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draft-ietf-krb-wg-preauth-framework-02.txt
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2004/10/25 20:55:18 1.1
Kerberos Working Group S. Hartman
Internet-Draft MIT
Expires: April 24, 2005 October 24, 2004
A Generalized Framework for Kerberos Pre-Authentication
draft-ietf-krb-wg-preauth-framework-02
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with
RFC 3668.
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 24, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004).
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
Hartman Expires April 24, 2005 [Page 1]
Internet-Draft Kerberos Preauth Framework October 2004
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.
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 Initial Preauth_Required Error . . . . . . . . . . . . 7
2.3 Client to KDC . . . . . . . . . . . . . . . . . . . . . . 8
2.4 KDC to Client . . . . . . . . . . . . . . . . . . . . . . 8
3. Pre-Authentication Facilities . . . . . . . . . . . . . . . . 10
3.1 Client Authentication . . . . . . . . . . . . . . . . . . 11
3.2 Strengthen Reply Key . . . . . . . . . . . . . . . . . . . 11
3.3 Replace Reply Key . . . . . . . . . . . . . . . . . . . . 12
3.4 Verify Response . . . . . . . . . . . . . . . . . . . . . 12
4. Requirements for Pre-Authentication Mechanisms . . . . . . . . 14
5. Tools for Use in Pre-Authentication Mechanisms . . . . . . . . 15
5.1 Combine Keys . . . . . . . . . . . . . . . . . . . . . . . 15
5.2 Signing Requests/Responses . . . . . . . . . . . . . . . . 15
5.3 Managing State for the KDC . . . . . . . . . . . . . . . . 15
5.4 PA-AUTHENTICATION-SET . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 19
9.2 Informative References . . . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 19
A. Todo List . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . 21
Hartman Expires April 24, 2005 [Page 2]
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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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Internet-Draft Kerberos Preauth Framework October 2004
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.
Hartman Expires April 24, 2005 [Page 4]
Internet-Draft Kerberos Preauth Framework October 2004
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
Hartman Expires April 24, 2005 [Page 5]
Internet-Draft Kerberos Preauth Framework October 2004
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 Initial 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; see Section 2.4
for details of that case. This
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
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 MAY include any initial
data for the mechanisms.
The KDC includes a a PA-AUTHENTICATION-SET padata element for each
authentication set; this element is defined in Section 5.4. This
element includes the pa-type and pa-value for the first mechanism in
the authentication set. It also includes the pa-type for each of
the other mechanisms. Associated with the second and following
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 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
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the KDC needs to expose ciphertext encrypted in a weak key before the
client has proven knowledge of that key.
2.3 Client to KDC
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