Kerberos Working Group S. Hartman
Internet-Draft MIT
Expires: January 17, 2005 July 19, 2004
A Generalized Framework for Kerberos Pre-Authentication
draft-ietf-krb-wg-preauth-framework-01
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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.
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 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Model for Pre-Authentication . . . . . . . . . . . . . . . . . 4
2.1 Information Managed by Model . . . . . . . . . . . . . . . 5
2.2 The Preauth_Required Error . . . . . . . . . . . . . . . . 7
2.3 Client to KDC . . . . . . . . . . . . . . . . . . . . . . 7
2.4 KDC to Client . . . . . . . . . . . . . . . . . . . . . . 8
3. Pre-Authentication Facilities . . . . . . . . . . . . . . . . 9
3.1 Client Authentication . . . . . . . . . . . . . . . . . . 10
3.2 Strengthen Reply Key . . . . . . . . . . . . . . . . . . . 10
3.3 Replace Reply Key . . . . . . . . . . . . . . . . . . . . 11
3.4 Verify Response . . . . . . . . . . . . . . . . . . . . . 11
4. Requirements for Pre-Authentication Mechanisms . . . . . . . . 12
5. Tools for Use in Pre-Authentication Mechanisms . . . . . . . . 13
5.1 Combine Keys . . . . . . . . . . . . . . . . . . . . . . . 13
5.2 Signing Requests/Responses . . . . . . . . . . . . . . . . 13
5.3 Managing State for the KDC . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 17
9.2 Informative References . . . . . . . . . . . . . . . . . . . 17
A. Todo List . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . 19
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1. Introduction
The core Kerberos specification 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 response. 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 the 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
pre-authentication mechanisms perform as well as how these functions
affect the state of the request and response. In addition several
common tools needed by pre-authentication mechanisms are provided.
Unlike [3], this framework is not complete--it does not describe all
the inputs and outputs for the pre-authentication mechanisms.
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.
This document should be read only after reading the documents
describing the Kerberos cryptography framework [3] and the core
Kerberos protocol [2]. This document freely uses terminology and
notation from these documents without reference or further
explanation.
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2. Model for Pre-Authentication
when a Kerberos client wishes to obtain a ticket using the
authentication server, it sends an initial AS request. If
pre-authentication is 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 a round-trip and send
an initial request with padata included. If the client includes the
wrong padata, the server MAY return KDC_ERR_PREAUTH_FAILED with no
indication of what padata should have been included. For
interoperability reasons, clients that include optimistic
pre-authentication MUST retry with no padata and examine the
KDC_ERR_PREAUTH_REQUIRED if they receive a KDC_ERR_PREAUTH_FAILED in
response to their initial optimistic request.
The 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
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 response. 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 5.3.
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 response. The padata 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. These 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 pre-authentication data at each step of
the AS request process.
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2.1 Information Managed by 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 response
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 response can be verified by the
client principal
o Whether the contents of the KDC response 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 [2], 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.
At the beginning of handling a message on both the client and 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. 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
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relatively weak forms of pre-authentication like encrypted timestamp.
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 KDC to enforce the security requirement stated in Section
3.3 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 3.3,
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 request 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 request is
authentic and has not been modified because it is encrypted in the
long-term key of the client. Only the KDC and client know that key.
So at the start of handling any message the KDC request is presumed
to be verified to the client principal. Any pre-authentication
mechanism that sets a new reply key not based on the principal's
long-term secret MUST either verify the KDC response some other way
or indicate that the response is not verified. If a mechanism
indicates that the response is not verified then the client
implementation MUST return an error unless a subsequent mechanism
verifies the response. The KDC needs to track this state so it can
avoid generating a response 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 response 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 response with a well-known
public key or providing a ticket for the client machine as a service
in addition to the requested ticket.
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2.2 The Preauth_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. This message MAY also be returned for pre-authentication
configurations that use multi-round-trip mechanisms. This error
contains a sequence of padata. Typically the padata contains the
pre-authentication type IDs of all the available pre-authentication
mechanisms. IN the initial error response, most mechanisms do not
contain data. If a mechanism requires multiple round trips or starts
with a challenge from the KDC to the client, then it will likely
contain data in the initial error response.
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 ciphertext encrypted in a weak key before the
client has proven knowledge of that key.
In order to generate the error response, 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 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 generated 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 mechanisms subsequent in the
generated response receive this state as input. After the padata is
generated, the error response is sent.
2.3 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 optimisticly
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 pdata in the
KDC_ERR_PREAUTH_REQUIRED.
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After processing the pdata 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. When the request is complete it is sent.
2.4 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 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 2.2,
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. The processing of the pre-authentication required
error is described in Section 2.2.
The KDC generates the pdata modifying the pre-authentication state as
necessary. Then it generates the final response, encrypting it in
the current pre-authentication reply key.
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3. 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 2mechanism 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 1.4.2 of the
Kerberos specification [2]), 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 preauth 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 clients have 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
3.2 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
2.1 can safely be ignored by a KDC that does not understand a
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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.
3.1 Client Authentication
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 [2] and the
single-use mechanism defined in [5]. 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
[6] implements Client Authentication along side 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.
3.2 Strengthen Reply Key
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 [5]. Typically these additional secrets are
converted into a Kerberos protocol key. Then they are combined with
the existing reply key as discussed in Section 5.1.
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
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
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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. XXX 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.
3.3 Replace Reply Key
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
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 Strengthen 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 3.2 if appropriate.
3.4 Verify Response
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4. Requirements for Pre-Authentication Mechanisms
State management for multi-round-trip mechs
Security interactions with other mechs
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5. Tools for Use in Pre-Authentication Mechanisms
5.1 Combine Keys
5.2 Signing Requests/Responses
5.3 Managing State for the KDC
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6. IANA Considerations
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7. Security Considerations
Very little of the AS request is authenticated. Same for padata
in the reply or error. Discuss implications
Table of security requirements stated elsewhere in the document
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8. Acknowledgements
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9. References
9.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[2] Neuman, C., Yu, T., Hartman, S. and K. Raeburn, "The Kerberos
Network Authentication Service (V5)",
draft-ietf-krb-wg-kerberos-clarifications-06.txt (work in
progress), June 2004.
[3] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", draft-ietf-krb-wg-crypto-03.txt (work in progress).
[4] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
9.2 Informative References
[5] 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.
[6] Tung, B., Neuman, C., Hur, M., Medvinsky, A. and S. Medvinsky,
"Public Key Cryptography for Initial Authentication in
Kerberos", draft-ietf-cat-kerberos-pk-init-19.txt (work in
progress), April 2004.
Author's Address
Sam hartman
MIT
EMail: hartmans@mit.edu
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Appendix A. Todo List
Flesh out sections that are still outlines
Discuss cookies and multiple-round-trip mechanisms.
Talk about checksum contributions from each mechanism
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