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Kerberos Working Group L. Zhu
Internet-Draft Microsoft Corporation
Updates: 4120 (if approved) S. Hartman
Intended status: Standards Track MIT
Expires: April 28, 2007 October 25, 2006
A Generalized Framework for Kerberos Pre-Authentication
draft-ietf-krb-wg-preauth-framework-04
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 April 28, 2007.
Copyright Notice
Copyright (C) The Internet Society (2006).
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
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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.
This document also provides common tools needed by multiple pre-
authentication mechanisms. One of such 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 straightforward to chain multiple authentication factors
or add a plugin to, for example, utilize a different key management
system, or support a new key agreement algorithm.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions 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 . . . . . . . . . . . . . . . . 11
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 . . . . . . . . . . . . . . 17
6.3. Managing States for the KDC . . . . . . . . . . . . . . . 17
6.4. Pre-authentication Set . . . . . . . . . . . . . . . . . . 18
6.5. Definition of Kerberos FAST Padata . . . . . . . . . . . . 19
6.5.1. FAST Armors . . . . . . . . . . . . . . . . . . . . . 20
6.5.2. FAST Request . . . . . . . . . . . . . . . . . . . . . 21
6.5.3. FAST Response . . . . . . . . . . . . . . . . . . . . 24
6.6. Authentication Strength Indication . . . . . . . . . . . . 27
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
8. Security Considerations . . . . . . . . . . . . . . . . . . . 27
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
10.1. Normative References . . . . . . . . . . . . . . . . . . . 28
10.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. ASN.1 module . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
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Intellectual Property and Copyright Statements . . . . . . . . . . 32
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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 appropriated. 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 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. FAST provides a protected channel between
the client and the KDC, and it also delivers 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 padata field Section 6.5 of
FAST. A pre-authentication type thus carried within FAST is called a
FAST factor. A FAST factor MUST NOT be used outside of FAST unless
its specification explicitly allows so. Note that FAST without a
FAST factor for authentication does NOT by itself authenticate the
client or the KDC.
New pre-authentication mechanisms SHOULD design FAST factors, instead
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of full-blown pre-authentication mechanisms.
A conversation consists of all messages that are necessary to
complete the mutual authentication between the client and the KDC. A
conversation is the smallest logic unit for messages exchanged
between the client and the KDC. The KDC need to manage mulitple
authentication sets frequently need to keep track of KDC states
during a convesation, standard solutions are provided for these
common problems.
This document should be read only after reading the documents
describing the Kerberos cryptography framework [RFC3961] and the core
Kerberos protocol [RFC4120]. This document freely uses terminology
and notation from these documents without reference or further
explanation.
2. Conventions 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 the shorthand of pre-authentication data.
A conversation is used to refer to all authentication messages
exchanged between the client and the KDC.
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
wrong padata, 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 as
that of the KDC_ERR_PREAUTH_REQUIRED error, 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
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single authentication session. Each exchange accumulates state and
hopefully brings the client closer to a successful authentication.
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
authentication session is trivial. For more complex authentication
sessions, 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 authentication session
o Whether the reply key has been replaced in this authentication
session
o Whether the contents of the KDC reply can be verified by the
client principal
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o Whether the contents of the KDC reply can be verified by the
client machine
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
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 handling 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 either directly to encrypt or
checksum some data or indirectly 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 used after it is replaced.
Initially the reply key has not been replaced. If a mechanism
implements the Replace Reply Key facility discussed in Section 4.3,
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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 authentication session.
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 handling any message 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 with a well-known public key or
providing a ticket for the client machine as a service in addition to
the requested ticket.
3.2. Initial Pre-authentication Required Error
Typically a client starts an authentication session 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 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
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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
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 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
choose 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 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 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
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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
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 a 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.
At this point the KDC decides whether it will issue a pre-
authentication required 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_PREAUTH_REQUIRED 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.
Mechanisms that are inappropriate for the client principal or the
request SHOULD also be 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
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response, encrypting it in the current pre-authentication reply key.
4. Pre-Authentication Facilities
Pre-Authentication mechanisms can be thought of as providing various
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.
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
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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.
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 [RFC4556]. 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.
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, mechanism designers are encouraged to include a
typed hole for additional padata in the message that proposes the
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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 interpreted as if they were included
immediately following the proposal. 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 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. Typically the reply key is used to protect the
padata. If you are only minimally increasing the strength of the
reply key, this may give the attacker access to something too close
to the original reply key. However, binding the padata to the new
reply key seems potentially important from a security standpoint.
There may also be objections to this from a double encryption
standpoint because we also recommend client authentication facilities
be tied to the reply key.
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 more common for both sides to know that the
facility is available by the time that the new key is available to be
used. However, mechanism designers can use a container for padata in
a proposal message as discussed in Section 4.2 if appropriate.
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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 need information such as, for
example, 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.
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 is used in FAST to provide
authentication information.
Significant problems have resulted in the specification of Kerberos
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
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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.
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 need 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
to it. 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.
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().
The function KRB-FX-CF2() produces a new key based on two existing
keys of the same enctype and it is base on a secure hash function and
the primitives encrypt(), random-to-key() and K-truncate() described
in [RFC3961].
KRB-FX-CF2(protocol key, protocol key, octet string) ->
(resulting key)
The KRB-FX-CF2() function takes two protocol keys and an octet string
as input, and output a new key of the same enctype.
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encrypt(B, initial-cipher-state, pepper) -> (state-1, cipher-text-1)
encrypt(A, initial-cipher-state, pepper) -> (state-2, cipher-text-2)
PRF+(H, cipher-text-1 | cipher-text-2) -> bitstring-1
K-truncate(cipher-text-1) -> bitstring-2
random-to-key(bitstring-2) -> final-key
KRB-FX-CF2(A, B, pepper) -> final-key
Where initial-cipher-state is defined in [RFC3961] and the key-
generation seed length K is specified by the enctype profile
[RFC3961]. The value of the parameter pepper is RECOMMENDED to be in
the form of contextID || SharedInfo per guidelines in [HKDF]. If the
value of pepper is too short for the encrypt() primitive, it MUST
first be padded with all zeroes to the next shortest length that
encryt() can operate on. PRF+() produces a bit-string of at least K
bits in length.
H is the secure hash function associated with the enctype. An
example of a secure hash function is SHA-256 [SHA2].
This document updates [RFC3961] to associate a secure hash function
with every enctype. Unless otherwise specified by the enctype
specification, the associated hash function is SHA-256. The
associated hash function for hmac-sha1-96-aes256 is SHA-512 [SHA2].
encryption type etype associated hash function
--------------------------------------------------------------
hmac-sha1-96-aes256 16 SHA-512 [SHA2]
PRF+ is defined as follows:
PRF+(secure hash function, octet string) -> (octet string)
PRF+(H, shared-info) -> H( 1 || shared-info ) ||
H( 2 || shared-info ) H ( 3 || shared-info ) || ...
Where the counter value 1, 2, 3 and so on are encoded as a one-octet
integer.
Mechanism designers MUST specify the pepper value when combining two
keys using KRB-FX-CF2().
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6.2. Protecting Requests/Responses
Mechanism designers SHOULD provide integrity protection of the
messages in a conversation whenever feasible
Sensitive data MUST be encrypted when sent over the wire. Non-
sensitive data that have privacy implications are encouraged to be
encrypted as well.
If there are more than one roundtrip for an authentication exchange,
mechanism designers SHOULD allow either the client or the KDC provide
a checksum of all the messages exchanged on the wire, that is then
verified by the receiver.
Primitives defined in [RFC3961] are RECOMMENDED for integrity
protection and confidentiality. Mechanisms based on these primitives
have the benefit of crypto-agility provided by [RFC3961]. The
advantage afforded by crypto-agility is the ability to avoid a multi-
year standardization and deployment cycle to fix a problem specific
to a particular algorithm, when real attacks do arise against that
algorithm.
New mechanisms MUST NOT be hard-wired to use a specific algorithm.
6.3. Managing States for the KDC
For any conversation that consists of more than two messages, the KDC
likely need to keep track of KDC states for incomplete authentication
exchanges and destroy the states of a conversation when the
authentication completes successful or fails, or the KDC times out.
When the KDC times out, the KDC returns an error message with the
code KDC_ERR_PREAUTH_TIMED_OUT.
KDC_ERR_PREAUTH_TIMED_OUT TBA
Upnon receipt of this error, the client MUST 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 need 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
state management.
PA_FX_COOKIE TBA
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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 {
Cookie [1] OCTET STRING,
-- Opaque data, for use to associate all the messages in a
-- single conversation between the client and the KDC.
-- This can be generated by either the client or the KDC.
-- The receiver MUST copy the exact Cookie encapsulated in
-- a PA_FX_COOKIE data element into the next message of the
-- same conversation.
...
}
The PA-FX-COOKIE structure contains an opaque cookie that is a logic
identifier of all the messages in a conversation.
The PA_FX_COOKIE can be initially sent by the client or the KDC, the
receiver MUST copy the Cookie into a PA_FX_COOKIE padata and include
it in the next message, if any, in the same conversation.
The content of the PA_FX_COOKIE padata is a local matter of the
sender. Implementations MUST NOT include any sensitive or private
data in the PA-FX-COOKIE structure.
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.
KDC_ERR_MORE_PREAUTH_DATA_NEEDED TBA
If a PA_FX_COOKIE is included in the client request, the KDC then
MUST copy the exact cookie into the response.
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 [1] Int32,
-- same as padata-type.
pa-hint [2] OCTET STRING,
-- hint data.
...
}
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.
When indicating which sets of padata are supported, the KDC includes
a PA-AUTHENTICATION-SET padata element for each authentication set.
The client sends the padata-value for the first mechanism it picks in
the 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.
6.5. Definition of Kerberos FAST Padata
The cipher text exposure of encrypted timestamp pre-authentication
data is a security concern for Kerberos. Attackers can lauch offline
dictionary attack using the cipher text. The FAST pre-authentication
padata is a tool to mitigate this threat. 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 using FAST messages. A pre-
authentication data element used within FAST is called a FAST factor.
A FAST factor capatures the minimal work required for extending
Kerberos to support a new authentication scheme. A FAST factor MUST
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NOT be used outside of FAST unless its specification explicitly
allows so. The typed holes in FAST messages can also be used as
generic ones 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.
A FAST mechanism factor when used within FAST to authenticate the
client or the KDC is a pre-authentication mechanism, as such the
specification of such a FAST factor SHOULD specify which facilities
it provides per Section 5.
Implementations of the pre-authentication framework SHOULD use
encrypted timestamp pre-authentication, if that is the mechanism to
authenticate the client, as a FAST factor to avoid security exposure.
The encrypted timestamp FAST factor MUST fill out the encrypted rep-
key-package field as described in Section 6.5.3. It provides the
following facilities: client-authentication, replacing-reply-key,
KDC-authentication. It does not provide the strengthening-reply-key
facility. The security considerations section of this document
provides an explaination why the security requirements are met.
FAST employs an armoring scheme. The armor can be a host Ticket
Granting Ticket (TGT), or an anonymous TGT obtained based on
anonymous PKINIT [KRB-ANON], or a pre-shared long term key such as a
host key. The rest of this section describes the types of armors and
the messages used by FAST.
6.5.1. FAST Armors
An armor key is used to encrypt pre-authentication data in the FAST
request and the response. The ArmorData structure is used to
identify the armor key. It contains the following two fields: the
armor-type identifies the type of armor data, and the armor-value as
an OCTET STRING contains the data.
KrbFastArmor ::= SEQUENCE {
armor-type [1] Int32,
-- Type of the armor.
armor-value [2] OCTET STRING,
-- Value of the armor.
...
}
The value of the armor key is a matter of the armor type
specification. The following types of armors are currently defined:
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FX_FAST_ARMOR_AP_REQUEST 1
FX_FAST_ARMOR_KEY_ID 2
Conforming implementations MUST implement the
FX_FAST_ARMOR_AP_REQUEST armor type.
6.5.1.1. Ticket-based Armors
The FX_FAST_ARMOR_AP_REQUEST armor type is based on a Kerberos TGT.
The armor-value field of an FX_FAST_ARMOR_AP_REQUEST armor contains
an AP-REQ encoded in DER. The subkey field in the AP-REQ MUST be
present. And the armor key is the subkey in the AP-REQ
authenticator.
If the client has a TGT for the expected KDC, it can use that ticket
to construct the AP-REQ. If not, the client can use anonymous PKINIT
as described in [KRB-ANON] to obtain a TGT anonymously and use that
to construct an FX_FAST_ARMOR_AP_REQUEST armor.
6.5.1.2. Key-based Armors
The FX_FAST_ARMOR_KEY_ID armor type is used to carry an identifier of
a key that is shared between the client host and the KDC. The
content and the encoding of the armor-data field of this armor type
is a local matter of the communicating client and the expected KDC.
The FX_FAST_ARMOR_KEY_ID armor is useful when the client host and the
KDC does have a shared key and it is beneficial to minimize the
number of messages exchanged between the client and the KDC, namely
by eliminating the messages for obtaining a host ticket based on the
host key.
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-REQUEST ::= CHOICE {
armored-data [1] KrbFastAmoredReq,
...
}
KrbFastAmoredReq ::= SEQUENCE {
armor [1] KrbFastArmor OPTIONAL,
-- Contains the armor that determines the armor key.
-- MUST be present in the initial AS-REQ in a converstation,
-- MUST be absent in any subsequent AS-REQ.
-- MUST be absent in TGS-REQ.
req-checksum [2] Checksum,
-- Checksum performed over the type KDC-REQ-BODY.
-- The checksum key is the armor key, and the checksum
-- type is the required checksum type for the enctype of
-- the armor key.
enc-fast-req [3] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is TBA.
...
}
The PA-FX-FAST-REQUEST contains a KrbFastAmoredReq structure. The
KrbFastAmoredReq encapsulates the encrypted padata.
The armor key is used to encrypt the KrbFastReq structure, and the
key usage number for that encryption is TBA. The armor field in the
KrbFastAmoredReq structure is filled to identify the armor key.
When a KrbFastAmoredReq is included in an AS request, the armor field
MUST be present in the initial AS-REQ in a converstation, specifying
the armor key being used. The armor field MUST be absent in any
subsequent AS-REQ of the same converstation. Thus the armor key is
specified explicitly in the initial AS-REQ in a converstation, and
implicitly thereafter.
When a KrbFastAmoredReq is included in a TGS request, the armor field
MUST be absent. In which case, the subkey in the AP-REQ
authenticator in the PA-TGS-REQ PA-DATA MUST be present, and the
armor key is implicitly that subkey.
The req-checksum field contains a checksum that is performed over the
type KDC-REQ-BODY 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.
The enc-fast-req field contains an encrypted KrbFastReq structure.
The KrbFastReq structure contains the following information:
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KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
timestamp [2] KerberosTime,
usec [3] Microseconds,
-- timestamp and usec represent the time of the client
-- host.
req-nonce [4] OCTET STRING,
-- At least 128 octets in length, randomly filled using
-- a PRNG by the client for each message request.
...
}
The fast-options field indicates various options that are to modify
the behavior of the KDC. The meanings of the options are as follows:
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 understand a critical option, it MUST
fail the request. Bit 16 and onward (with bit 16 included) are non-
critical options. The KDC conforming to this specification ignores
unknown non-critical options.
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 performed over the client-KDC messages. If the
anonymous option is set, the KDC implementing PA_FX_FAST MUST
identify the client as the anonymous principal in the KDC reply
and the error response. Thus this option is set by the client if
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it wishes to hide the client identity in the KDC response.
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 contain the AS specified in the error
response in order to complete client referrals. In many cases, it
is desirable to keep the client's involvement minimal. For
example, the client may contact the KDC via a satellite link that
has high latency, or the client has limited computational
capabilities. The kdc-referrals option is designed to minimize
the number of KDC response messages that the client need to
process. 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 ticket thus-obtained using
the reply key expected by the client. The kdc-referrals option
can be implemented when the KDC knows the reply key. KDC can
igore kdc-referrals option when it does not understand it or it
does not allow it based on local policy. The client MUST be able
to process the KDC responses when this option is not honored by
the KDC, unless otherwise specified.
The padata field contains a list of PA-DATA structures as described
in Section 5.2.7 in [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.
The timestamp and usec fields represent the time of the client host,
these fields have the same semantics as the corresponding-
identically-named fields in Section 5.6.1 of [RFC4120].
The req-nonce field is randomly filled using a PRNG by the client for
each message request. It MUST have at least 128 octets in length.
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 and/or the error response, when the
client and the KDC agreed upon the armor key. The corresponding
padata-value field [RFC4120] in the KDC response is the DER encoding
of the ASN.1 type PA-FX-FAST-REPLY.
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PA-FX-FAST-REPLY ::= CHOICE {
armored-data [1] KrbFastArmoredRep,
...
}
KrbFastArmoredRep ::= SEQUENCE {
enc-fast-rep [1] EncryptedData, -- KrbFastResponse --
-- The encryption key is the armor key in the request, and
-- the key usage number is 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 TBA.
The Kerberos client who does not receive a PA-FX-FAST-REPLY in the
KDC response MUST reject the reply based on local policy. 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-shooing purposes.
The KrbFastResponse structure contains the following information:
KrbFastResponse ::= SEQUENCE {
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
finish [2] KrbFastFinish 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.
rep-nonce [3] OCTET STRING,
-- At least 128 octets in length, randomly filled using
-- a PRNG by the KDC for each KDC response.
...
}
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 completing the exchange for
the FAST factors. They can also be used as generic typed-holes for
protocol extensibility.
The finish field contains a KrbFastFinish structure. It is filled by
the KDC when the client has been authenticated (the client
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authenticated state is marked), it MUST be absent otherwise.
Consequently this field can only be present in an AS-REP or a TGS-REP
when a ticket is returned. And typically the containing message with
the finish field present is the last one in a conversation.
The KrbFastFinish structure contains the following information:
KrbFastFinish ::= SEQUENCE {
authtime [1] KerberosTime,
usec [2] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
rep-key-package [3] EncryptedData OPTIONAL, -- EncryptionKey --
-- This, if present, replaces the reply key for AS and TGS.
-- The encryption key is the client key, unless otherwise
-- specified. The key usage number is TBA.
crealm [4] Realm,
cname [5] PrincipalName,
-- Contains the client realm and the client name.
checksum [6] Checksum,
-- Checksum performed over all the messages in the
-- conversation, except the containing message.
-- The checksum key is the ticket session key of the reply
-- ticket, and the checksum type is the required checksum
-- type of that key.
...
}
The timestamp and usec fields represent the time on the KDC when the
reply 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 rep-key-package field, if present, contains the reply key
encrypted using the client key unless otherwise specified. The key
usage number is TBA.
When the encrypted timestamp FAST factor is used in the request, the
rep-key-package field MUST be present and the client key is used to
encrypt the reply key enclosed in the KrbFastArmoredRep.
The cname and crealm fields identifies the authenticated client.
The checksum field contains a checksum of all the messages in the
conversation excluding and prior to the containing message. The
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checksum key is the ticket session key of the reply ticket, and the
checksum type is the required checksum type of the enctype of that
key.
The rep-nonce field is randomly filled using a PRNG by the KDC for
each KDC response, and it MUST have at least 128 octets in length.
The client MUST include a PA_FX_COOKIE as defined in Section 6.3, if
it includes a PA_FX_FAST in the request.
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
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 FAST factors, these are mini- and light-
weighted- pre-authentication mechanisms. A new IANA registry should
be setup for registering FAST factor IDs.
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
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for any given client principal.
Because the client secrets are known only to the client and the KDC,
the verification of the encrypted timestamp proves the client's
identity, the verification of the encrypted rep-key-package in the
KDC reply proves that the expected KDC responded. The encrypted
reply key is contained in the rep-key-package in the PA-FX-FAST-
REPLY. Therefore, the encrypted timestamp FAST factor as a pre-
authentication mechanism offers the following facilities: client-
authentication, replacing-reply-key, KDC-authentication. There is no
un-authenticated cleartext introduced by the encrypted timestamp FAST
factor.
9. Acknowledgements
Serveral suggestions from Jeffery Hutzman based on early revisions of
this documents led to significant improvements of this document.
10. References
10.1. Normative References
[KRB-ANON] Zhu, L., Leach, P. and Jaganathan, K., "Kerberos Anonymity
Support", draft-ietf-krb-wg-anon, work in progress.
[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.
[REFERALS] Raeburn, K. et al, "Generating KDC Referrals to Locate
Kerberos Realms", draft-ietf-krb-wg-kerberos-referrals,
work in progress.
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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.
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[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.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
Appendix A. ASN.1 module
KerberosPreauthFramework {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) preauth-framework(3)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
KerberosTime, PrincipalName, Realm, EncryptionKey, Checksum,
Int32, EncryptedData, PA-DATA
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 {
Cookie [1] OCTET STRING,
-- Opaque data, for use to associate all the messages in a
-- single conversation between the client and the KDC.
-- This can be generated by either the client or the KDC.
-- The receiver 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 [1] Int32,
-- same as padata-type.
pa-hint [2] OCTET STRING,
-- hint data.
...
}
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PA-FX-FAST-REQUEST ::= CHOICE {
armored-data [1] KrbFastAmoredReq,
...
}
KrbFastAmoredReq ::= SEQUENCE {
armor [1] KrbFastArmor OPTIONAL,
-- Contains the armor that determines the armor key.
-- MUST be present in AS-REQ.
-- MUST be absent in TGS-REQ.
req-checksum [2] Checksum,
-- Checksum performed over the type KDC-REQ-BODY.
-- The checksum key is the armor key, and the checksum
-- type is the required checksum type for the enctype of
-- the armor key.
enc-fast-req [3] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is TBA.
...
}
KrbFastArmor ::= SEQUENCE {
armor-type [1] Int32,
-- Type of the armor.
armor-value [2] OCTET STRING,
-- Value of the armor.
...
}
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
timestamp [2] KerberosTime,
usec [3] Microseconds,
-- timestamp and usec represent the time of the client
-- host.
req-nonce [4] OCTET STRING,
-- At least 128 octets in length, randomly filled using
-- a PRNG by the client for each message request.
...
}
FastOptions ::= KerberosFlags
-- reserved(0),
-- anonymous(1),
-- kdc-referrals(16)
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PA-FX-FAST-REPLY ::= CHOICE {
armored-data [1] KrbFastArmoredRep,
...
}
KrbFastArmoredRep ::= SEQUENCE {
enc-fast-rep [1] EncryptedData, -- KrbFastResponse --
-- The encryption key is the armor key in the request, and
-- the key usage number is TBA.
...
}
KrbFastResponse ::= SEQUENCE {
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
finish [2] KrbFastFinish 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.
rep-nonce [3] OCTET STRING,
-- At least 128 octets in length, randomly filled using
-- a PRNG by the KDC for each KDC response.
...
}
KrbFastFinish ::= SEQUENCE {
timestamp [1] KerberosTime,
usec [2] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
rep-key-package [3] EncryptedData OPTIONAL, -- EncryptionKey --
-- This, if present, replaces the reply key for AS and TGS.
-- The encryption key is the client key, unless otherwise
-- specified. The key usage number is TBA.
crealm [4] Realm,
cname [5] PrincipalName,
-- Contains the client realm and the client name.
checksum [6] Checksum,
-- Checksum performed over all the messages in the
-- conversation, except the containing message.
-- The checksum key is the ticket session key of the reply
-- ticket, and the checksum type is the required checksum
-- type of that key.
...
}
END
Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
Sam hartman
MIT
Email: hartmans@mit.edu
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