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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: September 6, 2007 March 5, 2007
A Generalized Framework for Kerberos Pre-Authentication
draft-ietf-krb-wg-preauth-framework-05
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 September 6, 2007.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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 these tools is a secure channel
between the client and the KDC with a reply key delivery mechanism;
this secure channel can be used to protect the authentication
exchange thus eliminate offline dictionary attacks. With these
tools, it is straightforward to chain multiple authentication
mechanisms, utilize a different key management system, or support a
new key agreement algorithm.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminologies Used in This Document . . . . . 5
3. Model for Pre-Authentication . . . . . . . . . . . . . . . . . 5
3.1. Information Managed by the Pre-authentication Model . . . 6
3.2. Initial Pre-authentication Required Error . . . . . . . . 8
3.3. Client to KDC . . . . . . . . . . . . . . . . . . . . . . 9
3.4. KDC to Client . . . . . . . . . . . . . . . . . . . . . . 10
4. Pre-Authentication Facilities . . . . . . . . . . . . . . . . 10
4.1. Client-authentication Facility . . . . . . . . . . . . . . 12
4.2. Strengthening-reply-key Facility . . . . . . . . . . . . . 12
4.3. Replacing-reply-key Facility . . . . . . . . . . . . . . . 13
4.4. KDC-authentication Facility . . . . . . . . . . . . . . . 14
5. Requirements for Pre-Authentication Mechanisms . . . . . . . . 14
6. Tools for Use in Pre-Authentication Mechanisms . . . . . . . . 15
6.1. Combining Keys . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Protecting Requests/Responses . . . . . . . . . . . . . . 16
6.3. Managing States for the KDC . . . . . . . . . . . . . . . 17
6.4. Pre-authentication Set . . . . . . . . . . . . . . . . . . 19
6.5. Definition of Kerberos FAST Padata . . . . . . . . . . . . 20
6.5.1. FAST and Encrypted Time Stamp . . . . . . . . . . . . 21
6.5.2. FAST Armors . . . . . . . . . . . . . . . . . . . . . 21
6.5.3. FAST Request . . . . . . . . . . . . . . . . . . . . . 22
6.5.4. FAST Response . . . . . . . . . . . . . . . . . . . . 26
6.5.5. Error Messages used with Kerberos FAST . . . . . . . . 28
6.6. Authentication Strength Indication . . . . . . . . . . . . 28
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
8. Security Considerations . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. ASN.1 module . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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1. Introduction
The core Kerberos specification [RFC4120] treats pre-authentication
data as an opaque typed hole in the messages to the KDC that may
influence the reply key used to encrypt the KDC reply. This
generality has been useful: pre-authentication data is used for a
variety of extensions to the protocol, many outside the expectations
of the initial designers. However, this generality makes designing
more common types of pre-authentication mechanisms difficult. Each
mechanism needs to specify how it interacts with other mechanisms.
Also, problems like combining a key with the long-term secret or
proving the identity of the user are common to multiple mechanisms.
Where there are generally well-accepted solutions to these problems,
it is desirable to standardize one of these solutions so mechanisms
can avoid duplication of work. In other cases, a modular approach to
these problems is appropriate. The modular approach will allow new
and better solutions to common pre-authentication problems to be used
by existing mechanisms as they are developed.
This document specifies a framework for Kerberos pre-authentication
mechanisms. It defines the common set of functions that pre-
authentication mechanisms perform as well as how these functions
affect the state of the request and reply. In addition several
common tools needed by pre-authentication mechanisms are provided.
Unlike [RFC3961], this framework is not complete--it does not
describe all the inputs and outputs for the pre-authentication
mechanisms. Pre-Authentication mechanism designers should try to be
consistent with this framework because doing so will make their
mechanisms easier to implement. Kerberos implementations are likely
to have plugin architectures for pre-authentication; such
architectures are likely to support mechanisms that follow this
framework plus commonly used extensions.
One of these common tools is the flexible authentication secure
tunneling (FAST) padata. 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 FAST messages as defined in
Section 6.5. A pre-authentication type carried within FAST is called
a FAST factor. Creating a FAST factor is the easiest path to create
a new pre-authentication mechanism. FAST factors are significantly
easier to analyze from a security standpoint than other pre-
authentication mechanisms.
Mechanism designers should design FAST factors, instead of new pre-
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authentication mechanisms outside of FAST.
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 and Terminologies 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 KDCs in order to authenticate
the client principal. A conversation as defined here consists of all
messages that are necessary to complete the authentication between
the client and the KDC. It is the smallest logic unit for 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, and
then retry.
The conventional KDC maintains no state between two requests;
subsequent requests may even be processed by a different KDC. On the
other hand, the client treats a series of exchanges with KDCs as a
single conversation. Each exchange accumulates state and hopefully
brings the client closer to a successful authentication.
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These models for state management are in apparent conflict. For many
of the simpler pre-authentication scenarios, the client uses one
round trip to find out what mechanisms the KDC supports. Then the
next request contains sufficient pre-authentication for the KDC to be
able to return a successful reply. For these simple scenarios, the
client only sends one request with pre-authentication data and so the
conversation is trivial. For more complex conversations, the KDC
needs to provide the client with a cookie to include in future
requests to capture the current state of the authentication session.
Handling of multiple round-trip mechanisms is discussed in
Section 6.3.
This framework specifies the behavior of Kerberos pre-authentication
mechanisms used to identify users or to modify the reply key used to
encrypt the KDC reply. The PA-DATA typed hole may be used to carry
extensions to Kerberos that have nothing to do with proving the
identity of the user or establishing a reply key. Such extensions
are outside the scope of this framework. However mechanisms that do
accomplish these goals should follow this framework.
This framework specifies the minimum state that a Kerberos
implementation needs to maintain while handling a request in order to
process pre-authentication. It also specifies how Kerberos
implementations process the padata at each step of the AS request
process.
3.1. Information Managed by the Pre-authentication Model
The following information is maintained by the client and KDC as each
request is being processed:
o The reply key used to encrypt the KDC reply
o How strongly the identity of the client has been authenticated
o Whether the reply key has been used in this conversation
o Whether the reply key has been replaced in this conversation
o Whether the contents of the KDC reply can be verified by the
client principal
Conceptually, the reply key is initially the long-term key of the
principal. However, principals can have multiple long-term keys
because of support for multiple encryption types, salts and
string2key parameters. As described in Section 5.2.7.5 of the
Kerberos protocol [RFC4120], the KDC sends PA-ETYPE-INFO2 to notify
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the client what types of keys are available. Thus in full
generality, the reply key in the pre-authentication model is actually
a set of keys. At the beginning of a request, it is initialized to
the set of long-term keys advertised in the PA-ETYPE-INFO2 element on
the KDC. If multiple reply keys are available, the client chooses
which one to use. Thus the client does not need to treat the reply
key as a set. At the beginning of a request, the client picks a
reply key to use.
KDC implementations MAY choose to offer only one key in the PA-ETYPE-
INFO2 element. Since the KDC already knows the client's list of
supported enctypes from the request, no interoperability problems are
created by choosing a single possible reply key. This way, the KDC
implementation avoids the complexity of treating the reply key as a
set.
When the padata in the request is verified by the KDC, then the
client is known to have that key, therefore the KDC SHOULD pick the
same key as the reply key.
At the beginning of handling a message on both the client and the
KDC, the client's identity is not authenticated. A mechanism may
indicate that it has successfully authenticated the client's
identity. This information is useful to keep track of on the client
in order to know what pre-authentication mechanisms should be used.
The KDC needs to keep track of whether the client is authenticated
because the primary purpose of pre-authentication is to authenticate
the client identity before issuing a ticket. The handling of
authentication strength using various authentication mechanisms is
discussed in Section 6.6.
Initially the reply key has not been used. A pre-authentication
mechanism that uses the reply key to encrypt or checksum some data in
the generation of new keys MUST indicate that the reply key is used.
This state is maintained by the client and the KDC to enforce the
security requirement stated in Section 4.3 that the reply key cannot
be replaced after it is used.
Initially the reply key has not been replaced. If a mechanism
implements the Replace Reply Key facility discussed in Section 4.3,
then the state MUST be updated to indicate that the reply key has
been replaced. Once the reply key has been replaced, knowledge of
the reply key is insufficient to authenticate the client. The reply
key is marked replaced in exactly the same situations as the KDC
reply is marked as not being verified to the client principal.
However, while mechanisms can verify the KDC reply to the client,
once the reply key is replaced, then the reply key remains replaced
for the remainder of the conversation.
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Without pre-authentication, the client knows that the KDC reply is
authentic and has not been modified because it is encrypted in a
long-term key of the client. Only the KDC and the client know that
key. So at the start of 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 that can be verified using a well-
known public key or providing a ticket for the client machine as a
service.
3.2. Initial Pre-authentication Required Error
Typically a client starts a conversation by sending an initial
request with no pre-authentication. If the KDC requires pre-
authentication, then it returns a KDC_ERR_PREAUTH_REQUIRED message.
After the first reply with the KDC_ERR_PREAUTH_REQUIRED error code,
the KDC returns the error code KDC_ERR_MORE_PREAUTH_DATA_NEEDED
(defined in Section 6.3) for pre-authentication configurations that
use multi-round-trip mechanisms; see Section 3.4 for details of that
case. [[anchor3: Is it desirable to define a new error code for this?
Probably but we need to call out to the WG.]]
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
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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 that a client has already received a
KDC_ERR_PREAUTH_REQUIRED from the KDC. If the client performs
optimistic pre-authentication then the client needs to optimistically
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 conforming to
this specification MUST NOT ignore the padata defined in Section 6.3.
Clients SHOULD process padata unrelated to this framework or other
means of authenticating the user. Clients SHOULD choose one
authentication set or mechanism that could lead to authenticating the
user and ignore the rest. Since the list of mechanisms offered by
the KDC is in the decreasing preference order, clients typically
choose the first mechanism or authentication set that the client can
usefully perform. If a client chooses to ignore a padata it MUST NOT
process the padata, allow the padata to affect the pre-authentication
state, nor respond to the padata.
For each padata the client chooses to process, the client processes
the padata and modifies the pre-authentication state as required by
that mechanism. Padata are processed in the order received from the
KDC.
After processing the padata in the KDC error, the client generates a
new request. It processes the pre-authentication mechanisms in the
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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 an AS reply.
There are many causes for an error to be generated that have nothing
to do with pre-authentication; they are discussed in the core
Kerberos specification.
From the standpoint of evaluating the pre-authentication, the KDC
first starts by initializing the pre-authentication state. It then
processes the padata in the request. As mentioned in Section 3.3,
the KDC MAY ignore padata that is inappropriate for the configuration
and MUST ignore padata of an unknown type.
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_MORE_PREAUTH_DATA_NEEDED error, the KDC
first starts by initializing the pre-authentication state. Then it
processes any padata in the client's request in the order provided by
the client. Mechanisms that are not understood by the KDC are
ignored. 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
response, encrypting it in the current pre-authentication reply key.
4. Pre-Authentication Facilities
Pre-Authentication mechanisms can be thought of as providing various
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conceptual facilities. This serves two useful purposes. First,
mechanism authors can choose only to solve one specific small
problem. It is often useful for a mechanism designed to offer key
management not to directly provide client authentication but instead
to allow one or more other mechanisms to handle this need. Secondly,
thinking about the abstract services that a mechanism provides yields
a minimum set of security requirements that all mechanisms providing
that facility must meet. These security requirements are not
complete; mechanisms will have additional security requirements based
on the specific protocol they employ.
A mechanism is not constrained to only offering one of these
facilities. While such mechanisms can be designed and are sometimes
useful, many pre-authentication mechanisms implement several
facilities. By combining multiple facilities in a single mechanism,
it is often easier to construct a secure, simple solution than by
solving the problem in full generality. Even when mechanisms provide
multiple facilities, they need to meet the security requirements for
all the facilities they provide. If the FAST factor approach is
used, it is likely that one or a small number of facilities can be
provided by a single mechanism without complicating the security
analysis.
According to Kerberos extensibility rules (Section 1.5 of the
Kerberos specification [RFC4120]), an extension MUST NOT change the
semantics of a message unless a recipient is known to understand that
extension. Because a client does not know that the KDC supports a
particular pre-authentication mechanism when it sends an initial
request, a pre-authentication mechanism MUST NOT change the semantics
of the request in a way that will break a KDC that does not
understand that mechanism. Similarly, KDCs MUST NOT send messages to
clients that affect the core semantics unless the client has
indicated support for the message.
The only state in this model that would break the interpretation of a
message is changing the expected reply key. If one mechanism changed
the reply key and a later mechanism used that reply key, then a KDC
that interpreted the second mechanism but not the first would fail to
interpret the request correctly. In order to avoid this problem,
extensions that change core semantics are typically divided into two
parts. The first part proposes a change to the core semantic--for
example proposes a new reply key. The second part acknowledges that
the extension is understood and that the change takes effect.
Section 4.2 discusses how to design mechanisms that modify the reply
key to be split into a proposal and acceptance without requiring
additional round trips to use the new reply key in subsequent pre-
authentication. Other changes in the state described in Section 3.1
can safely be ignored by a KDC that does not understand a mechanism.
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Mechanisms that modify the behavior of the request outside the scope
of this framework need to carefully consider the Kerberos
extensibility rules to avoid similar problems.
4.1. Client-authentication Facility
The client authentication facility proves the identity of a user to
the KDC before a ticket is issued. Examples of mechanisms
implementing this facility include the encrypted timestamp facility
defined in Section 5.2.7.2 of the Kerberos specification [RFC4120].
Mechanisms that provide this facility are expected to mark the client
as authenticated.
Mechanisms implementing this facility SHOULD require the client to
prove knowledge of the reply key before transmitting a successful KDC
reply. Otherwise, an attacker can intercept the pre-authentication
exchange and get a reply to attack. One way of proving the client
knows the reply key is to implement the Replace Reply Key facility
along with this facility. The PKINIT mechanism [RFC4556] implements
Client Authentication alongside Replace Reply Key.
If the reply key has been replaced, then mechanisms such as
encrypted-timestamp that rely on knowledge of the reply key to
authenticate the client MUST NOT be used.
4.2. Strengthening-reply-key Facility
Particularly, when dealing with keys based on passwords, it is
desirable to increase the strength of the key by adding additional
secrets to it. Examples of sources of additional secrets include the
results of a Diffie-Hellman key exchange or key bits from the output
of a smart card [KRB-WG.SAM]. Typically these additional secrets can
be first combined with the existing reply key and then converted to a
protocol key using tools defined in Section 6.1.
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 considered as an inner level. As
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with the outer level, one authentication set or mechanism is
typically chosen for client authentication, along with auxiliary
mechanisms such as KDC cookies, and other mechanisms are ignored.
[[anchor6: Containers like this need more thought. For example if
you are constructing an authentication set do you expect to use a
strengthen reply key mechanism in conjunction with something else, do
you include the something else in the hint of the strengthen
mechanism or as its own entry. It's easier to configure and express
the authentication set as its own entry. However if you do that' the
composition of the mechanisms looks in practice than it appears in
the authentication set.]] The party generating the proposal can
determine whether the padata were processed based on whether the
proposal for the reply key is accepted.
The specific formats of the proposal message, including where padata
are included is a matter for the mechanism specification. Similarly,
the format of the message accepting the proposal is mechanism-
specific.
Mechanisms implementing this facility and including a typed hole for
additional padata MUST checksum that padata using a keyed checksum or
encrypt the padata. [[anchor7: Why? I suspect there's an obvious
attack here but I need to work through it and add detail. In
particular, it seems that a checksum at the end should be
sufficient.]]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
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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.
4.4. KDC-authentication Facility
This facility verifies that the reply comes from the expected KDC.
In traditional Kerberos, the KDC and the client share a key, so if
the KDC reply can be decrypted then the client knows that a trusted
KDC responded. Note that the client machine cannot trust the client
unless the machine is presented with a service ticket for it
(typically the machine can retrieve this ticket by itself). However,
if the reply key is replaced, some mechanism is required to verify
the KDC. Pre-authentication mechanisms providing this facility allow
a client to determine that the expected KDC has responded even after
the reply key is replaced. They mark the pre-authentication state as
having been verified.
5. Requirements for Pre-Authentication Mechanisms
This section lists requirements for specifications of pre-
authentication mechanisms.
For each message in the pre-authentication mechanism, the
specification describes the pa-type value to be used and the contents
of the message. The processing of the message by the sender and
recipient is also specified. This specification needs to include all
modifications to the pre-authentication state.
Generally mechanisms have a message that can be sent in the error
data of the KDC_ERR_PREAUTH_REQUIRED error message or in an
authentication set. If the client needs information such as trusted
certificate authorities in order to determine if it can use the
mechanism, then this information should be in that message. In
addition, such mechanisms should also define a pa-hint to be included
in authentication sets. Often, the same information included in the
padata-value is appropriate to include in the pa-hint (as defined in
Section 6.4).
In order to ease security analysis the mechanism specification should
describe what facilities from this document are offered by the
mechanism. For each facility, the security consideration section of
the mechanism specification should show that the security
requirements of that facility are met. This requirement is
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applicable to any FAST factor that provides 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
plaintext is protected or discuss what harm an attacker could do by
modifying the plaintext. It is generally acceptable for an attacker
to be able to cause the protocol negotiation to fail by modifying
plaintext. More significant attacks should be evaluated carefully.
As discussed in Section 6.3, there is no guarantee that a client will
use the same KDCs for all messages in a conversation. The mechanism
specification needs to show why the mechanism is secure in this
situation. The hardest problem to deal with, especially for
challenge/response mechanisms is to make sure that the same response
cannot be replayed against two KDCs while allowing the client to talk
to any KDC.
6. Tools for Use in Pre-Authentication Mechanisms
This section describes common tools needed by multiple pre-
authentication mechanisms. By using these tools mechanism designers
can use a modular approach to specify mechanism details and ease
security analysis.
6.1. Combining Keys
Frequently a weak key 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.
Additional source of secrecy may come from hardware tokens.
This section provides standard ways to combine two keys into one.
KRB-FX-CF1() is defined to combine two pass-phrases.
KRB-FX-CF1(UTF-8 string, UTF-8 string) -> (UTF-8 string)
KRB-FX-CF1(x, y) -> x || y
Where || denotes concatenation. The strength of the final key is
roughly the total strength of the individual keys being combined
assuming that the string_to_key() function [RFC3961] uses all its
input evenly.
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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().
KRB-FX-CF2() combines two protocol keys based on the pseudo-random()
function defined in [RFC3961].
Given two input keys, K1 and K2, where K1 and K2 can be of two
different enctypes, the output key of KRB-FX-CF2(), K3, is derived as
follows:
KRB-FX-CF2(protocol key, protocol key, octet string,
octet string) -> (protocol key)
PRF+(K1, pepper1) -> octet-string-1
PRF+(K2, pepper2) -> octet-string-2
KRB-FX-CF2(K1, K2, pepper1, pepper2) ->
random-to-key(octet-string-1 ^ octet-string-2)
Where ^ denotes the exclusive-OR operation. PRF+() is defined as
follows:
PRF+(protocol key, octet string) -> (octet string)
PRF+(key, shared-info) -> pseudo-random( key, 1 || shared-info ) ||
pseudo-random( key, 2 || shared-info ) ||
pseudo-random( key, 3 || shared-info ) || ...
Here the counter value 1, 2, 3 and so on are encoded as a one-octet
integer. The pseudo-random() operation is specified by the enctype
of the protocol key. PRF+() uses the counter to generate enough bits
as needed by the random-to-key() [RFC3961] function for the
encryption type specified for the resulting key; unneeded bits are
removed from the tail.
Mechanism designers MUST specify the pepper values when combining two
keys using KRB-FX-CF2(). The pepper1 and pepper2 MUST be distinct so
that if the two keys being combined are the same, the resulting key
is not a trivial key.
6.2. Protecting Requests/Responses
Mechanism designers SHOULD protect clear text portions of pre-
authentication data. Various denial of service attacks and downgrade
attacks against Kerberos are possible unless plaintexts are somehow
protected against modification. An early design goal of Kerberos
Version 5 [RFC4120] was to avoid encrypting more of the
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authentication exchange that was required. (Version 4 doubly-
encrypted the encrypted part of a ticket in a KDC reply, for
example.) This minimization of encryption reduces the load on the
KDC and busy servers. Also, during the initial design of Version 5,
the existence of legal restrictions on the export of cryptography
made it desirable to minimize of the number of uses of encryption in
the protocol. Unfortunately, performing this minimization created
numerous instances of unauthenticated security-relevant plaintext
fields.
If there are more than one roundtrip for an authentication exchange,
mechanism designers need to allow either the client or the KDC to
provide a checksum of all the messages exchanged on the wire in the
conversation, and the checksum is then verified by the receiver.
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 that
is 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.
Note that data used by FAST factors (defined in Section 6.5) are
encrypted in a protected channel, in most cases, therefore no un-
authenticated-text issue is associated with these mechanisms.
However mechanism designers MUST consider the case carefully when the
KDC authentication is not provided by Kerberos FAST.
6.3. Managing States for the KDC
[[anchor11: Kerberos is stateless today. We can either maintain that
and store all the state in a cookie or change that and require
clients go to the same KDC for future requests. Consider how this
interacts with proxies. The rest of this section assumes we maintain
the current model.]] Kerberos KDCs are stateless. There is no
requirement that clients will choose the same KDC for the second
request in a conversation. Proxies or other intermediate nodes may
also influence KDC selection. So, each request from a client to a
KDC must include sufficient information that the KDC can regenerate
any needed state. This is accomplished by giving the client a
potentially long opaque cookie in responses to include in future
requests in the same conversation. The KDC MAY respond that a
conversation is too old and needs to restart by responding with a
KDC_ERR_PREAUTH_EXPIRED error.
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KDC_ERR_PREAUTH_EXPIRED TBA
When a client receives 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 needs to keep track of the challenge
issued for a client authentication request.
The PA-FX-COOKIE pdata type is defined in this section to facilitate
state management. This padata is sent by the KDC when the KDC
requires state for a future transaction. The client includes this
opaque token in the next message in the conversation. The token may
be relatively large; clients MUST be prepared for tokens somewhat
larger than the size of all messages in a conversation.
PA_FX_COOKIE TBA
-- Stateless cookie that is not tied to a specific KDC.
The corresponding padata-value field [RFC4120] contains the
Distinguished Encoding Rules (DER) [X60] [X690] encoding of the
following Abstract Syntax Notation One (ASN.1) type PA-FX-COOKIE:
PA-FX-COOKIE ::= SEQUENCE {
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 content of the PA_FX_COOKIE padata is a local matter of the KDC.
However the KDC MUST construct the token in such a manner that a
malicious client cannot subvert the authentication process by
manipulating the token. The KDC implementation needs to consider
expiration of tokens, key rollover and other security issues in token
design. The content of the Cookie field is likely specific to the
pre-authentication mechanisms used to authenticate the client. In
order to compute the finished field in the KrbFastRespons structure
as defined in Section 6.5.4, all the previous messages in the
conversation MUST be included in the Cookie. If a client
authentication response can be replayed to multiple KDCs via the
PA_FX_COOKIE mechanism, an expiration in the Cookie is RECOMMENDED to
prevent the response being presented indefinitely.
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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
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:
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. [[anchor12: What if you
have a padata type as the first member of a set that requires a
challenge. For example SAM assumes that the KDC sends a challenge to
the client initially. That's not a pa-hint; that's a pa-value. How
do you convey that data with this?]] [[anchor13: The PA-SET appears
only in the first message from the KDC to the client? In particular,
the client should not be prepared for the future authentication
mechanisms to change as the conversation progresses. I think this is
correct; we should discuss and if the WG agrees the text should
reflect this.]]
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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.
[[anchor14: It's much easier to design UIs if you can determine ahead
of time what all the elements of your dialogue will need to be. If
we mandate that the pa-hints need to be sufficient that you can
determine what information you will require from a user ahead of time
we can simplify the UI for login. I propose that we make this
requirement. WG agreement required.]]
6.5. Definition of Kerberos FAST Padata
The cipher text exposure when using the encrypted timestamp pre-
authentication data is a security concern for Kerberos. Attackers
can launch 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. A pre-authentication
data element used within FAST is called a FAST factor. A FAST factor
captures the minimal work required for extending Kerberos to support
a new authentication scheme.
A FAST factor MUST NOT be used outside of FAST unless its
specification explicitly allows so. The typed holes in FAST messages
can also be used as generic holes for other padata that are not
intended to prove the client's identity, or establish the reply key.
New pre-authentication mechanisms SHOULD be designed as FAST factors,
instead of full-blown pre-authentication mechanisms.
FAST factors that are pre-authentication mechanisms MUST meet the
requirements in Section 5.
FAST employs an armoring scheme. The armor can be a 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 armoring TGT can be a cross-realm TGT. The rest of
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this section describes the types of armors and the messages used by
FAST.
6.5.1. FAST and Encrypted Time Stamp
FAST provides new behavior for encrypted time stamp [RFC4120]. When
used as a FAST factor, this mechanism provides stronger security
guarantees.
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 6.5.4. 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 explanation why the security requirements are met.
6.5.2. 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 armor types are currently defined :
FX_FAST_ARMOR_AP_REQUEST 1
FX_FAST_ARMOR_KEY_ID 2
Conforming implementations MUST implement the
FX_FAST_ARMOR_AP_REQUEST armor type.
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6.5.2.1. 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. The armor key is the subkey in the AP-REQ authenticator.
The ticket in the AP-REQ MUST be for the TGT service of the target
KDC. Here are 3 ways in the decreasing preference order how an armor
TGT SHOULD be obtained:
1. If the client is authenticating from a host machine whose
Kerberos realm has a trust path to the client's realm, the host
machine obtains a TGT to the client's realm, and this ticket is
the armor ticket.
2. Otherwise, the client's host machine cannot obtain a host ticket
strictly based on RFC4120, but the KDC has a signing asymmetric
key that the client can verify its binding with the expected KDC,
the client then can use anonymous PKINIT to obtain a anonymous
TGT, and use that TGT to as the armor ticket.
3. Otherwise, the client uses anonymous PKINIT to get an anonymous
TGT without KDC authentication. Note that this mode of operation
is vulnerable to man-in-the-middle attacks at the time of
obtaining the initial anonymous TGT.
Because the KDC does not know if the client is able to trust the
ticket it has, the KDC and client MUST initialize the pre-
authentication state to an unverified KDC.
6.5.2.2. 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. [[anchor19: Do we believe this has sufficient value to
specify or do we want to assume all armor comes from tickets?]]
6.5.3. FAST Request
A padata type PA_FX_FAST is defined for the Kerberos FAST pre-
authentication padata. The corresponding padata-value field
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[RFC4120] contains the DER encoding of the ASN.1 type PA-FX-FAST-
REQUEST.
PA_FX_FAST TBA
-- Padata type for Kerberos FAST
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, the checksum
-- type is the required checksum type for the enctype of
-- the armor key, and the key usage number is
-- KEY_USAGE_FAST_REA_CHKSUM.
enc-fast-req [3] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is KEY_USAGE_FAST_ENC.
...
}
KEY_USAGE_FAST_REA_CHKSUM TBA
KEY_USAGE_FAST_ENC TBA
The PA-FX-FAST-REQUEST 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 KEY_USAGE_FAST_ARMOR.
KEY_USAGE_FAST_ARMOR TBA
The armor key is identified as follows:
o When a KrbFastAmoredReq is included in an AS request, the armor
field MUST be present in the initial AS-REQ in a conversation,
specifying the armor key being used. The armor field MUST be
absent in any subsequent AS-REQ of the same conversation. In
other words, the armor key is specified explicitly in the initial
AS-REQ in a conversation, and implicitly thereafter.
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o 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:
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
crealm [2] Realm OPTIONAL,
cname [3] PrincipalName OPTIONAL,
-- Contains the client realm and the client name.
-- If present, the client name and realm in the
-- AS_REQ KDC-REQ-BODY [RFC4120] MUST be ignored.
...
}
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. KDCs conforming to this specification ignores
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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 messages exchanged between the
client and the KDC. 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. Hence this
option is set by the client if it wishes to conceal 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 contact the AS specified in the error
response in order to complete client referrals. The kdc-referrals
option is designed to minimize the number of messages that need to
be processed by the client. This option is useful when, for
example, the client may contact the KDC via a satellite link that
has high latency, or the client has limited computational
capabilities. If the kdc-referrals option is set, the KDC that
honors this option acts as the client to follow AS referrals and
TGS referrals [REFERRALS], and return the 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. The
KDC can ignore kdc-referrals option when it does not understand it
or it does not allow this option based on local policy. The
client 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 of [RFC4120]. These PA-DATA structures can contain
FAST factors. They can also be used as generic typed-holes to
contain data not intended for proving the client's identity or
establishing a reply key, but for protocol extensibility.
The crealm field and the cname field identify the client principal in
the ticket request. If either the crealm field or the cname field is
present, the corresponding crealm or cname field in the KDC-REQ-BODY
[RFC4120] of an AS-REQ MUST be ignored. The client can fill in these
fields in the KrbFastReq structure and leaves the cname field and the
crealm field KDC-REQ-BODY absent, thus conceals its identity in the
AS-REQ.
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6.5.4. 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.
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 KEY_USAGE_FAST_REP.
...
}
KEY_USAGE_FAST_REP TBA
The PA-FX-FAST-REPLY structure contains a KrbFastArmoredRep
structure. The KrbFastArmoredRep structure encapsulates the padata
in the KDC reply in the encrypted form. The KrbFastResponse is
encrypted with the armor key used in the corresponding request, and
the key usage number is KEY_USAGE_FAST_REP.
The Kerberos client who does not receive a PA-FX-FAST-REPLY in the
KDC response MUST support a local policy that rejects the request.
Clients MAY also support policies that fall back to other mechanisms
or that do not use pre-authentication when FAST is unavailable. It
is important to consider the potential downgrade attacks when
deploying such a policy. The 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.
finished [2] KrbFastFinished OPTIONAL,
-- MUST be present if the client is authenticated,
-- absent otherwise.
-- Typically this is present if and only if the containing
-- message is the last one in a conversation.
...
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}
The padata field in the KrbFastResponse structure contains a list of
PA-DATA structures as described in Section 5.2.7 of [RFC4120]. These
PA-DATA structures are used to carry data advancing the exchange
specific for the FAST factors. They can also be used as generic
typed-holes for protocol extensibility.
The finished field contains a KrbFastFinished structure. It is
filled by the KDC in the final message in the conversation; 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.
The KrbFastFinished structure contains the following information:
KrbFastFinished ::= 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
-- KEY_USAGE_FAST_FINISHED.
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.
...
}
KEY_USAGE_FAST_REP_KEY TBA
KEY_USAGE_FAST_FINISHED TBA
The timestamp and usec fields represent the time on the KDC when the
reply ticket was generated, these fields have the same semantics as
the corresponding-identically-named fields in Section 5.6.1 of
[RFC4120]. The client MUST use the KDC's time in these fields
thereafter when using the returned ticket. Note that the KDC's time
in AS-REP may not match the authtime in the reply ticket if the kdc-
referrals option is requested and honored by the KDC.
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The rep-key-package field, if present, contains the reply key
encrypted using the client key unless otherwise specified. The key
usage number is KEY_USAGE_FAST_REP_KEY.
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 identify the authenticated client.
The checksum field contains a checksum of all the messages in the
conversation prior to the containing message (the containing message
is excluded). The checksum key is the ticket session key of the
reply ticket, the checksum type is the required checksum type of the
enctype of that key, and the key usage number is
KEY_USAGE_FAST_FINISHED.
6.5.5. Error Messages used with Kerberos FAST
If the Kerberos FAST padata was included in the request, unless
otherwise specified, the e-data field of the KRB-ERROR message
[RFC4120] contains the ASN.1 DER encoding of the type METHOD-DATA
[RFC4120], where a PA_FX_FAST padata element is included and it
contains the DER encoding of the type PA-FX-FAST-REPLY. If the
e-data field of the KRB-ERROR message contains the DER encoding of a
TYPED-DATA, a typed data element TD_FX_FAST SHOULD be included in the
e-data if the Kerberos FAST padata is included in the request, and
the corresponding data-value field [RFC4120] contains the ASN.1 DER
encoding of the type PA-FX-FAST-REPLY. In other words, the typed
data element type TD_FX_FAST is allocated to encapsulate the FAST
reply message in the error responses. If a PA-FX-FAST-REPLY is not
included in the error reply, it is a matter of the local policy on
the client to accept the information in the error message without
integrity protection. [[anchor21: Why do we want padata in arbitrary
error responses? What if the KDC cannot generate a fast reply
because for example no armor nor state cookie was included in a
request? Also, we need to confirm that the WG is OK with a pre-
authentication specification changing error returns for unrelated
errors.]]
TD_FX_FAST TBA
-- Typed data element type for Kerberos FAST
6.6. Authentication Strength Indication
Implementations that have pre-authentication mechanisms offering
significantly different strengths of client authentication MAY choose
to keep track of the strength of the authentication used as an input
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into policy decisions. For example, some principals might require
strong pre-authentication, while less sensitive principals can use
relatively weak forms of pre-authentication like encrypted timestamp.
An AuthorizationData data type AD-Authentication-Strength is defined
for this purpose.
AD-authentication-strength TBA
The corresponding ad-data field contains the DER encoding of the pre-
authentication data set as defined in Section 6.4. This set contains
all the pre-authentication mechanisms that were used to authenticate
the client. If only one pre-authentication mechanism was used to
authenticate the client, the pre-authentication set contains one
element.
The AD-authentication-strength element MUST be included in the AD-IF-
RELEVANT, thus it can be ignored if it is unknown to the receiver.
7. IANA Considerations
This document defines FAST factors, these are mini- and light-
weighted- pre-authentication mechanisms. A new IANA registry should
be setup for registering FAST factor IDs. The evaluation policy is
"Specification Required".
8. Security Considerations
The kdc-referrals option in the Kerberos FAST padata requests the KDC
to act as the client to follow referrals. This can overload the KDC.
To limit the damages of denied of service using this option, KDCs MAY
restrict the number of simultaneous active requests with this option
for any given client principal.
Because the client secrets are known only to the client and the KDC,
the verification of the 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 clear text introduced by the encrypted timestamp
FAST factor.
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9. Acknowledgements
Several 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.
[SHA2] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", Federal Information Processing
Standards Publication 180-2, August 2002.
[X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
10.2. Informative References
[EKE] Bellovin, S. M. and M. Merritt. "Augmented
Encrypted Key Exchange: A Password-Based Protocol Secure
Against Dictionary Attacks and Password File Compromise".
Proceedings of the 1st ACM Conference on Computer and
Communications Security, ACM Press, November 1993.
[HKDF] Dang, Q. and P. Polk, draft-dang-nistkdf, work in
progress.
[IEEE1363.2]
IEEE P1363.2: Password-Based Public-Key Cryptography,
2004.
[KRB-WG.SAM]
Hornstein, K., Renard, K., Neuman, C., and G. Zorn,
"Integrating Single-use Authentication Mechanisms with
Kerberos", draft-ietf-krb-wg-kerberos-sam-02.txt (work in
progress), October 2003.
[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 {
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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.
...
}
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, the checksum
-- type is the required checksum type for the enctype of
-- the armor key, and the key usage number is
-- KEY_USAGE_FAST_REA_CHKSUM.
enc-fast-req [3] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is KEY_USAGE_FAST_ENC.
...
}
KrbFastArmor ::= SEQUENCE {
armor-type [1] Int32,
-- Type of the armor.
armor-value [2] OCTET STRING,
-- Value of the armor.
...
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}
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
crealm [2] Realm OPTIONAL,
cname [3] PrincipalName OPTIONAL,
-- Contains the client realm and the client name.
-- If present, the client name and realm in the
-- AS_REQ KDC-REQ-BODY [RFC4120] MUST be ignored.
...
}
FastOptions ::= KerberosFlags
-- reserved(0),
-- anonymous(1),
-- kdc-referrals(16)
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 KEY_USAGE_FAST_REP.
...
}
KrbFastResponse ::= SEQUENCE {
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
finished [2] KrbFastFinished OPTIONAL,
-- MUST be present if the client is authenticated,
-- absent otherwise.
-- Typically this is present if and only if the containing
-- message is the last one in a conversation.
...
}
KrbFastFinished ::= SEQUENCE {
timestamp [1] KerberosTime,
usec [2] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
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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
-- KEY_USAGE_FAST_FINISHED.
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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