . . . . . . . . . . . . . 10
4. Pre-Authentication Facilities . . . . . . . . . . . . . . . . 10
4.1. Client-authentication Facility . . . . . . . . . . . . . . 12
4.2. Strengthening-reply-key Facility . . . . . . . . . . . . . 12
4.3. Replacing-reply-key Facility . . . . . . . . . . . . . . . 13
4.4. KDC-authentication Facility . . . . . . . . . . . . . . . 14
5. Requirements for Pre-Authentication Mechanisms . . . . . . . . 14
6. Tools for Use in Pre-Authentication Mechanisms . . . . . . . . 15
6.1. Combining Keys . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Protecting Requests/Responses . . . . . . . . . . . . . . 16
6.3. Managing States for the KDC . . . . . . . . . . . . . . . 17
6.4. Pre-authentication Set . . . . . . . . . . . . . . . . . . 19
6.5. Definition of Kerberos FAST Padata . . . . . . . . . . . . 22
6.5.1. FAST Armors . . . . . . . . . . . . . . . . . . . . . 23
6.5.2. FAST Request . . . . . . . . . . . . . . . . . . . . . 24
6.5.3. FAST Response . . . . . . . . . . . . . . . . . . . . 28
6.5.4. Authenticated Kerberos Error Messages using
Kerberos FAST . . . . . . . . . . . . . . . . . . . . 30
6.5.5. The Encrypted Challenge FAST Factor . . . . . . . . . 31
6.6. Authentication Strength Indication . . . . . . . . . . . . 32
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
10.1. Normative References . . . . . . . . . . . . . . . . . . . 34
10.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 35
A.1. Changes since 07 . . . . . . . . . . . . . . . . . . . . . 35
A.2. Changes since 06 . . . . . . . . . . . . . . . . . . . . . 35
Appendix B. ASN.1 module . . . . . . . . . . . . . . . . . . . . 35
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . . . 40
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1. Introduction
The core Kerberos specification [RFC4120] treats pre-authentication
data as an opaque typed hole in the messages to the KDC that may
influence the reply key used to encrypt the KDC reply. This
generality has been useful: pre-authentication data is used for a
variety of extensions to the protocol, many outside the expectations
of the initial designers. However, this generality makes designing
more common types of pre-authentication mechanisms difficult. Each
mechanism needs to specify how it interacts with other mechanisms.
Also, problems like combining a key with the long-term secret or
proving the identity of the user are common to multiple mechanisms.
Where there are generally well-accepted solutions to these problems,
it is desirable to standardize one of these solutions so mechanisms
can avoid duplication of work. In other cases, a modular approach to
these problems is appropriate. The modular approach will allow new
and better solutions to common pre-authentication problems to be used
by existing mechanisms as they are developed.
This document specifies a framework for Kerberos pre-authentication
mechanisms. It defines the common set of functions that pre-
authentication mechanisms perform as well as how these functions
affect the state of the request and reply. In addition several
common tools needed by pre-authentication mechanisms are provided.
Unlike [RFC3961], this framework is not complete--it does not
describe all the inputs and outputs for the pre-authentication
mechanisms. Pre-Authentication mechanism designers should try to be
consistent with this framework because doing so will make their
mechanisms easier to implement. Kerberos implementations are likely
to have plugin architectures for pre-authentication; such
architectures are likely to support mechanisms that follow this
framework plus commonly used extensions.
One of these common tools is the flexible authentication secure
tunneling (FAST) padata type. FAST provides a protected channel
between the client and the KDC, and it can optionally deliver a reply
key within the protected channel. Based on FAST, pre-authentication
mechanisms can extend Kerberos with ease, to support, for example,
password authenticated key exchange (PAKE) protocols with zero
knowledge password proof (ZKPP) [EKE] [IEEE1363.2]. Any pre-
authentication mechanism can be encapsulated in the FAST messages as
defined in Section 6.5. A pre-authentication type carried within
FAST is called a FAST factor. Creating a FAST factor is the easiest
path to create a new pre-authentication mechanism. FAST factors are
significantly easier to analyze from a security standpoint than other
pre-authentication mechanisms.
Mechanism designers should design FAST factors, instead of new pre-
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authentication mechanisms outside of FAST.
2. Conventions and Terminology Used in This Document
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 [RFC2119].
The word padata is used as a shorthand for pre-authentication data.
A conversation is the set of all authentication messages exchanged
between the client and the KDCs in order to authenticate the client
principal. A conversation as defined here consists of all messages
that are necessary to complete the authentication between the client
and the KDC.
Lastly, this document should be read only after reading the documents
describing the Kerberos cryptography framework [RFC3961] and the core
Kerberos protocol [RFC4120]. This document may freely use
terminology and notation from these documents without reference or
further explanation.
3. Model for Pre-Authentication
When a Kerberos client wishes to obtain a ticket using the
authentication server, it sends an initial Authentication Service
(AS) request. If pre-authentication is required but not being used,
then the KDC will respond with a KDC_ERR_PREAUTH_REQUIRED error.
Alternatively, if the client knows what pre-authentication to use, it
MAY optimize away a round-trip and send an initial request with
padata included in the initial request. If the client includes the
padata computed using the wrong pre-authentication mechanism or
incorrect keys, the KDC MAY return KDC_ERR_PREAUTH_FAILED with no
indication of what padata should have been included. In that case,
the client MUST retry with no padata and examine the error data of
the KDC_ERR_PREAUTH_REQUIRED error. If the KDC includes pre-
authentication information in the accompanying error data of
KDC_ERR_PREAUTH_FAILED, the client SHOULD process the error data, and
then retry.
The conventional KDC maintains no state between two requests;
subsequent requests may even be processed by a different KDC. On the
other hand, the client treats a series of exchanges with KDCs as a
single conversation. Each exchange accumulates state and hopefully
brings the client closer to a successful authentication.
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These models for state management are in apparent conflict. For many
of the simpler pre-authentication scenarios, the client uses one
round trip to find out what mechanisms the KDC supports. Then the
next request contains sufficient pre-authentication for the KDC to be
able to return a successful reply. For these simple scenarios, the
client only sends one request with pre-authentication data and so the
conversation is trivial. For more complex conversations, the KDC
needs to provide the client with a cookie to include in future
requests to capture the current state of the authentication session.
Handling of multiple round-trip mechanisms is discussed in
Section 6.3.
This framework specifies the behavior of Kerberos pre-authentication
mechanisms used to identify users or to modify the reply key used to
encrypt the KDC reply. The PA-DATA typed hole may be used to carry
extensions to Kerberos that have nothing to do with proving the
identity of the user or establishing a reply key. Such extensions
are outside the scope of this framework. However mechanisms that do
accomplish these goals should follow this framework.
This framework specifies the minimum state that a Kerberos
implementation needs to maintain while handling a request in order to
process pre-authentication. It also specifies how Kerberos
implementations process the padata at each step of the AS request
process.
3.1. Information Managed by the Pre-authentication Model
The following information is maintained by the client and KDC as each
request is being processed:
o The reply key used to encrypt the KDC reply
o How strongly the identity of the client has been authenticated
o Whether the reply key has been used in this conversation
o Whether the reply key has been replaced in this conversation
o Whether the contents of the KDC reply can be verified by the
client principal
Conceptually, the reply key is initially the long-term key of the
principal. However, principals can have multiple long-term keys
because of support for multiple encryption types, salts and
string2key parameters. As described in Section 5.2.7.5 of the
Kerberos protocol [RFC4120], the KDC sends PA-ETYPE-INFO2 to notify
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the client what types of keys are available. Thus in full
generality, the reply key in the pre-authentication model is actually
a set of keys. At the beginning of a request, it is initialized to
the set of long-term keys advertised in the PA-ETYPE-INFO2 element on
the KDC. If multiple reply keys are available, the client chooses
which one to use. Thus the client does not need to treat the reply
key as a set. At the beginning of a request, the client picks a
reply key to use.
KDC implementations MAY choose to offer only one key in the PA-ETYPE-
INFO2 element. Since the KDC already knows the client's list of
supported enctypes from the request, no interoperability problems are
created by choosing a single possible reply key. This way, the KDC
implementation avoids the complexity of treating the reply key as a
set.
When the padata in the request is verified by the KDC, then the
client is known to have that key, therefore the KDC SHOULD pick the
same key as the reply key.
At the beginning of handling a message on both the client and the
KDC, the client's identity is not authenticated. A mechanism may
indicate that it has successfully authenticated the client's
identity. This information is useful to keep track of on the client
in order to know what pre-authentication mechanisms should be used.
The KDC needs to keep track of whether the client is authenticated
because the primary purpose of pre-authentication is to authenticate
the client identity before issuing a ticket. The handling of
authentication strength using various authentication mechanisms is
discussed in Section 6.6.
Initially the reply key has not been used. A pre-authentication
mechanism that uses the reply key to encrypt or checksum some data in
the generation of new keys MUST indicate that the reply key is used.
This state is maintained by the client and the KDC to enforce the
security requirement stated in Section 4.3 that the reply key cannot
be replaced after it is used.
Initially the reply key has not been replaced. If a mechanism
implements the Replace Reply Key facility discussed in Section 4.3,
then the state MUST be updated to indicate that the reply key has
been replaced. Once the reply key has been replaced, knowledge of
the reply key is insufficient to authenticate the client. The reply
key is marked replaced in exactly the same situations as the KDC
reply is marked as not being verified to the client principal.
However, while mechanisms can verify the KDC reply to the client,
once the reply key is replaced, then the reply key remains replaced
for the remainder of the conversation.
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Without pre-authentication, the client knows that the KDC reply is
authentic and has not been modified because it is encrypted in a
long-term key of the client. Only the KDC and the client know that
key. So at the start of a conversation, the KDC reply is presumed to
be verified using the client principal's long-term key. Any pre-
authentication mechanism that sets a new reply key not based on the
principal's long-term secret MUST either verify the KDC reply some
other way or indicate that the reply is not verified. If a mechanism
indicates that the reply is not verified then the client
implementation MUST return an error unless a subsequent mechanism
verifies the reply. The KDC needs to track this state so it can
avoid generating a reply that is not verified.
The typical Kerberos request does not provide a way for the client
machine to know that it is talking to the correct KDC. Someone who
can inject packets into the network between the client machine and
the KDC and who knows the password that the user will give to the
client machine can generate a KDC reply that will decrypt properly.
So, if the client machine needs to authenticate that the user is in
fact the named principal, then the client machine needs to do a TGS
request for itself as a service. Some pre-authentication mechanisms
may provide a way for the client to authenticate the KDC. Examples
of this include signing the reply that can be verified using a well-
known public key or providing a ticket for the client machine as a
service.
3.2. Initial Pre-authentication Required Error
Typically a client starts a conversation by sending an initial
request with no pre-authentication. If the KDC requires pre-
authentication, then it returns a KDC_ERR_PREAUTH_REQUIRED message.
After the first reply with the KDC_ERR_PREAUTH_REQUIRED error code,
the KDC returns the error code KDC_ERR_MORE_PREAUTH_DATA_NEEDED
(defined in Section 6.3) for pre-authentication configurations that
use multi-round-trip mechanisms; see Section 3.4 for details of that
case.
The KDC needs to choose which mechanisms to offer the client. The
client needs to be able to choose what mechanisms to use from the
first message. For example consider the KDC that will accept
mechanism A followed by mechanism B or alternatively the single
mechanism C. A client that supports A and C needs to know that it
should not bother trying A.
Mechanisms can either be sufficient on their own or can be part of an
authentication set--a group of mechanisms that all need to
successfully complete in order to authenticate a client. Some
mechanisms may only be useful in authentication sets; others may be
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useful alone or in authentication sets. For the second group of
mechanisms, KDC policy dictates whether the mechanism will be part of
an authentication set or offered alone. For each mechanism that is
offered alone, the KDC includes the pre-authentication type ID of the
mechanism in the padata sequence returned in the
KDC_ERR_PREAUTH_REQUIRED error.
The KDC SHOULD NOT send data that is encrypted in the long-term
password-based key of the principal. Doing so has the same security
exposures as the Kerberos protocol without pre-authentication. There
are few situations where pre-authentication is desirable and where
the KDC needs to expose cipher text encrypted in a weak key before
the client has proven knowledge of that key.
3.3. Client to KDC
This description assumes that a client has already received a
KDC_ERR_PREAUTH_REQUIRED from the KDC. If the client performs
optimistic pre-authentication then the client needs to optimistically
guess values for the information it would normally receive from that
error response.
The client starts by initializing the pre-authentication state as
specified. It then processes the padata in the
KDC_ERR_PREAUTH_REQUIRED.
When processing the response to the KDC_ERR_PREAUTH_REQUIRED, the
client MAY ignore any padata it chooses unless doing so violates a
specification to which the client conforms. Clients conforming to
this specification MUST NOT ignore the padata defined in Section 6.3.
Clients SHOULD process padata unrelated to this framework or other
means of authenticating the user. Clients SHOULD choose one
authentication set or mechanism that could lead to authenticating the
user and ignore the rest. Since the list of mechanisms offered by
the KDC is in the decreasing preference order, clients typically
choose the first mechanism or authentication set that the client can
usefully perform. If a client chooses to ignore a padata it MUST NOT
process the padata, allow the padata to affect the pre-authentication
state, nor respond to the padata.
For each padata the client chooses to process, the client processes
the padata and modifies the pre-authentication state as required by
that mechanism. Padata are processed in the order received from the
KDC.
After processing the padata in the KDC error, the client generates a
new request. It processes the pre-authentication mechanisms in the
order in which they will appear in the next request, updating the
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state as appropriate. The request is sent when it is complete.
3.4. KDC to Client
When a KDC receives an AS request from a client, it needs to
determine whether it will respond with an error or an AS reply.
There are many causes for an error to be generated that have nothing
to do with pre-authentication; they are discussed in the core
Kerberos specification.
From the standpoint of evaluating the pre-authentication, the KDC
first starts by initializing the pre-authentication state. It then
processes the padata in the request. As mentioned in Section 3.3,
the KDC MAY ignore padata that is inappropriate for the configuration
and MUST ignore padata of an unknown type. The KDC MUST NOT ignore
padata of types used in previous messages. For example, if a KDC
issues a KDC_ERR_PREAUTH_REQUIRED error including padata of type x,
then the KDC cannot ignore padata of type x received in an AS-REQ
message from the client.
At this point the KDC decides whether it will issue an error or a
reply. Typically a KDC will issue a reply if the client's identity
has been authenticated to a sufficient degree.
In the case of a KDC_ERR_MORE_PREAUTH_DATA_NEEDED error, the KDC
first starts by initializing the pre-authentication state. Then it
processes any padata in the client's request in the order provided by
the client. Mechanisms that are not understood by the KDC are
ignored. Next, it generates padata for the error response, modifying
the pre-authentication state appropriately as each mechanism is
processed. The KDC chooses the order in which it will generate
padata (and thus the order of padata in the response), but it needs
to modify the pre-authentication state consistently with the choice
of order. For example, if some mechanism establishes an
authenticated client identity, then the subsequent mechanisms in the
generated response receive this state as input. After the padata is
generated, the error response is sent. Typically the errors with the
code KDC_ERR_MORE_PREAUTH_DATA_NEEDED in a converstation will include
KDC state as discussed in Section 6.3.
To generate a final reply, the KDC generates the padata modifying the
pre-authentication state as necessary. Then it generates the final
response, encrypting it in the current pre-authentication reply key.
4. Pre-Authentication Facilities
Pre-Authentication mechanisms can be thought of as providing various
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conceptual facilities. This serves two useful purposes. First,
mechanism authors can choose only to solve one specific small
problem. It is often useful for a mechanism designed to offer key
management not to directly provide client authentication but instead
to allow one or more other mechanisms to handle this need. Secondly,
thinking about the abstract services that a mechanism provides yields
a minimum set of security requirements that all mechanisms providing
that facility must meet. These security requirements are not
complete; mechanisms will have additional security requirements based
on the specific protocol they employ.
A mechanism is not constrained to only offering one of these
facilities. While such mechanisms can be designed and are sometimes
useful, many pre-authentication mechanisms implement several
facilities. By combining multiple facilities in a single mechanism,
it is often easier to construct a secure, simple solution than by
solving the problem in full generality. Even when mechanisms provide
multiple facilities, they need to meet the security requirements for
all the facilities they provide. If the FAST factor approach is
used, it is likely that one or a small number of facilities can be
provided by a single mechanism without complicating the security
analysis.
According to Kerberos extensibility rules (Section 1.5 of the
Kerberos specification [RFC4120]), an extension MUST NOT change the
semantics of a message unless a recipient is known to understand that
extension. Because a client does not know that the KDC supports a
particular pre-authentication mechanism when it sends an initial
request, a pre-authentication mechanism MUST NOT change the semantics
of the request in a way that will break a KDC that does not
understand that mechanism. Similarly, KDCs MUST NOT send messages to
clients that affect the core semantics unless the client has
indicated support for the message.
The only state in this model that would break the interpretation of a
message is changing the expected reply key. If one mechanism changed
the reply key and a later mechanism used that reply key, then a KDC
that interpreted the second mechanism but not the first would fail to
interpret the request correctly. In order to avoid this problem,
extensions that change core semantics are typically divided into two
parts. The first part proposes a change to the core semantic--for
example proposes a new reply key. The second part acknowledges that
the extension is understood and that the change takes effect.
Section 4.2 discusses how to design mechanisms that modify the reply
key to be split into a proposal and acceptance without requiring
additional round trips to use the new reply key in subsequent pre-
authentication. Other changes in the state described in Section 3.1
can safely be ignored by a KDC that does not understand a mechanism.
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Mechanisms that modify the behavior of the request outside the scope
of this framework need to carefully consider the Kerberos
extensibility rules to avoid similar problems.
4.1. Client-authentication Facility
The client authentication facility proves the identity of a user to
the KDC before a ticket is issued. Examples of mechanisms
implementing this facility include the encrypted timestamp facility
defined in Section 5.2.7.2 of the Kerberos specification [RFC4120].
Mechanisms that provide this facility are expected to mark the client
as authenticated.
Mechanisms implementing this facility SHOULD require the client to
prove knowledge of the reply key before transmitting a successful KDC
reply. Otherwise, an attacker can intercept the pre-authentication
exchange and get a reply to attack. One way of proving the client
knows the reply key is to implement the Replace Reply Key facility
along with this facility. The PKINIT mechanism [RFC4556] implements
Client Authentication alongside Replace Reply Key.
If the reply key has been replaced, then mechanisms such as
encrypted-timestamp that rely on knowledge of the reply key to
authenticate the client MUST NOT be used.
4.2. Strengthening-reply-key Facility
Particularly, when dealing with keys based on passwords, it is
desirable to increase the strength of the key by adding additional
secrets to it. Examples of sources of additional secrets include the
results of a Diffie-Hellman key exchange or key bits from the output
of a smart card [KRB-WG.SAM]. Typically these additional secrets can
be first combined with the existing reply key and then converted to a
protocol key using tools defined in Section 6.1.
Typically a mechanism implementing this facility will know that the
other side of the exchange supports the facility before the reply key
is changed. For example, a mechanism might need to learn the
certificate for a KDC before encrypting a new key in the public key
belonging to that certificate. However, if a mechanism implementing
this facility wishes to modify the reply key before knowing that the
other party in the exchange supports the mechanism, it proposes
modifying the reply key. The other party then includes a message
indicating that the proposal is accepted if it is understood and
meets policy. In many cases it is desirable to use the new reply key
for client authentication and for other facilities. Waiting for the
other party to accept the proposal and actually modify the reply key
state would add an additional round trip to the exchange. Instead,
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mechanism designers are encouraged to include a typed hole for
additional padata in the message that proposes the reply key change.
The padata included in the typed hole are generated assuming the new
reply key. If the other party accepts the proposal, then these
padata are considered as an inner level. As with the outer level,
one authentication set or mechanism is typically chosen for client
authentication, along with auxiliary mechanisms such as KDC cookies,
and other mechanisms are ignored. When mechanisms include such a
container, the hint provided for use in authentication sets MUST
contain a sequence of inner mechanisms along with hints for those
mechanisms. The party generating the proposal can determine whether
the padata were processed based on whether the proposal for the reply
key is accepted.
The specific formats of the proposal message, including where padata
are included is a matter for the mechanism specification. Similarly,
the format of the message accepting the proposal is mechanism-
specific.
Mechanisms implementing this facility and including a typed hole for
additional padata MUST checksum that padata using a keyed checksum or
encrypt the padata. This requirement protects against modification
of the contents of the typed hole. By modifying these contents an
attacker might be able to choose which mechanism is used to
authenticate the client, or to convince a party to provide text
encrypted in a key that the attacker had manipulated. It is
important that mechanisms strengthen the reply key enough that using
it to checksum padata is appropriate.
4.3. Replacing-reply-key Facility
The Replace Reply Key facility replaces the key in which a successful
AS reply will be encrypted. This facility can only be used in cases
where knowledge of the reply key is not used to authenticate the
client. The new reply key MUST be communicated to the client and the
KDC in a secure manner. Mechanisms implementing this facility MUST
mark the reply key as replaced in the pre-authentication state.
Mechanisms implementing this facility MUST either provide a mechanism
to verify the KDC reply to the client or mark the reply as unverified
in the pre-authentication state. Mechanisms implementing this
facility SHOULD NOT be used if a previous mechanism has used the
reply key.
As with the strengthening-reply-key facility, Kerberos extensibility
rules require that the reply key not be changed unless both sides of
the exchange understand the extension. In the case of this facility
it will likely be the case for both sides to know that the facility
is available by the time that the new key is available to be used.
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However, mechanism designers can use a container for padata in a
proposal message as discussed in Section 4.2 if appropriate.
4.4. KDC-authentication Facility
This facility verifies that the reply comes from the expected KDC.
In traditional Kerberos, the KDC and the client share a key, so if
the KDC reply can be decrypted then the client knows that a trusted
KDC responded. Note that the client machine cannot trust the client
unless the machine is presented with a service ticket for it
(typically the machine can retrieve this ticket by itself). However,
if the reply key is replaced, some mechanism is required to verify
the KDC. Pre-authentication mechanisms providing this facility allow
a client to determine that the expected KDC has responded even after
the reply key is replaced. They mark the pre-authentication state as
having been verified.
5. Requirements for Pre-Authentication Mechanisms
This section lists requirements for specifications of pre-
authentication mechanisms.
For each message in the pre-authentication mechanism, the
specification describes the pa-type value to be used and the contents
of the message. The processing of the message by the sender and
recipient is also specified. This specification needs to include all
modifications to the pre-authentication state.
Generally mechanisms have a message that can be sent in the error
data of the KDC_ERR_PREAUTH_REQUIRED error message or in an
authentication set. If the client needs information such as trusted
certificate authorities in order to determine if it can use the
mechanism, then this information should be in that message. In
addition, such mechanisms should also define a pa-hint to be included
in authentication sets. Often, the same information included in the
padata-value is appropriate to include in the pa-hint (as defined in
Section 6.4).
In order to ease security analysis the mechanism specification should
describe what facilities from this document are offered by the
mechanism. For each facility, the security consideration section of
the mechanism specification should show that the security
requirements of that facility are met. This requirement is
applicable to any FAST factor that provides authentication
information.
Significant problems have resulted in the specification of Kerberos
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protocols because much of the KDC exchange is not protected against
authentication. The security considerations section should discuss
unauthenticated plaintext attacks. It should either show that
plaintext is protected or discuss what harm an attacker could do by
modifying the plaintext. It is generally acceptable for an attacker
to be able to cause the protocol negotiation to fail by modifying
plaintext. More significant attacks should be evaluated carefully.
As discussed in Section 6.3, there is no guarantee that a client will
use the same KDCs for all messages in a conversation. The mechanism
specification needs to show why the mechanism is secure in this
situation. The hardest problem to deal with, especially for
challenge/response mechanisms is to make sure that the same response
cannot be replayed against two KDCs while allowing the client to talk
to any KDC.
6. Tools for Use in Pre-Authentication Mechanisms
This section describes common tools needed by multiple pre-
authentication mechanisms. By using these tools mechanism designers
can use a modular approach to specify mechanism details and ease
security analysis.
6.1. Combining Keys
Frequently a weak key needs to be combined with a stronger key before
use. For example, passwords are typically limited in size and
insufficiently random, therefore it is desirable to increase the
strength of the keys based on passwords by adding additional secrets.
Additional source of secrecy may come from hardware tokens.
This section provides standard ways to combine two keys into one.
KRB-FX-CF1() is defined to combine two pass-phrases.
KRB-FX-CF1(UTF-8 string, UTF-8 string) -> (UTF-8 string)
KRB-FX-CF1(x, y) -> x || y
Where || denotes concatenation. The strength of the final key is
roughly the total strength of the individual keys being combined
assuming that the string_to_key() function [RFC3961] uses all its
input evenly.
An example usage of KRB-FX-CF1() is when a device provides random but
short passwords, the password is often combined with a personal
identification number (PIN). The password and the PIN can be
combined using KRB-FX-CF1().
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KRB-FX-CF2() combines two protocol keys based on the pseudo-random()
function defined in [RFC3961].
Given two input keys, K1 and K2, where K1 and K2 can be of two
different enctypes, the output key of KRB-FX-CF2(), K3, is derived as
follows:
KRB-FX-CF2(protocol key, protocol key, octet string,
octet string) -> (protocol key)
PRF+(K1, pepper1) -> octet-string-1
PRF+(K2, pepper2) -> octet-string-2
KRB-FX-CF2(K1, K2, pepper1, pepper2) ->
random-to-key(octet-string-1 ^ octet-string-2)
Where ^ denotes the exclusive-OR operation. PRF+() is defined as
follows:
PRF+(protocol key, octet string) -> (octet string)
PRF+(key, shared-info) -> pseudo-random( key, 1 || shared-info ) ||
pseudo-random( key, 2 || shared-info ) ||
pseudo-random( key, 3 || shared-info ) || ...
Here the counter value 1, 2, 3 and so on are encoded as a one-octet
integer. The pseudo-random() operation is specified by the enctype
of the protocol key. PRF+() uses the counter to generate enough bits
as needed by the random-to-key() [RFC3961] function for the
encryption type specified for the resulting key; unneeded bits are
removed from the tail.
Mechanism designers MUST specify the values for the input parameter
pepper1 and pepper2 when combining two keys using KRB-FX-CF2(). The
pepper1 and pepper2 MUST be distinct so that if the two keys being
combined are the same, the resulting key is not a trivial key.
6.2. Protecting Requests/Responses
Mechanism designers SHOULD protect clear text portions of pre-
authentication data. Various denial of service attacks and downgrade
attacks against Kerberos are possible unless plaintexts are somehow
protected against modification. An early design goal of Kerberos
Version 5 [RFC4120] was to avoid encrypting more of the
authentication exchange that was required. (Version 4 doubly-
encrypted the encrypted part of a ticket in a KDC reply, for
example.) This minimization of encryption reduces the load on the
KDC and busy servers. Also, during the initial design of Version 5,
the existence of legal restrictions on the export of cryptography
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made it desirable to minimize of the number of uses of encryption in
the protocol. Unfortunately, performing this minimization created
numerous instances of unauthenticated security-relevant plaintext
fields.
If there is more than one roundtrip for an authentication exchange,
mechanism designers need to allow either the client or the KDC to
provide a checksum of all the messages exchanged on the wire in the
conversation, and the checksum is then verified by the receiver.
New mechanisms MUST NOT be hard-wired to use a specific algorithm.
Primitives defined in [RFC3961] are RECOMMENDED for integrity
protection and confidentiality. Mechanisms based on these primitives
are crypto-agile as the result of using [RFC3961] along with
[RFC4120]. The advantage afforded by crypto-agility is the ability
to avoid a multi-year standardization and deployment cycle to fix a
problem that is specific to a particular algorithm, when real attacks
do arise against that algorithm.
Note that data used by FAST factors (defined in Section 6.5) is
encrypted in a protected channel, thus they do not share the un-
authenticated-text issues with mechanisms designed as full-blown pre-
authentication mechanisms.
6.3. Managing States for the KDC
Kerberos KDCs are stateless. There is no requirement that clients
will choose the same KDC for the second request in a conversation.
Proxies or other intermediate nodes may also influence KDC selection.
So, each request from a client to a KDC must include sufficient
information that the KDC can regenerate any needed state. This is
accomplished by giving the client a potentially long opaque cookie in
responses to include in future requests in the same conversation.
The KDC MAY respond that a conversation is too old and needs to
restart by responding with a KDC_ERR_PREAUTH_EXPIRED error.
KDC_ERR_PREAUTH_EXPIRED TBA
When a client receives this error, the client SHOULD abort the
existing conversation, and restart a new one.
An example, where more than one message from the client is needed, is
when the client is authenticated based on a challenge-response
scheme. In that case, the KDC needs to keep track of the challenge
issued for a client authentication request.
The PA-FX-COOKIE pdata type is defined in this section to facilitate
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state management. This padata is sent by the KDC when the KDC
requires state for a future transaction. The client includes this
opaque token in the next message in the conversation. The token may
be relatively large; clients MUST be prepared for tokens somewhat
larger than the size of all messages in a conversation.
PA_FX_COOKIE TBA
-- Stateless cookie that is not tied to a specific KDC.
The corresponding padata-value field [RFC4120] contains the
Distinguished Encoding Rules (DER) [X60] [X690] encoding of the
following Abstract Syntax Notation One (ASN.1) type PA-FX-COOKIE:
PA-FX-COOKIE ::= SEQUENCE {
conversationId [0] OCTET STRING,
-- Contains the identifier of this conversation. This field
-- must contain the same value for all the messages
-- within the same conversation.
enc-binding-key [1] EncryptedData OPTIONAL,
-- EncryptionKey --
-- This field is present when and only when a FAST
-- padata as defined in Section 6.5 is included.
-- The encrypted data, when decrypted, contains an
-- EncryptionKey structure.
-- This encryption key is encrypted using the armor key
-- (defined in Section 6.5.1), and the key usage for the
-- encryption is KEY_USAGE_FAST_BINDING_KEY.
-- Present only once in a converstation.
cookie [2] OCTET STRING OPTIONAL,
-- Opaque data, for use to associate all the messages in
-- a single conversation between the client and the KDC.
-- This is generated by the KDC and the client MUST copy
-- the exact cookie encapsulated in a PA_FX_COOKIE data
-- element into the next message of the same conversation.
...
}
KEY_USAGE_FAST_BINDING_KEY TBA
The conversationId field contains a sufficiently-long rand number
that uniquely identifies the conversation. If a PA_FX_COOKIE padata
is present in one message, a PA_FX_COOKIE structure MUST be present
in all subsequent messages of the same converstation between the
client and the KDC, with the same conversationId value.
The enc-binding-key field is present when and only when a FAST padata
(defined in Section 6.5) is included. The enc-binding-key field is
present only once in a conversation. It MUST be ignored if it is
present in a subsequent message of the same conversation. The
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encrypted data, when decrypted, contains an EncryptionKey structure
that is called the binding key. The binding key is encrypted using
the armor key (defined in Section 6.5.1), and the key usage for the
encryption is KEY_USAGE_FAST_BINDING_KEY.
If a Kerberos FAST padata as defined in Section 6.5 is included in
one message, it MUST be included in all subsequent messages of the
same conversation.
When FAST padata as defined Section 6.5 is included, the PA-FX-COOKIE
padata MUST be included.
The cookie token is generated by the KDC and the client MUST copy the
exact cookie encapsulated in a PA_FX_COOKIE data element into the
next message of the same conversation. The content of the cookie
field is a local matter of the KDC. However the KDC MUST construct
the cookie token in such a manner that a malicious client cannot
subvert the authentication process by manipulating the token. The
KDC implementation needs to consider expiration of tokens, key
rollover and other security issues in token design. The content of
the cookie field is likely specific to the pre-authentication
mechanisms used to authenticate the client. If a client
authentication response can be replayed to multiple KDCs via the
PA_FX_COOKIE mechanism, an expiration in the cookie is RECOMMENDED to
prevent the response being presented indefinitely.
If at least one more message for a mechanism or a mechanism set is
expected by the KDC, the KDC returns a
KDC_ERR_MORE_PREAUTH_DATA_NEEDED error with a PA_FX_COOKIE to
identify the conversation with the client according to Section 6.5.4.
KDC_ERR_MORE_PREAUTH_DATA_NEEDED TBA
6.4. Pre-authentication Set
If all mechanisms in a group need to successfully complete in order
to authenticate a client, the client and the KDC SHOULD use the
PA_AUTHENTICATION_SET padata element.
A PA_AUTHENTICATION_SET padata element contains the ASN.1 DER
encoding of the PA-AUTHENTICATION-SET structure:
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PA-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM
PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE {
pa-type [0] Int32,
-- same as padata-type.
pa-hint [1] OCTET STRING OPTIONAL,
pa-value [2] OCTET STRING OPTIONAL,
...
}
The pa-type field of the PA-AUTHENTICATION-SET-ELEM structure
contains the corresponding value of padata-type in PA-DATA [RFC4120].
Associated with the pa-type is a pa-hint, which is an octet-string
specified by the pre-authentication mechanism. This hint may provide
information for the client which helps it determine whether the
mechanism can be used. For example a public-key mechanism might
include the certificate authorities it trusts in the hint info. Most
mechanisms today do not specify hint info; if a mechanism does not
specify hint info the KDC MUST NOT send a hint for that mechanism.
To allow future revisions of mechanism specifications to add hint
info, clients MUST ignore hint info received for mechanisms that the
client believes do not support hint info. The pa-value element of
the PA-AUTHENTICATION-SET-ELEM sequence is included to carry the
first padata-value from the KDC to the client. If the client chooses
this authentication set then the client MUST process this pa-value.
The pa-value element MUST be absent for all but the first entry in
the authentication set. Clients MUST ignore pa-value for the second
and following entries in the authentication set.
If the client chooses an authentication set, then its AS-REQ message
MUST contain a PA_AUTHENTICATION_SET_SELECTED padata element. This
element contains the encoding of the PA-AUTHENTICATION-SET sequence
received from the KDC corresponding to the authentication set that is
chosen. The client MUST use the same octet values received from the
KDC; it cannot re-encode the sequence. This allows KDCs to use bit-
wise comparison to identify the selected authentication set. The
PA_AUTHENTICATION_SET_SELECTED padata element MUST come before any
padata elements from the authentication set in the padata sequence in
the AS-REQ message. The client MAY cache authentication sets from
prior messages and use them to construct an optimistic initial AS-
REQ. If the KDC receives a PA_AUTHENTICATION_SET_SELECTED padata
element that does not correspond to an authentication set that it
would offer, then the KDC returns the
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET error. The edata in this
error contains a sequence of padata just as for the
KDC_ERR_PREAUTH_REQUIRED error.
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PA_AUTHENTICATION_SET_SELECTED TBA
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET TBA
The PA-AUTHENTICATION-SET appears only in the first message from the
KDC to the client. In particular, the client MAY fail if the
authentication mechanism sets change as the conversation progresses.
Clients MAY assume that the hints provided in the authentication set
contain enough information that the client knows what user interface
elements need to be displayed during the entire authentication
conversation. Exceptional circumstances such as expired passwords or
expired accounts may require that additional user interface be
displayed. Mechanism designers need to carefully consider the design
of their hints so that the client has this information. This way,
clients can construct necessary dialogue boxes or wizards based on
the authentication set and can present a coherent user interface.
Current standards for user interface do not provide an acceptable
experience when the client has to ask additional questions later in
the conversation.
When indicating which sets of pre-authentication mechanisms are
supported, the KDC includes a PA-AUTHENTICATION-SET padata element
for each pre-authentication mechanism set.
The client sends the padata-value for the first mechanism it picks in
the pre-authentication set, when the first mechanism completes, the
client and the KDC will proceed with the second mechanism, and so on
until all mechanisms complete successfully. The PA_FX_COOKIE as
defined in Section 6.3 MUST be sent by the KDC along with the first
message that contains a PA-AUTHENTICATION-SET, in order to keep track
of KDC states.
Before the authentication succeeds and a ticket is returned, the
message that the client sends is an AS_REQ and the message that the
KDC sends is a KRB-ERROR message. The error code in the KRB-ERROR
message from the KDC is KDC_ERR_MORE_PREAUTH_DATA_NEEDED as defined
in Section 6.3 and the accompanying e-data contains the DER encoding
of ASN.1 type METHOD-DATA. The KDC includes the padata elements in
the METHOD-DATA. If there is no padata, the e-data field is absent
in the KRB-ERROR message.
If the client sends the last message for a given mechanism, then the
KDC sends the first message for the next mechanism. If the next
mechanism does not start with a KDC-side challenge, then the KDC
includes a padata item with the appropriate pa-type and an empty pa-
data.
If the KDC sends the last message for a particular mechanism, the KDC
also includes the first padata for the next mechanism.
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6.5. Definition of Kerberos FAST Padata
As described in [RFC4120], Kerberos is vulnerable to offline
dictionary attacks. An attacker can request an AS-REP and try
various passwords to see if they can decrypt the resulting ticket.
RFC 4120 provides the entrypted timestap pre-authentication method
that ameliorates the situation somewhat by requiring that an attacker
observe a successful authentication. However stronger security is
desired in many environments. The Kerberos FAST pre-authentication
padata defined in this section provides a tool to significantly
reduce vulnerability to offline dictionary attack. When combined
with encrypted timestamp, FAST requires an attacker to mount a
successful man-in-the-middle attack to observe ciphertext. When
combined with host keys, FAST can even protect against active
attacks. FAST also provides solutions to common problems for pre-
authentication mechanisms such as binding of the request and the
reply, freshness guarantee of the authentication. FAST itself,
however, does not authenticate the client or the KDC, instead, it
provides a typed hole to allow pre-authentication data be tunneled.
A pre-authentication data element used within FAST is called a FAST
factor. A FAST factor captures the minimal work required for
extending Kerberos to support a new pre-authentication scheme.
A FAST factor MUST NOT be used outside of FAST unless its
specification explicitly allows so. The typed holes in FAST messages
can also be used as generic holes for other padata that are not
intended to prove the client's identity, or establish the reply key.
New pre-authentication mechanisms SHOULD be designed as FAST factors,
instead of full-blown pre-authentication mechanisms.
FAST factors that are pre-authentication mechanisms MUST meet the
requirements in Section 5.
FAST employs an armoring scheme. The armor can be a Ticket Granting
Ticket (TGT) obtained by the client's machine using the host keys to
pre-authenticate with the KDC, or an anonymous TGT obtained based on
anonymous PKINIT [KRB-ANON] [RFC4556].
The rest of this section describes the types of armors and the syntax
of the messages used by FAST. Conforming implementations MUST
support Kerberos FAST padata.
Any FAST armor scheme MUST provide a fresh armor key for each
conversation. Clients and KDCs can assume that if a message is
encrypted and integrity protected with a given armor key then it is
part of the conversation using that armor key.
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6.5.1. FAST Armors
An armor key is used to encrypt pre-authentication data in the FAST
request and the response. The KrbFastArmor structure is defined to
identify the armor key. This structure contains the following two
fields: the armor-type identifies the type of armors, and the armor-
value as an OCTET STRING contains the description of the armor scheme
and the armor key.
KrbFastArmor ::= SEQUENCE {
armor-type [0] Int32,
-- Type of the armor.
armor-value [1] OCTET STRING,
-- Value of the armor.
...
}
The value of the armor key is a matter of the armor type
specification. Only one armor type is defined in this document.
FX_FAST_ARMOR_AP_REQUEST TBA
The FX_FAST_ARMOR_AP_REQUEST armor is based on Kerberos tickets.
Conforming implementations MUST implement the
FX_FAST_ARMOR_AP_REQUEST armor type.
6.5.1.1. Ticket-based Armors
This is a ticket-based armoring scheme. The armor-type is
FX_FAST_ARMOR_AP_REQUEST, the armor-value contains an ASN.1 DER
encoded AP-REQ. The ticket in the AP-REQ is called an armor ticket
or an armor TGT. The subkey field in the AP-REQ MUST be present.
The armor key is the subkey in the AP-REQ authenticator.
The server name field of the armor ticket MUST identify the TGS of
the target realm. Here are three ways in the decreasing preference
order how an armor TGT SHOULD be obtained:
1. If the client is authenticating from a host machine whose
Kerberos realm has a trust path to the client's realm, the host
machine obtains a TGT by pre-authenticating intitialy the realm
of the host machine using the host keys. If the client's realm
is different than the realm of the local host, the machine then
obtains a cross-realm TGT to the client's realm as the armor
ticket. Otherwise, the host's primary TGT is the armor ticket.
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2. If the client's host machine cannot obtain a host ticket strictly
based on RFC4120, but the KDC has an asymmetric signing key that
the client can verify the binding between the public key of the
signing key and the expected KDC, the client can use anonymous
PKINIT [KRB-ANON] [RFC4556] to authenticate the KDC and obtain an
anonymous TGT as the armor ticket. The armor key can be a cross-
team TGT obtained based on the initial primary TGT obtained using
anonymous PKINIT with KDC authentication.
3. Otherwise, the client uses anonymous PKINIT to get an anonymous
TGT without KDC authentication and that TGT is the armor ticket.
Note that this mode of operation is vulnerable to man-in-the-
middle attacks at the time of obtaining the initial anonymous
armor TGT. The armor key can be a cross-team TGT obtained based
on the initial primary TGT obtained using anonymous PKINIT
without KDC authentication.
Because the KDC does not know if the client is able to trust the
ticket it has, the KDC MUST initialize the pre-authentication state
to an unverified KDC.
6.5.2. FAST Request
A padata type PA_FX_FAST is defined for the Kerberos FAST pre-
authentication padata. The corresponding padata-value field
[RFC4120] contains the DER encoding of the ASN.1 type PA-FX-FAST-
REQUEST.
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PA_FX_FAST TBA
-- Padata type for Kerberos FAST
PA-FX-FAST-REQUEST ::= CHOICE {
armored-data [0] KrbFastArmoredReq,
...
}
KrbFastArmoredReq ::= SEQUENCE {
armor [0] KrbFastArmor OPTIONAL,
-- Contains the armor that identifies the armor key.
-- MUST be present in AS-REQ.
-- MUST be absent in TGS-REQ.
req-checksum [1] Checksum,
-- Checksum performed over the type KDC-REQ-BODY for
-- the req-body field of the KDC-REQ structure defined in
-- [RFC4120]
-- The checksum key is the armor key, the checksum
-- type is the required checksum type for the enctype of
-- the armor key, and the key usage number is
-- KEY_USAGE_FAST_REA_CHKSUM.
enc-fast-req [2] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is KEY_USAGE_FAST_ENC.
...
}
KEY_USAGE_FAST_REA_CHKSUM TBA
KEY_USAGE_FAST_ENC TBA
The PA-FX-FAST-REQUEST structure contains a KrbFastArmoredReq type.
The KrbFastArmoredReq encapsulates the encrypted padata.
The enc-fast-req field contains an encrypted KrbFastReq structure.
The armor key is used to encrypt the KrbFastReq structure, and the
key usage number for that encryption is KEY_USAGE_FAST_ARMOR.
KEY_USAGE_FAST_ARMOR TBA
The armor key is selected as follows:
o In an AS request, the armor field in the KrbFastArmoredReq
structure MUST be present and the armor key is identified
according to the specification of the armor type.
o In a TGS request, the armor field in the KrbFastArmoredReq
structure MUST NOT be present and the subkey in the AP-REQ
authenticator in the PA-TGS-REQ PA-DATA MUST be present. In this
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case, the armor key is that subkey in the AP-REQ authenticator.
The req-checksum field contains a checksum that is performed over the
type KDC-REQ-BODY for the req-body field of the KDC-REQ [RFC4120]
structure of the containing message. The checksum key is the armor
key, and the checksum type is the required checksum type for the
enctype of the armor key per [RFC3961]. This checksum is included in
order to bind the FAST data to the outer request. A KDC that
implements FAST will ignore the outer request, but including a
checksum is relatively cheap and may prevent confusing behavior.
The KrbFastReq structure contains the following information:
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
req-body [2] KDC-REQ-BODY,
-- Contains the KDC request body as defined in Section
-- 5.4.1 of [RFC4120].
-- This req-body field is preferred over the outer field
-- in the KDC request.
...
}
The fast-options field indicates various options that are to modify
the behavior of the KDC. The following options are defined:
FastOptions ::= KerberosFlags
-- reserved(0),
-- anonymous(1),
-- kdc-referrals(16)
Bits Name Description
-----------------------------------------------------------------
0 RESERVED Reserved for future expansion of this field.
1 anonymous Requesting the KDC to hide client names in
the KDC response, as described next in this
section.
16 kdc-referrals Requesting the KDC to follow referrals, as
described next in this section.
Bits 1 through 15 (with bit 2 and bit 15 included) are critical
options. If the KDC does not support a critical option, it MUST fail
the request with KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS (there is no
accompanying e-data defined in this document for this error code).
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Bit 16 and onward (with bit 16 included) are non-critical options.
KDCs conforming to this specification ignores unknown non-critical
options.
KDC_ERR_UNKNOWN_FAST_OPTIONS TBA
The anonymous Option
The Kerberos response defined in [RFC4120] contains the client
identity in clear text, This makes traffic analysis
straightforward. The anonymous option is designed to complicate
traffic analysis. If the anonymous option is set, the KDC
implementing PA_FX_FAST MUST identify the client as the anonymous
principal [KRB-ANON] in the KDC reply and the error response.
Hence this option is set by the client if it wishes to conceal the
client identity in the KDC response. A conforming KD ignores the
client principal name in the outer KDC-REQ-BODY field, and
identifies the client using the cname and crealm fields in the
req-body field of the KrbFastReq structure.
The kdc-referrals Option
The Kerberos client described in [RFC4120] has to request referral
TGTs along the authentication path in order to get a service
ticket for the target service. The Kerberos client described in
the [REFERRALS] need to contact the AS specified in the error
response in order to complete client referrals. The kdc-referrals
option is designed to minimize the number of messages that need to
be processed by the client. This option is useful when, for
example, the client may contact the KDC via a satellite link that
has high network latency, or the client has limited computational
capabilities. If the kdc-referrals option is set, the KDC that
honors this option acts as the client to follow AS referrals and
TGS referrals [REFERRALS], and return the service ticket to the
named server principal in the client request using the reply key
expected by the client. The kdc-referrals option can be
implemented when the KDC knows the reply key. The KDC can ignore
kdc-referrals option when it does not understand it or it does not
allow this option based on local policy. The client SHOULD be
able to process the KDC responses when this option is not honored
by the KDC.
The padata field contains a list of PA-DATA structures as described
in Section 5.2.7 of [RFC4120]. These PA-DATA structures can contain
FAST factors. They can also be used as generic typed-holes to
contain data not intended for proving the client's identity or
establishing a reply key, but for protocol extensibility.
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The KDC-REQ-BODY in the FAST structure is used in preference to the
KDC-REQ-BODY outside of the FAST pre-authentication. The outer KDC-
REQ-BODY structure SHOULD be filled in for backwards compatibility
with KDCs that do not support FAST. A conforming KDC ignores the
outer KDC-REQ-BODY field in the KDC request.
6.5.3. FAST Response
The KDC that supports the PA_FX_FAST padata MUST include a PA_FX_FAST
padata element in the KDC reply. In the case of an error, the
PA_FX_FAST padata is included in the KDC responses according to
Section 6.5.4.
The corresponding padata-value field [RFC4120] for the PA_FX_FAST in
the KDC response contains the DER encoding of the ASN.1 type PA-FX-
FAST-REPLY.
PA-FX-FAST-REPLY ::= CHOICE {
armored-data [0] KrbFastArmoredRep,
...
}
KrbFastArmoredRep ::= SEQUENCE {
enc-fast-rep [0] EncryptedData, -- KrbFastResponse --
-- The encryption key is the armor key in the request, and
-- the key usage number is KEY_USAGE_FAST_REP.
...
}
KEY_USAGE_FAST_REP TBA
The PA-FX-FAST-REPLY structure contains a KrbFastArmoredRep
structure. The KrbFastArmoredRep structure encapsulates the padata
in the KDC reply in the encrypted form. The KrbFastResponse is
encrypted with the armor key used in the corresponding request, and
the key usage number is KEY_USAGE_FAST_REP.
The Kerberos client who does not receive a PA-FX-FAST-REPLY in the
KDC response MUST support a local policy that rejects the response.
Clients MAY also support policies that fall back to other mechanisms
or that do not use pre-authentication when FAST is unavailable. It
is important to consider the potential downgrade attacks when
deploying such a policy.
The KrbFastResponse structure contains the following information:
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KrbFastResponse ::= SEQUENCE {
padata [0] SEQUENCE OF PA-DATA,
-- padata typed holes.
rep-key [1] EncryptionKey OPTIONAL,
-- This, if present, replaces the reply key for AS and TGS.
-- MUST be absent in KRB-ERROR.
finished [2] KrbFastFinished OPTIONAL,
-- MUST be present if the client is authenticated,
-- absent otherwise.
-- Typically this is present if and only if the containing
-- message is the last one in a conversation.
...
}
The padata field in the KrbFastResponse structure contains a list of
PA-DATA structures as described in Section 5.2.7 of [RFC4120]. These
PA-DATA structures are used to carry data advancing the exchange
specific for the FAST factors. They can also be used as generic
typed-holes for protocol extensibility.
The rep-key field, if present, contains the reply key that is used to
encrypted the KDC reply. The rep-key field MUST be absent in the
case where an error occurs. The enctype of the rep-key is the
strongest mutually supported by the KDC and the client.
The finished field contains a KrbFastFinished structure. It is
filled by the KDC in the final message in the conversation; it MUST
be absent otherwise. In other words, this field can only be present
in an AS-REP or a TGS-REP when a ticket is returned.
The KrbFastFinished structure contains the following information:
KrbFastFinished ::= SEQUENCE {
timestamp [0] KerberosTime,
usec [1] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
crealm [2] Realm,
cname [3] PrincipalName,
-- Contains the client realm and the client name.
checksum [4] Checksum,
-- Checksum performed over all the messages in the
-- conversation, except the containing message.
-- The checksum key is the binding key as defined in
-- Section 6.3, and the checksum type is the required
-- checksum type of the binding key.
...
}
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KEY_USAGE_FAST_FINISHED TBA
The timestamp and usec fields represent the time on the KDC when the
reply ticket was generated, these fields have the same semantics as
the corresponding-identically-named fields in Section 5.6.1 of
[RFC4120]. The client MUST use the KDC's time in these fields
thereafter when using the returned ticket. Note that the KDC's time
in AS-REP may not match the authtime in the reply ticket if the kdc-
referrals option is requested and honored by the KDC.
The cname and crealm fields identify the authenticated client.
The checksum field contains a checksum of all the messages in the
conversation prior to the containing message (the containing message
is excluded). The checksum key is the binding key as defined in
Section 6.3, and the checksum type is the required checksum type of
the enctype of that key, and the key usage number is
KEY_USAGE_FAST_FINISHED. [[anchor9: Examples would be good here; what
all goes into the checksum?]]
When FAST padata is included, the PA-FX-COOKIE padata as defined in
Section 6.3 MUST also be included if the KDC expects at least one
more message from the client in order to complete the authentication.
6.5.4. Authenticated Kerberos Error Messages using Kerberos FAST
If the Kerberos FAST padata was included in the request, unless
otherwise specified, the e-data field of the KRB-ERROR message
[RFC4120] contains the ASN.1 DER encoding of the type METHOD-DATA
[RFC4120] and a PA_FX_FAST is included in the METHOD-DATA. The KDC
MUST include all the padata elements such as PA-ETYPE-INFO2 and
padata elments that indicate acceptable pre-authentication mechanisms
[RFC4120] and in the KrbFastResponse structure.
If the Kerberos FAST padata is included in the request but not
included in the error reply, it is a matter of the local policy on
the client to accept the information in the error message without
integrity protection. The Kerberos client MAY process an error
message without a PA-FX-FAST-REPLY, if that is only intended to
return better error information to the application, typically for
trouble-shooting purposes.
In the cases where the e-data field of the KRB-ERROR message is
expected to carry a TYPED-DATA [RFC4120] element, the
PA_FX_TYPED_DATA padata is included in the KrbFastResponse structure
to encapsulate the TYPED-DATA [RFC4120] elements. For example, the
TD_TRUSTED_CERTIFIERS structure is expected to be in the KRB-ERROR
message when the error code is KDC_ERR_CANT_VERIFY_CERTIFICATE
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[RFC4556].
PA_FX_TYPED_DATA TBA
-- This is the padata element that encapsulates a TYPED-DATA
-- structure.
The corresponding padata-value for the PA_FX_TYPED_DATA padata type
contains the DER encoding of the ASN.1 type TYPED-DATA [RFC4120].
6.5.5. The Encrypted Challenge FAST Factor
The encrypted challenge FAST factor authenticates a client using the
client's long-term key. This factor works similarly to the encrypted
time stamp pre-authentication option described in [RFC4120]. The
client encrypts a structure containing a timestamp in the challenge
key. The challenge key is KRB-FX-CF2(long_term_key, armor_key,
"challengelongterm", "challengearmor"). Because the armor key is
fresh and random, the challenge key is fresh and random. The only
purpose of the timestamp is to limit the validity of the
authentication so that a request cannot be replayed. A client MAY
base the timestamp based on the KDC time in a KDC error and need not
maintain accurate time synchronization itself. If a client bases its
time on an untrusted source, an attacker may trick the client into
producing an authentication request that is valid at some future
time. The attacker may be able to use this authentication request to
make it appear that a client has authenticated at that future time.
If ticket-based armor is used, then the lifetime of the ticket will
limit the window in which an attacker can make the client appear to
have authenticated. For many situations, the ability of an attacker
to cause a client to appear to have authenticated is not a
significant concern; the ability to avoid requiring time
synchronization on clients is more valuable.
The client sends a padata of type PA_ENCRYPTED_CHALLENGE the
corresponding padata-value contains the DER encoding of ASN.1 type
EncryptedChallenge.
EncryptedChallenge ::= EncryptedData
-- Encrypted PA-ENC-TS-ENC, encrypted in the challenge key
-- using key usage KEY_USAGE_ENC_CHALLENGE_CLIENT for the
-- client and KEY_USAGE_ENC_CHALLENGE_KDC for the KDC.
PA_ENCRYPTED_CHALLENGE TBA
KEY_USAGE_ENC_CHALLENGE_CLIENT TBA
KEY_USAGE_ENC_CHALLENGE_KDC TBA
The client includes some time stamp reasonably close to the KDC's
current time and encrypts it in the challenge key. Clients MAY use
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the current time; doing so prevents the exposure where an attacker
can cause a client to appear to authenticate in the future. The
client sends the request including this factor.
On receiving an AS-REQ containing the PA_ENCRYPTED_CHALLENGE fast
factor, the KDC decrypts the timestamp. If the decryption fails the
KDC SHOULD return KDC_ERR_PREAUTH_FAILED, including etype-info2 in
the error [[anchor11: Or should this be KRB_APP_ERR_MODIFIED?]]. The
KDC confirms that the timestamp falls within its current clock skew
returning KRB_APP_ERR_SKEW if not. The KDC then SHOULD check to see
if the encrypted challenge is a replay. The KDC MUST NOT consider
two encrypted challenges replays simply because the time stamps are
the same; to be a replay, the ciphertext MUST be identical. It is
not clear that RFC 3961 prevents encryption systems for which an
attacker can transform one ciphertext into a different ciphertext
yielding an identical plaintext. So, it may not be safe to base
replay detection on the ciphertext in the general case. However the
FAST tunnel provides integrity protection so requiring ciphertext be
identical is secure in this instance. Allowing clients to re-use
time stamps avoids requiring that clients maintain state about which
time stamps have been used.
If the KDC accepts the encrypted challenge, it MUST include a padata
element of type PA_ENCRYPTED_CHALLENGE. The KDC encrypts its current
time in the challenge key. The KDC MUST replace the reply key before
issuing a ticket. [[anchor12: I'd like to say that the KDC replaces
its reply key by this point. However we need to decide at what
points the FAST mechanism for replacing the reply key can be used and
how that interacts with this.]]The client MUST check that the
timestamp decrypts properly. The client MAY check that the timestamp
is in some reasonable skew of the current time. The client MUST NOT
require that the timestamp be identical to the timestamp in the
issued credentials or the returned message.
The encrypted challenge FAST factor provides the following
facilities: client-authentication, KDC authentication. It does not
provide the strengthening-reply-key facility. The security
considerations section of this document provides an explanation why
the security requirements are met.
Conforming implementations MUST support the encrypted challenge FAST
factor.
6.6. Authentication Strength Indication
Implementations that have pre-authentication mechanisms offering
significantly different strengths of client authentication MAY choose
to keep track of the strength of the authentication used as an input
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into policy decisions. For example, some principals might require
strong pre-authentication, while less sensitive principals can use
relatively weak forms of pre-authentication like encrypted timestamp.
An AuthorizationData data type AD-Authentication-Strength is defined
for this purpose.
AD-authentication-strength TBA
The corresponding ad-data field contains the DER encoding of the pre-
authentication data set as defined in Section 6.4. This set contains
all the pre-authentication mechanisms that were used to authenticate
the client. If only one pre-authentication mechanism was used to
authenticate the client, the pre-authentication set contains one
element.
The AD-authentication-strength element MUST be included in the AD-IF-
RELEVANT, thus it can be ignored if it is unknown to the receiver.
7. IANA Considerations
This document defines several new pa-data types, key usages and error
codes. In addition it would be good to track which pa-data items are
only to be used as FAST factors.
8. Security Considerations
The kdc-referrals option in the Kerberos FAST padata requests the KDC
to act as the client to follow referrals. This can overload the KDC.
To limit the damages of denied of service using this option, KDCs MAY
restrict the number of simultaneous active requests with this option
for any given client principal.
Because the client secrets are known only to the client and the KDC,
the verification of the authenticated timestamp proves the client's
identity, the verification of the authenticated timestamp in the KDC
reply proves that the expected KDC responded. The encrypted reply
key is contained in the rep-key in the PA-FX-FAST-REPLY. Therefore,
the authenticated timestamp FAST factor as a pre-authentication
mechanism offers the following facilities: client-authentication,
replacing-reply-key, KDC-authentication. There is no un-
authenticated clear text introduced by the authenticated timestamp
FAST factor.
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9. Acknowledgements
Sam Hartman would like to thank the MIT Kerberos Consortium for its
funding of his time on this project prior to April 2008.
Several suggestions from Jeffery Hutzman based on early revisions of
this documents led to significant improvements of this document.
The proposal to ask one KDC to chase down the referrals and return
the final ticket is based on requirements in [ID.CROSS].
Joel Webber had a proposal for a mechanism similar to FAST that
created a protected tunnel for Kerberos pre-authentication.
10. References
10.1. Normative References
[KRB-ANON]
Zhu, L. and P. Leach, "Kerberos Anonymity Support",
draft-ietf-krb-wg-anon-04.txt (work in progress), 2007.
[REFERRALS]
Raeburn, K. and L. Zhu, "Generating KDC Referrals to
Locate Kerberos Realms",
draft-ietf-krb-wg-kerberos-referrals-10.txt (work in
progress), 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
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[SHA2] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", Federal Information Processing
Standards Publication 180-2, August 2002.
[X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
10.2. Informative References
[EKE] Bellovin, S. M. and M. Merritt. "Augmented
Encrypted Key Exchange: A Password-Based Protocol Secure
Against Dictionary Attacks and Password File Compromise".
Proceedings of the 1st ACM Conference on Computer and
Communications Security, ACM Press, November 1993.
[HKDF] Dang, Q. and P. Polk, draft-dang-nistkdf, work in
progress.
[IEEE1363.2]
IEEE P1363.2: Password-Based Public-Key Cryptography,
2004.
[ID.CROSS]
Sakane, S., Zrelli, S., and M. Ishiyama , "Problem
Statement on the Operation of Kerberos in a Specific
System", draft-sakane-krb-cross-problem-statement-02.txt
(work in progress), April 2007.
[KRB-WG.SAM]
Hornstein, K., Renard, K., Neuman, C., and G. Zorn,
"Integrating Single-use Authentication Mechanisms with
Kerberos", draft-ietf-krb-wg-kerberos-sam-02.txt (work in
progress), October 2003.
Appendix A. Change History
RFC editor, please remove this section before publication.
A.1. Changes since 07
Propose replacement of authenticated timestamp with encrypted
challenge. The desire to avoid clients needing time
synchronization and to simply the factor.
Add a requirement that any FAST armor scheme must provide a fresh
key for each conversation. This allows us to assume that anything
encrypted/integrity protected in the right key is fresh and not
subject to cross-conversation cut&paste.
Removed heartbeat padata. The KDC will double up messages if it
needs to; the client simply sends its message and waits for the
next response.
Define PA_AUTHENTICATION_SET_SELECTED
Clarify a KDC cannot ignore padata is has clamed to support
A.2. Changes since 06
Note that even for replace reply key it is likely that the side
using the mechanism will know that the other side supports it.
Since it is reasonablly unlikely we'll need a container mechanism
other than FAST itself, we don't need to optimize for that case.
So, we want to optimize for implementation simplicity. Thus if
you do have such a container mechanism interacting with
authentication sets we'll assume that the hint need to describe
hints for all contained mechanisms. This closes out a long-
standing issue.
Write up what Sam believes is the consensus on UI and prompts in
the authentication set: clients MAY assume that they have all the
UI information they need.
Appendix B. ASN.1 module
KerberosPreauthFramework {
iso(1) identified-organization(3) dod(6) internet(1)
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security(5) kerberosV5(2) modules(4) preauth-framework(3)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
KerberosTime, PrincipalName, Realm, EncryptionKey, Checksum,
Int32, EncryptedData, PA-ENC-TS-ENC, PA-DATA, KDC-REQ-BODY,
Microseconds, KerberosFlags
FROM KerberosV5Spec2 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) kerberosV5(2)
modules(4) krb5spec2(2) };
-- as defined in RFC 4120.
PA-FX-COOKIE ::= SEQUENCE {
conversationId [0] OCTET STRING,
-- Contains the identifier of this conversation. This field
-- must contain the same value for all the messages
-- within the same conversation.
enc-binding-key [1] EncryptedData OPTIONAL,
-- EncryptionKey --
-- This field is present when and only when a FAST
-- padata as defined in Section 6.5 is included.
-- The encrypted data, when decrypted, contains an
-- EncryptionKey structure.
-- This encryption key is encrypted using the armor key
-- (defined in Section 6.5.1), and the key usage for the
-- encryption is KEY_USAGE_FAST_BINDING_KEY.
-- Present only once in a converstation.
cookie [2] OCTET STRING OPTIONAL,
-- Opaque data, for use to associate all the messages in
-- a single conversation between the client and the KDC.
-- This is generated by the KDC and the client MUST copy
-- the exact cookie encapsulated in a PA_FX_COOKIE data
-- element into the next message of the same conversation.
...
}
PA-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM
PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE {
pa-type [0] Int32,
-- same as padata-type.
pa-hint [1] OCTET STRING OPTIONAL,
pa-value [2] OCTET STRING OPTIONAL,
...
}
KrbFastArmor ::= SEQUENCE {
armor-type [0] Int32,
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-- Type of the armor.
armor-value [1] OCTET STRING,
-- Value of the armor.
...
}
PA-FX-FAST-REQUEST ::= CHOICE {
armored-data [0] KrbFastArmoredReq,
...
}
KrbFastArmoredReq ::= SEQUENCE {
armor [0] KrbFastArmor OPTIONAL,
-- Contains the armor that identifies the armor key.
-- MUST be present in AS-REQ.
-- MUST be absent in TGS-REQ.
req-checksum [1] Checksum,
-- Checksum performed over the type KDC-REQ-BODY for
-- the req-body field of the KDC-REQ structure defined in
-- [RFC4120]
-- The checksum key is the armor key, the checksum
-- type is the required checksum type for the enctype of
-- the armor key, and the key usage number is
-- KEY_USAGE_FAST_REA_CHKSUM.
enc-fast-req [2] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is KEY_USAGE_FAST_ENC.
...
}
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
req-body [2] KDC-REQ-BODY,
-- Contains the KDC request body as defined in Section
-- 5.4.1 of [RFC4120].
-- This req-body field is preferred over the outer field
-- in the KDC request.
...
}
FastOptions ::= KerberosFlags
-- reserved(0),
-- anonymous(1),
-- kdc-referrals(16)
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PA-FX-FAST-REPLY ::= CHOICE {
armored-data [0] KrbFastArmoredRep,
...
}
KrbFastArmoredRep ::= SEQUENCE {
enc-fast-rep [0] EncryptedData, -- KrbFastResponse --
-- The encryption key is the armor key in the request, and
-- the key usage number is KEY_USAGE_FAST_REP.
...
}
KrbFastResponse ::= SEQUENCE {
padata [0] SEQUENCE OF PA-DATA,
-- padata typed holes.
rep-key [1] EncryptionKey OPTIONAL,
-- This, if present, replaces the reply key for AS and TGS.
-- MUST be absent in KRB-ERROR.
finished [2] KrbFastFinished OPTIONAL,
-- MUST be present if the client is authenticated,
-- absent otherwise.
-- Typically this is present if and only if the containing
-- message is the last one in a conversation.
...
}
KrbFastFinished ::= SEQUENCE {
timestamp [0] KerberosTime,
usec [1] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
crealm [2] Realm,
cname [3] PrincipalName,
-- Contains the client realm and the client name.
checksum [4] Checksum,
-- Checksum performed over all the messages in the
-- conversation, except the containing message.
-- The checksum key is the binding key as defined in
-- Section 6.3, and the checksum type is the required
-- checksum type of the binding key.
...
}
EncryptedChallenge ::= EncryptedData
-- Encrypted PA-ENC-TS-ENC, encrypted in the challenge key
-- using key usage KEY_USAGE_ENC_CHALLENGE_CLIENT for the
-- client and KEY_USAGE_ENC_CHALLENGE_KDC for the KDC.
END
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Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
Sam hartman
Painless Security
Email: hartmans-ietf@mit.edu
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