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Internet Engineering Task Force (IETF) S. Hartman
Request for Comments: 6113 Painless Security
Updates: 4120 L. Zhu
Category: Standards Track Microsoft Corporation
ISSN: 2070-1721 April 2011
A Generalized Framework for Kerberos Pre-Authentication
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 facility called pre-authentication.
Pre-authentication mechanisms can use this facility to extend the
Kerberos protocol and prove the identity of a principal.
This document describes a more formal model for this facility. 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 key distribution center with a reply key
strengthening mechanism; this secure channel can be used to protect
the authentication exchange and thus eliminate offline dictionary
attacks. With these tools, it is relatively straightforward to chain
multiple authentication mechanisms, utilize a different key
management system, or support a new key agreement algorithm.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6113.
Hartman & Zhu Standards Track [Page 1]
RFC 6113 Kerberos Preauth Framework April 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hartman & Zhu Standards Track [Page 2]
RFC 6113 Kerberos Preauth Framework April 2011
Table of Contents
1. Introduction ....................................................4
1.1. Conventions and Terminology Used in This Document ..........5
1.2. Conformance Requirements ...................................5
2. Model for Pre-Authentication ....................................6
2.1. Information Managed by the Pre-Authentication Model ........7
2.2. Initial Pre-Authentication Required Error ..................9
2.3. Client to KDC .............................................10
2.4. KDC to Client .............................................11
3. Pre-Authentication Facilities ..................................12
3.1. Client Authentication Facility ............................13
3.2. Strengthening Reply Key Facility ..........................13
3.3. Replace Reply Key Facility ................................14
3.4. KDC Authentication Facility ...............................15
4. Requirements for Pre-Authentication Mechanisms .................15
4.1. Protecting Requests/Responses .............................16
5. Tools for Use in Pre-Authentication Mechanisms .................17
5.1. Combining Keys ............................................17
5.2. Managing States for the KDC ...............................19
5.3. Pre-Authentication Set ....................................20
5.4. Definition of Kerberos FAST Padata ........................23
5.4.1. FAST Armors ........................................24
5.4.2. FAST Request .......................................26
5.4.3. FAST Response ......................................30
5.4.4. Authenticated Kerberos Error Messages Using
Kerberos FAST ......................................33
5.4.5. Outer and Inner Requests ...........................34
5.4.6. The Encrypted Challenge FAST Factor ................34
5.5. Authentication Strength Indication ........................36
6. Assigned Constants .............................................37
6.1. New Errors ................................................37
6.2. Key Usage Numbers .........................................37
6.3. Authorization Data Elements ...............................37
6.4. New PA-DATA Types .........................................37
7. IANA Considerations ............................................38
7.1. Pre-Authentication and Typed Data .........................38
7.2. Fast Armor Types ..........................................40
7.3. FAST Options ..............................................40
8. Security Considerations ........................................41
9. Acknowledgements ...............................................42
10. References ....................................................43
10.1. Normative References .....................................43
10.2. Informative References ...................................43
Appendix A. Test Vectors for KRB-FX-CF2 ...........................45
Appendix B. ASN.1 Module ..........................................46
Hartman & Zhu Standards Track [Page 3]
RFC 6113 Kerberos Preauth Framework April 2011
1. Introduction
The core Kerberos specification [RFC4120] treats pre-authentication
data (padata) as an opaque typed hole in the messages to the key
distribution center (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, tasks such as
combining a key with the long-term secrets 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 plug-in architectures for pre-authentication; such
architectures are likely to support mechanisms that follow this
framework plus commonly used extensions. This framework also
facilitates combining multiple pre-authentication mechanisms, each of
which may represent an authentication factor, into a single multi-
factor pre-authentication mechanism.
One of these common tools is the flexible authentication secure
tunneling (FAST) padata type. FAST provides a protected channel
between the client and the key distribution center (KDC), and it can
optionally deliver key material used to strengthen the 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 5.4. A pre-authentication type carried within
FAST is called a "FAST factor". Creating a FAST factor is the
Hartman & Zhu Standards Track [Page 4]
RFC 6113 Kerberos Preauth Framework April 2011
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-
authentication mechanisms outside of FAST.
1.1. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document should be read only after reading the documents
describing the Kerberos cryptography framework [RFC3961] and the core
Kerberos protocol [RFC4120]. This document may freely use
terminology and notation from these documents without reference or
further explanation.
The word padata is used as a shorthand for pre-authentication data.
A conversation is the set of all authentication messages exchanged
between the client and the client's Authentication Service (AS) 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 client's AS. In the
Ticket Granting Service (TGS) exchange, a conversation consists of
the request message and the reply message. The term conversation is
defined here for both AS and TGS for convenience of discussion. See
Section 5.2 for specific rules on the extent of a conversation in the
AS-REQ case. Prior to this framework, implementations needed to use
implementation-specific heuristics to determine the extent of a
conversation.
If the KDC reply in an AS exchange is verified, the KDC is
authenticated by the client. In this document, verification of the
KDC reply is used as a synonym of authentication of the KDC.
1.2. Conformance Requirements
This section summarizes the mandatory-to-implement subset of this
specification as a convenience to implementors. The actual
requirements and their context are stated in the body of the
document.
Clients conforming to this specification MUST support the padata
defined in Section 5.2.
Hartman & Zhu Standards Track [Page 5]
RFC 6113 Kerberos Preauth Framework April 2011
Conforming implementations MUST support Kerberos FAST padata
(Section 5.4). Conforming implementations MUST implement the
FX_FAST_ARMOR_AP_REQUEST armor type.
Conforming implementations MUST support the encrypted challenge FAST
factor (Section 5.4.6).
2. Model for Pre-Authentication
When a Kerberos client wishes to obtain a ticket, it sends an initial
Authentication Service (AS) request to the KDC. If pre-
authentication is required but not being used, then the KDC will
respond with a KDC_ERR_PREAUTH_REQUIRED error [RFC4120].
Alternatively, if the client knows what pre-authentication to use, it
MAY optimize away a round trip and send an initial request with
padata included in the initial request. If the client includes the
padata computed using the wrong pre-authentication mechanism or
incorrect keys, the KDC MAY return KDC_ERR_PREAUTH_FAILED with no
indication of what padata should have been included. In that case,
the client MUST retry with no padata and examine the error data of
the KDC_ERR_PREAUTH_REQUIRED error. If the KDC includes pre-
authentication information in the accompanying error data of
KDC_ERR_PREAUTH_FAILED, the client SHOULD process the error data and
then retry.
The conventional KDC maintains no state between two requests;
subsequent requests may even be processed by a different KDC. On the
other hand, the client treats a series of exchanges with KDCs as a
single conversation. Each exchange accumulates state and hopefully
brings the client closer to a successful authentication.
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 5.2.
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
Hartman & Zhu Standards Track [Page 6]
RFC 6113 Kerberos Preauth Framework April 2011
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.
2.1. 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 origin of the KDC reply can be verified by the client
(i.e., whether the KDC is authenticated to the client)
Conceptually, the reply key is initially the long-term key of the
principal. However, principals can have multiple long-term keys
because of support for multiple encryption types, salts, and
string2key parameters. As described in Section 5.2.7.5 of the
Kerberos protocol [RFC4120], the KDC sends PA-ETYPE-INFO2 to notify
the client what types of keys are available. Thus, in full
generality, the reply key in the pre-authentication model is actually
a set of keys. At the beginning of a request, it is initialized to
the set of long-term keys advertised in the PA-ETYPE-INFO2 element on
the KDC. If multiple reply keys are available, the client chooses
which one to use. Thus, the client does not need to treat the reply
key as a set. At the beginning of a request, the client picks a 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
Hartman & Zhu Standards Track [Page 7]
RFC 6113 Kerberos Preauth Framework April 2011
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 are 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. It is useful to keep track of this information 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 5.5.
Initially, the reply key is not 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 3.3 that the reply key SHOULD
NOT be replaced after it is used.
Initially, the reply key is not 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 as 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.
Without pre-authentication, the client knows that the KDC reply is
authentic and has not been modified because it is encrypted in a
long-term key of the client. Only the KDC and the client know that
key. So, at the start of a conversation, the KDC reply is presumed
to be verified using the client's long-term key. It should be noted
that in this document, verifying the KDC reply means authenticating
the KDC, and these phrases are used interchangeably. 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
Hartman & Zhu Standards Track [Page 8]
RFC 6113 Kerberos Preauth Framework April 2011
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.
In this specification, KDC verification/authentication refers to the
level of authentication of the KDC to the client provided by RFC
4120. There is a stronger form of KDC verification that, while
sometimes important in Kerberos deployments, is not addressed in this
specification: 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 machine 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.
2.2. 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_REQUIRED
(defined in Section 5.2) for pre-authentication configurations that
use multi-round-trip mechanisms; see Section 2.4 for details of that
case.
The KDC needs to choose which mechanisms to offer the client. The
client needs to be able to choose what mechanisms to use from the
first message. For example, consider the KDC that will accept
mechanism A followed by mechanism B or alternatively the single
mechanism C. A client that supports A and C needs to know that it
should not bother trying A.
Mechanisms can either be sufficient on their own or can be part of an
authentication set -- a group of mechanisms that all need to
successfully complete in order to authenticate a client. Some
mechanisms may only be useful in authentication sets; others may be
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, offered alone, or both. For each mechanism
that is offered alone (even if it is also offered in an
authentication set), the KDC includes the pre-authentication type ID
Hartman & Zhu Standards Track [Page 9]
RFC 6113 Kerberos Preauth Framework April 2011
of the mechanism in the padata sequence returned in the
KDC_ERR_PREAUTH_REQUIRED error. Mechanisms that are only offered as
part of an authentication set are not directly represented in the
padata sequence returned in the KDC_ERR_PREAUTH_REQUIRED error,
although they are represented in the PA-AUTHENTICATION-SET sequence.
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 the KDC needs to expose cipher text
encrypted in a weak key before the client has proven knowledge of
that key, and where pre-authentication is desirable.
2.3. 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 guess values
for the information it would normally receive from that error
response or use cached information obtained in prior interactions
with the KDC.
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 5.2.
Clients SHOULD choose one authentication set or mechanism that could
lead to authenticating the user and ignore other such mechanisms.
However, this rule does not affect the processing of padata unrelated
to this framework; clients SHOULD process such padata normally.
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 padata, it MUST NOT process the padata, allow the
padata to affect the pre-authentication state, or respond to the
padata.
For each instance of padata the client chooses to process, the client
processes the padata and modifies the pre-authentication state as
required by that mechanism.
Hartman & Zhu Standards Track [Page 10]
RFC 6113 Kerberos Preauth Framework April 2011
After processing the padata in the KDC error, the client generates a
new request. It processes the pre-authentication mechanisms in the
order in which they will appear in the next request, updating the
state as appropriate. The request is sent when it is complete.
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 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. If a PA-
FX-COOKIE pre-authentication data item is present, it is processed
first; see Section 5.2 for a definition. It then processes the
padata in the request. As mentioned in Section 2.3, the KDC MAY
ignore padata that are inappropriate for the configuration and MUST
ignore padata of an unknown type. The KDC MUST NOT ignore padata of
types used in previous messages. For example, if a KDC issues a
KDC_ERR_PREAUTH_REQUIRED error including padata of type x, then the
KDC cannot ignore padata of type x received in an AS-REQ message from
the client.
At this point, the KDC decides whether it will issue an error or a
reply. Typically, a KDC will issue a reply if the client's identity
has been authenticated to a sufficient degree.
In the case of a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error, the KDC
first starts by initializing the pre-authentication state. Then, it
processes any padata in the client's request in the order provided by
the client. Mechanisms that are not understood by the KDC are
ignored. 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 are
generated, the error response is sent. Typically, the errors with
the code KDC_ERR_MORE_PREAUTH_DATA_REQUIRED in a conversation will
include KDC state, as discussed in Section 5.2.
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.
Hartman & Zhu Standards Track [Page 11]
RFC 6113 Kerberos Preauth Framework April 2011
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 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. It is often easier to construct a secure, simple
solution by combining multiple facilities in a single mechanism 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 3.2 discusses how to design mechanisms that modify the reply
key to be split into a proposal and acceptance without requiring
Hartman & Zhu Standards Track [Page 12]
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additional round trips to use the new reply key in subsequent pre-
authentication. Other changes in the state described in Section 2.1
can safely be ignored by a KDC that does not understand a mechanism.
Mechanisms that modify the behavior of the request outside the scope
of this framework need to carefully consider the Kerberos
extensibility rules to avoid similar problems.
3.1. 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 Public Key Cryptography for Initial
Authentication in Kerberos (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.
3.2. 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 5.1.
Typically, a mechanism implementing this facility will know that the
other side of the exchange supports the facility before the reply key
is changed. For example, a mechanism might need to learn the
certificate for a KDC before encrypting a new key in the public key
belonging to that certificate. However, if a mechanism implementing
this facility wishes to modify the reply key before knowing that the
other party in the exchange supports the mechanism, it proposes
modifying the reply key. The other party then includes a message
indicating that the proposal is accepted if it is understood and
Hartman & Zhu Standards Track [Page 13]
RFC 6113 Kerberos Preauth Framework April 2011
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 with the outer
level, one authentication set or mechanism is typically chosen for
client authentication, along with auxiliary mechanisms such as KDC
cookies, and other mechanisms are ignored. When mechanisms include
such a container, the hint provided for use in authentication sets
(as defined in Section 5.3) MUST contain a sequence of inner
mechanisms along with hints for those mechanisms. The party
generating the proposal can determine whether the padata were
processed based on whether the proposal for the reply key is
accepted.
The specific formats of the proposal message, including where padata
are included, is a matter for the mechanism specification.
Similarly, the format of the message accepting the proposal is
mechanism specific.
Mechanisms implementing this facility and including a typed hole for
additional padata MUST checksum that padata using a keyed checksum or
encrypt the padata. This requirement protects against modification
of the contents of the typed hole. By modifying these contents, an
attacker might be able to choose which mechanism is used to
authenticate the client, or to convince a party to provide text
encrypted in a key that the attacker had manipulated. It is
important that mechanisms strengthen the reply key enough that using
it to checksum padata is appropriate.
3.3. Replace 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. This facility MUST NOT be used if there can
be a man-in-the-middle between the client and the KDC. 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.
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As with the Strengthening Reply Key facility, Kerberos extensibility
rules require that the reply key not be changed unless both sides of
the exchange understand the extension. In the case of this facility,
it will likely be the case for both sides to know that the facility
is available by the time that the new key is available to be used.
However, mechanism designers can use a container for padata in a
proposal message, as discussed in Section 3.2, if appropriate.
3.4. 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.
4. 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 5.3).
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 considerations section of
the mechanism specification should show that the security
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requirements of that facility are met. This requirement is
applicable to any FAST factor that provides authentication
information.
Significant problems have resulted in the specification of Kerberos
protocols because much of the KDC exchange is not protected against
alteration. 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 5.2, 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.
4.1. Protecting Requests/Responses
Mechanism designers SHOULD protect cleartext portions of pre-
authentication data. Various denial-of-service attacks and downgrade
attacks against Kerberos are possible unless plaintexts are somehow
protected against modification. An early design goal of Kerberos
Version 5 [RFC4120] was to avoid encrypting more of the
authentication exchange than 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.
Mechanisms MUST guarantee that by the end of a successful
authentication exchange, both the client and the KDC have verified
all the plaintext sent by the other party. If there is more than one
round trip in the exchange, mechanisms MUST additionally guarantee
that no individual messages were reordered or replayed from a
previous exchange. Strategies for accomplishing this include using
message authentication codes (MACs) to protect the plaintext as it is
sent including some form of nonce or cookie to allow for the chaining
of state from one message to the next or exchanging a MAC of the
entire conversation after a key is established.
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Mechanism designers need to provide a strategy for updating
cryptographic algorithms, such as defining a new pre-authentication
type for each algorithm or taking advantage of the client's list of
supported RFC 3961 encryption types to indicate the client's support
for cryptographic algorithms.
Primitives defined in [RFC3961] are RECOMMENDED for integrity
protection and confidentiality. Mechanisms based on these primitives
are crypto-agile as the result of using [RFC3961] along with
[RFC4120]. The advantage afforded by crypto-agility is the ability
to incrementally deploy a fix specific to a particular algorithm thus
avoid a multi-year standardization and deployment cycle, when real
attacks do arise against that algorithm.
Note that data used by FAST factors (defined in Section 5.4) is
encrypted in a protected channel; thus, they do not share the un-
authenticated-text issues with mechanisms designed as full-blown pre-
authentication mechanisms.
5. 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.
5.1. Combining Keys
Frequently, a weak key needs to be combined with a stronger key
before use. For example, passwords are typically limited in size and
insufficiently random: therefore, it is desirable to increase the
strength of the keys based on passwords by adding additional secrets.
An 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 passphrases.
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. Unless otherwise specified, the resulting
enctype of KRB-FX-CF2 is the enctype of k1. The pseudo-random()
operation is the RFC 3961 pseudo-random() operation for the
corresponding input key; the random-to-key() operation is the RFC
3961 random-to-key operation for the resulting key.
Mechanism designers MUST specify the values for the input parameter
pepper1 and pepper2 when combining two keys using KRB-FX-CF2(). The
pepper1 and pepper2 MUST be distinct so that if the two keys being
combined are the same, the resulting key is not a trivial key.
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5.2. Managing States for the KDC
Kerberos KDCs are stateless in that there is no requirement that
clients will choose the same KDC for the second request in a
conversation. Proxies or other intermediate nodes may also influence
KDC selection. So, each request from a client to a KDC must include
sufficient information that the KDC can regenerate any needed state.
This is accomplished by giving the client a potentially long opaque
cookie in responses to include in future requests in the same
conversation. The KDC MAY respond that a conversation is too old and
needs to restart by responding with a KDC_ERR_PREAUTH_EXPIRED error.
KDC_ERR_PREAUTH_EXPIRED 90
When a client receives this error, the client SHOULD abort the
existing conversation, and restart a new one.
An example, where more than one message from the client is needed, is
when the client is authenticated based on a challenge/response
scheme. In that case, the KDC needs to keep track of the challenge
issued for a client authentication request.
The PA-FX-COOKIE padata type is defined in this section to facilitate
state management in the AS exchange. These padata are 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 133
-- Stateless cookie that is not tied to a specific KDC.
The corresponding padata-value field [RFC4120] contains an opaque
token that will be echoed by the client in its response to an error
from the KDC.
The cookie token is generated by the KDC and transmitted in a PA-FX-
COOKIE pre-authentication data item of a KRB-ERROR message. The
client MUST copy the exact cookie encapsulated in a PA-FX-COOKIE data
element into the next message of the same conversation. The content
of the cookie field is a local matter of the KDC. As a result, it is
not generally possible to mix KDC implementations from different
vendors in the same realm. However, the KDC MUST construct the
cookie token in such a manner that a malicious client cannot subvert
the authentication process by manipulating the token. The KDC
implementation needs to consider expiration of tokens, key rollover,
and other security issues in token design. The content of the cookie
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field is likely specific to the pre-authentication mechanisms used to
authenticate the client. If a client authentication response can be
replayed to multiple KDCs via the PA-FX-COOKIE mechanism, an
expiration in the cookie is RECOMMENDED to prevent the response being
presented indefinitely. Implementations need to consider replay both
of an entire conversation and of messages within a conversation when
designing what information is stored in a cookie and how pre-
authentication mechanisms are implemented.
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_REQUIRED error with a PA-FX-COOKIE to
identify the conversation with the client, according to Section 2.2.
The cookie is not expected to stay constant for a conversation: the
KDC is expected to generate a new cookie for each message.
KDC_ERR_MORE_PREAUTH_DATA_REQUIRED 91
A client MAY throw away the state associated with a conversation and
begin a new conversation by discarding its state and not including a
cookie in the first message of a conversation. KDCs that comply with
this specification MUST include a cookie in a response when the
client can continue the conversation. In particular, a KDC MUST
include a cookie in a KDC_ERR_PREAUTH_REQUIRED or
KDC_ERR_MORE_PREAUTH_DATA_REQUIRED. KDCs SHOULD include a cookie in
errors containing additional information allowing a client to retry.
One reasonable strategy for meeting these requirements is to always
include a cookie in KDC errors.
A KDC MAY indicate that it is terminating a conversation by not
including a cookie in a response. When FAST is used, clients can
assume that the absence of a cookie means that the KDC is ending the
conversation. Similarly, if a cookie is seen at all during a
conversation, clients MAY assume that the absence of a cookie in a
future message means that the KDC is ending the conversation.
Clients also need to deal with KDCs, prior to this specification,
that do not include cookies; if neither cookies nor FAST are used in
a conversation, the absence of a cookie is not a strong indication
that the KDC is terminating the conversation.
5.3. 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.
PA-AUTHENTICATION-SET 134
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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 [0] Int32,
-- same as padata-type.
pa-hint [1] OCTET STRING OPTIONAL,
pa-value [2] OCTET STRING OPTIONAL,
...
}
The pa-type field of the PA-AUTHENTICATION-SET-ELEM structure
contains the corresponding value of padata-type in PA-DATA [RFC4120].
Associated with the pa-type is a pa-hint, which is an octet string
specified by the pre-authentication mechanism. This hint may provide
information for the client that helps it determine whether the
mechanism can be used. For example, a public-key mechanism might
include the certificate authorities it trusts in the hint info. Most
mechanisms today do not specify hint info; if a mechanism does not
specify hint info, the KDC MUST NOT send a hint for that mechanism.
To allow future revisions of mechanism specifications to add hint
info, clients MUST ignore hint info received for mechanisms that the
client believes do not support hint info. The pa-value element of
the PA-AUTHENTICATION-SET-ELEM sequence is included to carry the
first padata-value from the KDC to the client. If the client chooses
this authentication set, then the client MUST process this pa-value.
The pa-value element MUST be absent for all but the first entry in
the authentication set. Clients MUST ignore the pa-value for the
second and following entries in the authentication set.
If the client chooses an authentication set, then its first AS-REQ
message MUST contain a PA-AUTH-SET-SELECTED padata element. This
element contains the encoding of the PA-AUTHENTICATION-SET sequence
received from the KDC corresponding to the authentication set that is
chosen. The client MUST use the same octet values received from the
KDC; it cannot re-encode the sequence. This allows KDCs to use bit-
wise comparison to identify the selected authentication set.
Permitting bit-wise comparison may limit the ability to use certain
pre-authentication mechanisms that generate a dynamic challenge in an
authentication set with optimistic selection of an authentication
set. As with other optimistic pre-authentication failures, the KDC
MAY return KDC_ERR_PREAUTH_FAILED with a new list of pre-
authentication mechanisms (including authentication sets) if
optimistic pre-authentication fails. The PA-AUTH-SET-SELECTED padata
element MUST come before any padata elements from the authentication
set in the padata sequence in the AS-REQ message. The client MAY
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cache authentication sets from prior messages and use them to
construct an optimistic initial AS-REQ. If the KDC receives a PA-
AUTH-SET-SELECTED padata element that does not correspond to an
authentication set that it would offer, then the KDC returns the
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET error. The e-data in this
error contains a sequence of padata just as for the
KDC_ERR_PREAUTH_REQUIRED error.
PA-AUTH-SET-SELECTED 135
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET 92
The PA-AUTHENTICATION-SET appears only in the first message from the
KDC to the client. In particular, the client MAY fail if the
authentication mechanism sets change as the conversation progresses.
Clients MAY assume that the hints provided in the authentication set
contain enough information that the client knows what user interface
elements need to be displayed during the entire authentication
conversation. Exceptional circumstances, such as expired passwords
or expired accounts, may require that additional user interface be
displayed. Mechanism designers need to carefully consider the design
of their hints so that the client has this information. This way,
clients can construct necessary dialogue boxes or wizards based on
the authentication set and can present a coherent user interface.
Current standards for user interfaces do not provide an acceptable
experience when the client has to ask additional questions later in
the conversation.
When indicating which sets of pre-authentication mechanisms are
supported, the KDC includes a PA-AUTHENTICATION-SET padata element
for each pre-authentication mechanism set.
The client sends the padata-value for the first mechanism it picks in
the pre-authentication set, when the first mechanism completes, the
client and the KDC will proceed with the second mechanism, and so on
until all mechanisms complete successfully. The PA-FX-COOKIE, as
defined in Section 5.2, MUST be sent by the KDC. One reason for this
requirement is so that the conversation can continue if the
conversation involves multiple KDCs. KDCs MUST support clients that
do not include a cookie because they optimistically choose an
authentication set, although they MAY always return a
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET and include a cookie in that
message. Clients that support PA-AUTHENTICATION-SET MUST support PA-
FX-COOKIE.
Before the authentication succeeds and a ticket is returned, the
message that the client sends is an AS-REQ and the message that the
KDC sends is a KRB-ERROR message. The error code in the KRB-ERROR
message from the KDC is KDC_ERR_MORE_PREAUTH_DATA_REQUIRED as defined
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in Section 5.2 and the accompanying e-data contains the DER encoding
of ASN.1 type METHOD-DATA. The KDC includes the padata elements in
the METHOD-DATA. If there are no padata, the e-data field is absent
in the KRB-ERROR message.
If the client sends the last message for a given mechanism, then the
KDC sends the first message for the next mechanism. If the next
mechanism does not start with a KDC-side challenge, then the KDC
includes a padata item with the appropriate pa-type and an empty pa-
data.
If the KDC sends the last message for a particular mechanism, the KDC
also includes the first padata for the next mechanism.
5.4. Definition of Kerberos FAST Padata
As described in [RFC4120], Kerberos is vulnerable to offline
dictionary attacks. An attacker can request an AS-REP and try
various passwords to see if they can decrypt the resulting ticket.
RFC 4120 provides the encrypted timestamp pre-authentication method
that ameliorates the situation somewhat by requiring that an attacker
observe a successful authentication. However, stronger security is
desired in many environments. The Kerberos FAST pre-authentication
padata defined in this section provides a tool to significantly
reduce vulnerability to offline dictionary attacks. When combined
with encrypted challenge, FAST requires an attacker to mount a
successful man-in-the-middle attack to observe ciphertext. When
combined with host keys, FAST can even protect against active
attacks. FAST also provides solutions to common problems for pre-
authentication mechanisms such as binding of the request and the
reply and freshness guarantee of the authentication. FAST itself,
however, does not authenticate the client or the KDC; instead, it
provides a typed hole to allow pre-authentication data be tunneled.
A pre-authentication data element used within FAST is called a "FAST
factor". A FAST factor captures the minimal work required for
extending Kerberos to support a new pre-authentication scheme.
A FAST factor MUST NOT be used outside of FAST unless its
specification explicitly allows so. The typed holes in FAST messages
can also be used as generic holes for other padata that are not
intended to prove the client's identity, or establish the reply key.
New pre-authentication mechanisms SHOULD be designed as FAST factors,
instead of full-blown pre-authentication mechanisms.
FAST factors that are pre-authentication mechanisms MUST meet the
requirements in Section 4.
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FAST employs an armoring scheme. The armor can be a Ticket Granting
Ticket (TGT) obtained by the client's machine using the host keys to
pre-authenticate with the KDC, or an anonymous TGT obtained based on
anonymous PKINIT [RFC6112] [RFC4556].
The rest of this section describes the types of armors and the syntax
of the messages used by FAST. Conforming implementations MUST
support Kerberos FAST padata.
Any FAST armor scheme MUST provide a fresh armor key for each
conversation. Clients and KDCs can assume that if a message is
encrypted and integrity protected with a given armor key, then it is
part of the conversation using that armor key.
All KDCs in a realm MUST support FAST if FAST is offered by any KDC
as a pre-authentication mechanism.
5.4.1. FAST Armors
An armor key is used to encrypt pre-authentication data in the FAST
request and the response. The KrbFastArmor structure is defined to
identify the armor key. This structure contains the following two
fields: the armor-type identifies the type of armors and the armor-
value is an OCTET STRING that contains the description of the armor
scheme and the armor key.
KrbFastArmor ::= SEQUENCE {
armor-type [0] Int32,
-- Type of the armor.
armor-value [1] OCTET STRING,
-- Value of the armor.
...
}
The value of the armor key is a matter of the armor type
specification. Only one armor type is defined in this document.
FX_FAST_ARMOR_AP_REQUEST 1
The FX_FAST_ARMOR_AP_REQUEST armor is based on Kerberos tickets.
Conforming implementations MUST implement the
FX_FAST_ARMOR_AP_REQUEST armor type. If a FAST KDC receives an
unknown armor type it MUST respond with KDC_ERR_PREAUTH_FAILED.
An armor type may be appropriate for use in armoring AS requests,
armoring TGS requests, or both. TGS armor types MUST authenticate
the client to the KDC, typically by binding the TGT sub-session key
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to the armor key. As discussed below, it is desirable for AS armor
types to authenticate the KDC to the client, but this is not
required.
FAST implementations MUST maintain state about whether the armor
mechanism authenticates the KDC. If it does not, then a FAST factor
that authenticates the KDC MUST be used if the reply key is replaced.
5.4.1.1. Ticket-Based Armors
This is a ticket-based armoring scheme. The armor-type is
FX_FAST_ARMOR_AP_REQUEST, the armor-value contains an ASN.1 DER
encoded AP-REQ. The ticket in the AP-REQ is called an armor ticket
or an armor TGT. The subkey field in the AP-REQ MUST be present.
The armor key is defined by the following function:
armor_key = KRB-FX-CF2( subkey, ticket_session_key,
"subkeyarmor", "ticketarmor" )
The 'ticket_session_key' is the session key from the ticket in the
ap-req. The 'subkey' is the ap-req subkey. This construction
guarantees that both the KDC (through the session key) and the client
(through the subkey) contribute to the armor key.
The server name field of the armor ticket MUST identify the TGS of
the target realm. Here are three common 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 an authentication path to the client's realm,
the host machine obtains a TGT by using the host keys. If the
client's realm is different than the realm of the local host, the
machine then obtains a cross-realm TGT to the client's realm as
the armor ticket. Otherwise, the host's primary TGT is the armor
ticket.
2. If the client's host machine cannot obtain a host ticket strictly
based on RFC 4120, but the KDC has an asymmetric signing key
whose binding with the expected KDC can be verified by the
client, the client can use anonymous PKINIT [RFC6112] [RFC4556]
to authenticate the KDC and obtain an anonymous TGT as the armor
ticket. The armor ticket can also be a cross-realm TGT obtained
based on the initial primary TGT obtained using anonymous PKINIT
with KDC authentication.
3. Otherwise, the client uses anonymous PKINIT to get an anonymous
TGT without KDC authentication and that TGT is the armor ticket.
Note that this mode of operation is vulnerable to man-in-the-
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middle attacks at the time of obtaining the initial anonymous
armor TGT.
If anonymous PKINIT is used to obtain the armor ticket, the KDC
cannot know whether its signing key can be verified by the client;
hence, the KDC MUST be marked as unverified from the KDC's point of
view while the client could be able to authenticate the KDC by
verifying the KDC's signing key is bound with the expected KDC. The
client needs to carefully consider the risk and benefit tradeoffs
associated with active attacks before exposing cipher text encrypted
using the user's long-term secrets when the armor does not
authenticate the KDC.
The TGS MUST reject a request if there is an AD-fx-fast-armor (71)
element in the authenticator of the pa-tgs-req padata or if the
ticket in the authenticator of a pa-tgs-req contains the AD-fx-fast-
armor authorization data element. These tickets and authenticators
MAY be used as FAST armor tickets but not to obtain a ticket via the
TGS. This authorization data is used in a system where the
encryption of the user's pre-authentication data is performed in an
unprivileged user process. A privileged process can provide to the
user process a host ticket, an authenticator for use with that
ticket, and the sub-session key contained in the authenticator. In
order for the host process to ensure that the host ticket is not
accidentally or intentionally misused, (i.e., the user process might
use the host ticket to authenticate as the host), it MUST include a
critical authorization data element of the type AD-fx-fast-armor when
providing the authenticator or in the enc-authorization-data field of
the TGS request used to obtain the TGT. The corresponding ad-data
field of the AD-fx-fast-armor element is empty.
This armor type is only valid for AS requests; implicit armor,
described below in TGS processing, is the only supported way to
establish an armor key for the TGS at this time.
5.4.2. FAST Request
A padata type PA-FX-FAST is defined for the Kerberos FAST pre-
authentication padata. The corresponding padata-value field
[RFC4120] contains the DER encoding of the ASN.1 type PA-FX-FAST-
REQUEST. As with all pre-authentication types, the KDC SHOULD
advertise PA-FX-FAST in a PREAUTH_REQUIRED error. KDCs MUST send the
advertisement of PA-FX-FAST with an empty pa-value. Clients MUST
ignore the pa-value of PA-FX-FAST in an initial PREAUTH_REQUIRED
error. FAST is not expected to be used in an authentication set:
clients will typically use FAST padata if available and this decision
should not depend on what other pre-authentication methods are
available. As such, no pa-hint is defined for FAST at this time.
Hartman & Zhu Standards Track [Page 26]
RFC 6113 Kerberos Preauth Framework April 2011
PA-FX-FAST 136
-- Padata type for Kerberos FAST
PA-FX-FAST-REQUEST ::= CHOICE {
armored-data [0] KrbFastArmoredReq,
...
}
KrbFastArmoredReq ::= SEQUENCE {
armor [0] KrbFastArmor OPTIONAL,
-- Contains the armor that identifies the armor key.
-- MUST be present in AS-REQ.
req-checksum [1] Checksum,
-- For AS, contains the checksum performed over the type
-- KDC-REQ-BODY for the req-body field of the KDC-REQ
-- structure;
-- For TGS, contains the checksum performed over the type
-- AP-REQ in the PA-TGS-REQ padata.
-- 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_REQ_CHKSUM.
enc-fast-req [2] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is KEY_USAGE_FAST_ENC.
...
}
KEY_USAGE_FAST_REQ_CHKSUM 50
KEY_USAGE_FAST_ENC 51
The PA-FX-FAST-REQUEST structure contains a KrbFastArmoredReq type.
The KrbFastArmoredReq encapsulates the encrypted padata.
The enc-fast-req field contains an encrypted KrbFastReq structure.
The armor key is used to encrypt the KrbFastReq structure, and the
key usage number for that encryption is KEY_USAGE_FAST_ENC.
The armor key is selected as follows:
o In an AS request, the armor field in the KrbFastArmoredReq
structure MUST be present and the armor key is identified
according to the specification of the armor type.
Hartman & Zhu Standards Track [Page 27]
RFC 6113 Kerberos Preauth Framework April 2011
o There are two possibilities for armor for a TGS request. If the
ticket presented in the PA-TGS-REQ authenticator is a TGT, then
the client SHOULD NOT include the armor field in the Krbfastreq
and a subkey MUST be included in the PA-TGS-REQ authenticator. In
this case, the armor key is the same armor key that would be
computed if the TGS-REQ authenticator was used in an
FX_FAST_ARMOR_AP_REQUEST armor. Clients MAY present a non-TGT in
the PA-TGS-REQ authenticator and omit the armor field, in which
case the armor key is the same that would be computed if the
authenticator were used in an FX_FAST_ARMOR_AP_REQUEST armor.
This is the only case where a ticket other than a TGT can be used
to establish an armor key; even though the armor key is computed
the same as an FX_FAST_ARMOR_AP_REQUEST, a non-TGT cannot be used
as an armor ticket in FX_FAST_ARMOR_AP_REQUEST. Alternatively, a
client MAY use an armor type defined in the future for use with
the TGS request.
The req-checksum field contains a checksum computed differently for
AS and TGS. For an AS-REQ, it is performed over the type KDC-REQ-
BODY for the req-body field of the KDC-REQ structure of the
containing message; for a TGS-REQ, it is performed over the type AP-
REQ in the PA-TGS-REQ padata of the TGS request. The checksum key is
the armor key, and the checksum type is the required checksum type
for the enctype of the armor key per [RFC3961]. This checksum MUST
be a keyed checksum and it is included in order to bind the FAST
padata to the outer request. A KDC that implements FAST will ignore
the outer request, but including a checksum is relatively cheap and
may prevent confusing behavior.
The KrbFastReq structure contains the following information:
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
req-body [2] KDC-REQ-BODY,
-- Contains the KDC request body as defined in Section
-- 5.4.1 of [RFC4120].
-- This req-body field is preferred over the outer field
-- in the KDC request.
...
}
Hartman & Zhu Standards Track [Page 28]
RFC 6113 Kerberos Preauth Framework April 2011
The fast-options field indicates various options that are to modify
the behavior of the KDC. The following options are defined:
FastOptions ::= KerberosFlags
-- reserved(0),
-- hide-client-names(1),
Bits Name Description
-----------------------------------------------------------------
0 RESERVED Reserved for future expansion of this
field.
1 hide-client-names Requesting the KDC to hide client
names in the KDC response, as
described next in this section.
16 kdc-follow-referrals reserved [REFERRALS].
Bits 1 through 15 inclusive (with bit 1 and bit 15 included) are
critical options. If the KDC does not support a critical option, it
MUST fail the request with KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS, and
there is no accompanying e-data defined in this document for this
error code. Bit 16 and onward (with bit 16 included) are non-
critical options. KDCs conforming to this specification ignore
unknown non-critical options.
KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS 93
The hide-client-names Option
The Kerberos response defined in [RFC4120] contains the client
identity in cleartext. This makes traffic analysis
straightforward. The hide-client-names option is designed to
complicate traffic analysis. If the hide-client-names option is
set, the KDC implementing PA-FX-FAST MUST identify the client as
the anonymous principal [RFC6112] in the KDC reply and the error
response. Hence, this option is set by the client if it wishes to
conceal the client identity in the KDC response. A conforming KDC
ignores the client principal name in the outer KDC-REQ-BODY field,
and identifies the client using the cname and crealm fields in the
req-body field of the KrbFastReq structure.
The kdc-follow-referrals Option
This option is reserved for [REFERRALS].
Hartman & Zhu Standards Track [Page 29]
RFC 6113 Kerberos Preauth Framework April 2011
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. If the KDC
supports the PA-FX-FAST-REQUEST padata, unless otherwise specified,
the client MUST place any padata that is otherwise in the outer KDC
request body into this field. In a TGS request, PA-TGS-REQ padata is
not included in this field and it is present in the outer KDC request
body.
The KDC-REQ-BODY in the FAST structure is used in preference to the
KDC-REQ-BODY outside of the FAST pre-authentication. The outer KDC-
REQ-BODY structure SHOULD be filled in for backwards compatibility
with KDCs that do not support FAST. A conforming KDC ignores the
outer KDC-REQ-BODY field in the KDC request. Pre-authentication data
methods such as [RFC4556] that include a checksum of the KDC-REQ-BODY
should checksum the KDC-REQ-BODY in the FAST structure.
In a TGS request, a client MAY include the AD-fx-fast-used authdata
either in the pa-tgs-req authenticator or in the authorization data
in the pa-tgs-req ticket. If the KDC receives this authorization
data but does not find a FAST padata, then it MUST return
KRB_APP_ERR_MODIFIED.
5.4.3. FAST Response
The KDC that supports the PA-FX-FAST padata MUST include a PA-FX-FAST
padata element in the KDC reply. In the case of an error, the PA-FX-
FAST padata is included in the KDC responses according to
Section 5.4.4.
The corresponding padata-value field [RFC4120] for the PA-FX-FAST in
the KDC response contains the DER encoding of the ASN.1 type PA-FX-
FAST-REPLY.
PA-FX-FAST-REPLY ::= CHOICE {
armored-data [0] KrbFastArmoredRep,
...
}
KrbFastArmoredRep ::= SEQUENCE {
enc-fast-rep [0] EncryptedData, -- KrbFastResponse --
-- The encryption key is the armor key in the request, and
-- the key usage number is KEY_USAGE_FAST_REP.
...
}
KEY_USAGE_FAST_REP 52
Hartman & Zhu Standards Track [Page 30]
RFC 6113 Kerberos Preauth Framework April 2011
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 MUST support a local policy that rejects the
response if PA-FX-FAST-REPLY is not included in the response.
Clients MAY also support policies that fall back to other mechanisms
or that do not use pre-authentication when FAST is unavailable. It
is important to consider the potential downgrade attacks when
deploying such a policy.
The KrbFastResponse structure contains the following information:
KrbFastResponse ::= SEQUENCE {
padata [0] SEQUENCE OF PA-DATA,
-- padata typed holes.
strengthen-key [1] EncryptionKey OPTIONAL,
-- This, if present, strengthens the reply key for AS and
-- TGS. MUST be present for TGS.
-- MUST be absent in KRB-ERROR.
finished [2] KrbFastFinished OPTIONAL,
-- Present in AS or TGS reply; absent otherwise.
nonce [3] UInt32,
-- Nonce from the client request.
...
}
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. Unless otherwise specified,
the KDC MUST include any padata that are otherwise in the outer KDC-
REP or KDC-ERROR structure into this field. The padata field in the
KDC reply structure outside of the PA-FX-FAST-REPLY structure
typically includes only the PA-FX-FAST-REPLY padata.
The strengthen-key field provides a mechanism for the KDC to
strengthen the reply key. If set, the strengthen-key value MUST be
randomly generated to have the same etype as that of the reply key
before being strengthened, and then the reply key is strengthened
after all padata items are processed. Let padata-reply-key be the
reply key after padata processing.
reply-key = KRB-FX-CF2(strengthen-key, padata-reply-key,
"strengthenkey", "replykey")
Hartman & Zhu Standards Track [Page 31]
RFC 6113 Kerberos Preauth Framework April 2011
The strengthen-key field MAY be set in an AS reply; it MUST be set in
a TGS reply; it must be absent in an error reply. The strengthen key
is required in a TGS reply so that an attacker cannot remove the FAST
PADATA from a TGS reply, causing the KDC to appear not to support
FAST.
The finished field contains a KrbFastFinished structure. It is
filled by the KDC in the final message in the conversation. This
field is present in an AS-REP or a TGS-REP when a ticket is returned,
and it is not present in an error reply.
The KrbFastFinished structure contains the following information:
KrbFastFinished ::= SEQUENCE {
timestamp [0] KerberosTime,
usec [1] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
crealm [2] Realm,
cname [3] PrincipalName,
-- Contains the client realm and the client name.
ticket-checksum [4] Checksum,
-- checksum of the ticket in the KDC-REP using the armor
-- and the key usage is KEY_USAGE_FAST_FINISH.
-- The checksum type is the required checksum type
-- of the armor key.
...
}
KEY_USAGE_FAST_FINISHED 53
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. The client need not
confirm that the timestamp returned is within allowable clock skew:
the armor key guarantees that the reply is fresh. The client MAY
trust the timestamp returned.
The cname and crealm fields identify the authenticated client. If
facilities described in [REFERRALS] are used, the authenticated
client may differ from the client in the FAST request.
The ticket-checksum is a checksum of the issued ticket. The checksum
key is the armor key, and the checksum type is the required checksum
type of the enctype of that key, and the key usage number is
KEY_USAGE_FAST_FINISHED.
Hartman & Zhu Standards Track [Page 32]
RFC 6113 Kerberos Preauth Framework April 2011
When FAST padata is included, the PA-FX-COOKIE padata as defined in
Section 5.2 MUST be included in the padata sequence in the
KrbFastResponse sequence if the KDC expects at least one more message
from the client in order to complete the authentication.
The nonce field in the KrbFastResponse contains the value of the
nonce field in the KDC-REQ of the corresponding client request and it
binds the KDC response with the client request. The client MUST
verify that this nonce value in the reply matches with that of the
request and reject the KDC reply otherwise. To prevent the response
from one message in a conversation from being replayed to a request
in another message, clients SHOULD use a new nonce for each message
in a conversation.
5.4.4. Authenticated Kerberos Error Messages Using Kerberos FAST
If the Kerberos FAST padata was included in the request, unless
otherwise specified, the e-data field of the KRB-ERROR message
[RFC4120] contains the ASN.1 DER encoding of the type METHOD-DATA
[RFC4120] and a PA-FX-FAST is included in the METHOD-DATA. The KDC
MUST include all the padata elements such as PA-ETYPE-INFO2 and
padata elements that indicate acceptable pre-authentication
mechanisms [RFC4120] in the KrbFastResponse structure.
The KDC MUST also include a PA-FX-ERROR padata item in the
KRBFastResponse structure. The padata-value element of this sequence
is the ASN.1 DER encoding of the type KRB-ERROR. The e-data field
MUST be absent in the PA-FX-ERROR padata. All other fields should be
the same as the outer KRB-ERROR. The client ignores the outer error
and uses the combination of the padata in the KRBFastResponse and the
error information in the PA-FX-ERROR.
PA-FX-ERROR 137
If the Kerberos FAST padata is included in the request but not
included in the error reply, it is a matter of the local policy on
the client to accept the information in the error message without
integrity protection. However, the client SHOULD process the KDC
errors as the result of the KDC's inability to accept the AP_REQ
armor and potentially retry another request with a different armor
when applicable. The Kerberos client MAY process an error message
without a PA-FX-FAST-REPLY, if that is only intended to return better
error information to the application, typically for trouble-shooting
purposes.
In the cases where the e-data field of the KRB-ERROR message is
expected to carry a TYPED-DATA [RFC4120] element, that information
should be transmitted in a pa-data element within the KRBFastResponse
Hartman & Zhu Standards Track [Page 33]
RFC 6113 Kerberos Preauth Framework April 2011
structure. The padata-type is the same as the data-type would be in
the typed data element and the padata-value is the same as the data-
value. As discussed in Section 7, data-types and padata-types are
drawn from the same namespace. For example, the
TD_TRUSTED_CERTIFIERS structure is expected to be in the KRB-ERROR
message when the error code is KDC_ERR_CANT_VERIFY_CERTIFICATE
[RFC4556].
5.4.5. Outer and Inner Requests
Typically, a client will know that FAST is being used before a
request containing PA-FX-FAST is sent. So, the outer AS request
typically only includes one pa-data item: PA-FX-FAST. The client MAY
include additional pa-data, but the KDC MUST ignore the outer request
body and any padata besides PA-FX-FAST if and only if PA-FX-FAST is
processed. In the case of the TGS request, the outer request should
include PA-FX-FAST and PA-TGS-REQ.
When an AS generates a response, all padata besides PA-FX-FAST should
be included in PA-FX-FAST. The client MUST ignore other padata
outside of PA-FX-FAST.
5.4.6. The Encrypted Challenge FAST Factor
The encrypted challenge FAST factor authenticates a client using the
client's long-term key. This factor works similarly to the encrypted
timestamp pre-authentication option described in [RFC4120]. The word
"challenge" is used instead of "timestamp" because while the
timestamp is used as an initial challenge, if the KDC and client do
not have synchronized time, then the KDC can provide updated time to
the client to use as a challenge. The client encrypts a structure
containing a timestamp in the challenge key. The challenge key used
by the client to send a message to the KDC is KRB-FX-
CF2(armor_key,long_term_key, "clientchallengearmor",
"challengelongterm"). The challenge key used by the KDC encrypting
to the client is KRB-FX-CF2(armor_key, long_term_key,
"kdcchallengearmor", "challengelongterm"). Because the armor key is
fresh and random, the challenge key is fresh and random. The only
purpose of the timestamp is to limit the validity of the
authentication so that a request cannot be replayed. A client MAY
base the timestamp on the KDC time in a KDC error and need not
maintain accurate time synchronization itself. If a client bases its
time on an untrusted source, an attacker may trick the client into
producing an authentication request that is valid at some future
time. The attacker may be able to use this authentication request to
make it appear that a client has authenticated at that future time.
If ticket-based armor is used, then the lifetime of the ticket will
limit the window in which an attacker can make the client appear to
Hartman & Zhu Standards Track [Page 34]
RFC 6113 Kerberos Preauth Framework April 2011
have authenticated. For many situations, the ability of an attacker
to cause a client to appear to have authenticated is not a
significant concern; the ability to avoid requiring time
synchronization on clients is more valuable.
The client sends a padata of type PA-ENCRYPTED-CHALLENGE. The
corresponding padata-value contains the DER encoding of ASN.1 type
EncryptedChallenge.
EncryptedChallenge ::= EncryptedData
-- Encrypted PA-ENC-TS-ENC, encrypted in the challenge key
-- using key usage KEY_USAGE_ENC_CHALLENGE_CLIENT for the
-- client and KEY_USAGE_ENC_CHALLENGE_KDC for the KDC.
PA-ENCRYPTED-CHALLENGE 138
KEY_USAGE_ENC_CHALLENGE_CLIENT 54
KEY_USAGE_ENC_CHALLENGE_KDC 55
The client includes some timestamp reasonably close to the KDC's
current time and encrypts it in the challenge key in a PA-ENC-TS-ENC
structure (see Section 5.2.7.2 in RFC 4120). Clients MAY use the
current time; doing so prevents the exposure where an attacker can
cause a client to appear to authenticate in the future. The client
sends the request including this factor.
On receiving an AS-REQ containing the PA-ENCRYPTED-CHALLENGE FAST
factor, the KDC decrypts the timestamp. If the decryption fails the
KDC SHOULD return KDC_ERR_PREAUTH_FAILED, including PA-ETYPE-INFO2 in
the KRBFastResponse in the error. The KDC confirms that the
timestamp falls within its current clock skew returning
KRB_APP_ERR_SKEW if not. The KDC then SHOULD check to see if the
encrypted challenge is a replay. The KDC MUST NOT consider two
encrypted challenges replays simply because the timestamps are the
same; to be a replay, the ciphertext MUST be identical. Allowing
clients to reuse timestamps avoids requiring that clients maintain
state about which timestamps have been used.
If the KDC accepts the encrypted challenge, it MUST include a padata
element of type PA-ENCRYPTED-CHALLENGE. The KDC encrypts its current
time in the challenge key. The KDC MUST strengthen the reply key
before issuing a ticket. The client MUST check that the timestamp
decrypts properly. The client MAY check that the timestamp is within
the window of acceptable clock skew for the client. The client MUST
NOT require that the timestamp be identical to the timestamp in the
issued credentials or the returned message.
Hartman & Zhu Standards Track [Page 35]
RFC 6113 Kerberos Preauth Framework April 2011
The encrypted challenge FAST factor provides the following
facilities: Client Authentication and KDC Authentication. This FAST
factor also takes advantage of the FAST facility to strengthen the
reply key. It does not provide the Replace Reply Key facility. The
Security Considerations section of this document provides an
explanation why the security requirements are met.
The encrypted challenge FAST factor can be useful in an
authentication set. No pa-hint is defined because the only
information needed by this mechanism is information contained in the
PA-ETYPE-INFO2 pre-authentication data. KDCs are already required to
send PA-ETYPE-INFO2. If KDCs were not required to send PA-ETYPE-
INFO2 then that information would need to be part of a hint for
encrypted challenge.
Conforming implementations MUST support the encrypted challenge FAST
factor.
5.5. Authentication Strength Indication
Implementations that have pre-authentication mechanisms offering
significantly different strengths of client authentication MAY choose
to keep track of the strength of the authentication used as an input
into policy decisions. For example, some principals might require
strong pre-authentication, while less sensitive principals can use
relatively weak forms of pre-authentication like encrypted timestamp.
An AuthorizationData data type AD-Authentication-Strength is defined
for this purpose.
AD-authentication-strength 70
The corresponding ad-data field contains the DER encoding of the pre-
authentication data set as defined in Section 5.3. 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. Unless otherwise specified, the hint and value fields of
the members of this sequence MUST be empty. In order to permit
mechanisms to carry additional information about strength in these
fields in the future, clients and application servers MUST ignore
non-empty hint and value fields for mechanisms unless the
implementation is updated with the interpretation of these fields for
a given pre-authentication mechanism in this authorization element.
The AD-authentication-strength element MUST be included in the AD-
KDC-ISSUED container so that the KDC integrity protects its contents.
This element can be ignored if it is unknown to the receiver.
Hartman & Zhu Standards Track [Page 36]
RFC 6113 Kerberos Preauth Framework April 2011
6. Assigned Constants
The pre-authentication framework and FAST involve using a number of
Kerberos protocol constants. This section lists protocol constants
first introduced in this specification drawn from registries not
managed by IANA. Many of these registries would best be managed by
IANA; that is a known issue that is out of scope for this document.
The constants described in this section have been accounted for and
will appear in the next revision of the Kerberos core specification
or in a document creating IANA registries.
Section 7 creates IANA registries for a different set of constants
used by the extensions described in this document.
6.1. New Errors
KDC_ERR_PREAUTH_EXPIRED 90
KDC_ERR_MORE_PREAUTH_DATA_REQUIRED 91
KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET 92
KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS 93
6.2. Key Usage Numbers
KEY_USAGE_FAST_REQ_CHKSUM 50
KEY_USAGE_FAST_ENC 51
KEY_USAGE_FAST_REP 52
KEY_USAGE_FAST_FINISHED 53
KEY_USAGE_ENC_CHALLENGE_CLIENT 54
KEY_USAGE_ENC_CHALLENGE_KDC 55
6.3. Authorization Data Elements
AD-authentication-strength 70
AD-fx-fast-armor 71
AD-fx-fast-used 72
6.4. New PA-DATA Types
PA-FX-COOKIE 133
PA-AUTHENTICATION-SET 134
PA-AUTH-SET-SELECTED 135
PA-FX-FAST 136
PA-FX-ERROR 137
PA-ENCRYPTED-CHALLENGE 138
Hartman & Zhu Standards Track [Page 37]
RFC 6113 Kerberos Preauth Framework April 2011
7. IANA Considerations
This document creates a number of IANA registries. These registries
are all located under Kerberos Parameters on http://www.iana.org.
See [RFC5226] for descriptions of the registration policies used in
this section.
7.1. Pre-Authentication and Typed Data
RFC 4120 defines pre-authentication data, which can be included in a
KDC request or response in order to authenticate the client or extend
the protocol. In addition, it defines Typed-Data, which is an
extension mechanism for errors. Both pre-authentication data and
typed data are carried as a 32-bit signed integer along with an octet
string. The encoding of typed data and pre-authentication data is
slightly different. However, the types for pre-authentication data
and typed-data are drawn from the same namespace. By convention,
registrations starting with TD- are typed data and registrations
starting with PA- are pre-authentication data. It is important that
these data types be drawn from the same namespace, because some
errors where it would be desirable to include typed data require the
e-data field to be formatted as pre-authentication data.
When Kerberos FAST is used, pre-authentication data encoding is
always used.
There is one apparently conflicting registration between typed data
and pre-authentication data. PA-GET-FROM-TYPED-DATA and TD-PADATA
are both assigned the value 22. However, this registration is simply
a mechanism to include an element of the other encoding. The use of
both should be deprecated.
This document creates a registry for pre-authentication and typed
data. The registration procedures are as follows. Expert review for
pre-authentication mechanisms designed to authenticate users, KDCs,
or establish the reply key. The expert first determines that the
purpose of the method is to authenticate clients, KDCs, or to
establish the reply key. If so, expert review is appropriate. The
expert evaluates the security and interoperability of the
specification.
IETF review is required if the expert believes that the pre-
authentication method is broader than these three areas. Pre-
authentication methods that change the Kerberos state machine or
otherwise make significant changes to the Kerberos protocol should be
Standards Track RFCs. A concern that a particular method needs to be
a Standards Track RFC may be raised as an objection during IETF
review.
Hartman & Zhu Standards Track [Page 38]
RFC 6113 Kerberos Preauth Framework April 2011
Several of the registrations indicated below were made at a time when
the Kerberos protocol was less mature and do not meet the current
requirements for this registry. These registrations are included in
order to accurately document what is known about the use of these
protocol code points and to avoid conflicts.
Type Value Reference
----------------------------------------------------------------------
PA-TGS-REQ 1 [RFC4120]
PA-ENC-TIMESTAMP 2 [RFC4120]
PA-PW-SALT 3 [RFC4120]
[reserved] 4 [RFC6113]
PA-ENC-UNIX-TIME 5 (deprecated) [RFC4120]
PA-SANDIA-SECUREID 6 [RFC4120]
PA-SESAME 7 [RFC4120]
PA-OSF-DCE 8 [RFC4120]
PA-CYBERSAFE-SECUREID 9 [RFC4120]
PA-AFS3-SALT 10 [RFC4120] [RFC3961]
PA-ETYPE-INFO 11 [RFC4120]
PA-SAM-CHALLENGE 12 [KRB-WG.SAM]
PA-SAM-RESPONSE 13 [KRB-WG.SAM]
PA-PK-AS-REQ_OLD 14 [PK-INIT-1999]
PA-PK-AS-REP_OLD 15 [PK-INIT-1999]
PA-PK-AS-REQ 16 [RFC4556]
PA-PK-AS-REP 17 [RFC4556]
PA-PK-OCSP-RESPONSE 18 [RFC4557]
PA-ETYPE-INFO2 19 [RFC4120]
PA-USE-SPECIFIED-KVNO 20 [RFC4120]
PA-SVR-REFERRAL-INFO 20 [REFERRALS]
PA-SAM-REDIRECT 21 [KRB-WG.SAM]
PA-GET-FROM-TYPED-DATA 22 (embedded in typed data) [RFC4120]
TD-PADATA 22 (embeds padata) [RFC4120]
PA-SAM-ETYPE-INFO 23 (sam/otp) [KRB-WG.SAM]
PA-ALT-PRINC 24 (crawdad@fnal.gov) [HW-AUTH]
PA-SERVER-REFERRAL 25 [REFERRALS]
PA-SAM-CHALLENGE2 30 (kenh@pobox.com) [KRB-WG.SAM]
PA-SAM-RESPONSE2 31 (kenh@pobox.com) [KRB-WG.SAM]
PA-EXTRA-TGT 41 Reserved extra TGT [RFC6113]
TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS
TD-KRB-PRINCIPAL 102 PrincipalName
TD-KRB-REALM 103 Realm
TD-TRUSTED-CERTIFIERS 104 [RFC4556]
TD-CERTIFICATE-INDEX 105 [RFC4556]
TD-APP-DEFINED-ERROR 106 Application specific [RFC6113]
TD-REQ-NONCE 107 INTEGER [RFC6113]
TD-REQ-SEQ 108 INTEGER [RFC6113]
TD_DH_PARAMETERS 109 [RFC4556]
TD-CMS-DIGEST-ALGORITHMS 111 [ALG-AGILITY]
Hartman & Zhu Standards Track [Page 39]
RFC 6113 Kerberos Preauth Framework April 2011
TD-CERT-DIGEST-ALGORITHMS 112 [ALG-AGILITY]
PA-PAC-REQUEST 128 [MS-KILE]
PA-FOR_USER 129 [MS-KILE]
PA-FOR-X509-USER 130 [MS-KILE]
PA-FOR-CHECK_DUPS 131 [MS-KILE]
PA-AS-CHECKSUM 132 [MS-KILE]
PA-FX-COOKIE 133 [RFC6113]
PA-AUTHENTICATION-SET 134 [RFC6113]
PA-AUTH-SET-SELECTED 135 [RFC6113]
PA-FX-FAST 136 [RFC6113]
PA-FX-ERROR 137 [RFC6113]
PA-ENCRYPTED-CHALLENGE 138 [RFC6113]
PA-OTP-CHALLENGE 141 (gareth.richards@rsa.com) [OTP-PREAUTH]
PA-OTP-REQUEST 142 (gareth.richards@rsa.com) [OTP-PREAUTH]
PA-OTP-CONFIRM 143 (gareth.richards@rsa.com) [OTP-PREAUTH]
PA-OTP-PIN-CHANGE 144 (gareth.richards@rsa.com) [OTP-PREAUTH]
PA-EPAK-AS-REQ 145 (sshock@gmail.com) [RFC6113]
PA-EPAK-AS-REP 146 (sshock@gmail.com) [RFC6113]
PA_PKINIT_KX 147 [RFC6112]
PA_PKU2U_NAME 148 [PKU2U]
PA-SUPPORTED-ETYPES 165 [MS-KILE]
PA-EXTENDED_ERROR 166 [MS-KILE]
7.2. Fast Armor Types
FAST armor types are defined in Section 5.4.1. A FAST armor type is
a signed 32-bit integer. FAST armor types are assigned by standards
action.
Type Name Description
------------------------------------------------------------
0 Reserved.
1 FX_FAST_ARMOR_AP_REQUEST Ticket armor using an ap-req.
7.3. FAST Options
A FAST request includes a set of bit flags to indicate additional
options. Bits 0-15 are critical; other bits are non-critical.
Assigning bits greater than 31 may require special support in
implementations. Assignment of FAST options requires standards
action.
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RFC 6113 Kerberos Preauth Framework April 2011
Type Name Description
-------------------------------------------------------------------
0 RESERVED Reserved for future expansion of this
field.
1 hide-client-names Requesting the KDC to hide client
names in the KDC response
16 kdc-follow-referrals Reserved.
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 denial of service using this option, KDCs MAY
restrict the number of simultaneous active requests with this option
for any given client principal.
Regarding the facilities provided by the Encrypted Challenge FAST
factor, the challenge key is derived from the client secrets and
because the client secrets are known only to the client and the KDC,
the verification of the EncryptedChallenge structure proves the
client's identity, the verification of the EncryptedChallenge
structure in the KDC reply proves that the expected KDC responded.
Therefore, the Encrypted Challenge FAST factor as a pre-
authentication mechanism offers the following facilities: Client
Authentication and KDC Authentication. There is no un-authenticated
cleartext introduced by the Encrypted Challenge FAST factor.
FAST provides an encrypted tunnel over which pre-authentication
conversations can take place. In addition, FAST optionally
authenticates the KDC to the client. It is the responsibility of
FAST factors to authenticate the client to the KDC. Care MUST be
taken to design FAST factors such that they are bound to the
conversation. If this is not done, a man-in-the-middle may be able
to cut&paste a FAST factor from one conversation to another. The
easiest way to do this is to bind each FAST factor to the armor key
that is guaranteed to be unique for each conversation.
The anonymous PKINIT mode for obtaining an armor ticket does not
always authenticate the KDC to the client before the conversation
begins. Tracking the KDC verified state guarantees that by the end
of the conversation, the client has authenticated the KDC. However,
FAST factor designers need to consider the implications of using
their factor when the KDC has not yet been authenticated. If this
proves problematic in an environment, then the particular FAST factor
should not be used with anonymous PKINIT.
Existing pre-authentication mechanisms are believed to be at least as
secure when used with FAST as they are when used outside of FAST.
Hartman & Zhu Standards Track [Page 41]
RFC 6113 Kerberos Preauth Framework April 2011
One part of this security is making sure that when pre-authentication
methods checksum the request, they checksum the inner request rather
than the outer request. If the mechanism checksummed the outer
request, a man-in-the-middle could observe it outside a FAST tunnel
and then cut&paste it into a FAST exchange where the inner rather
than outer request would be used to select attributes of the issued
ticket. Such attacks would typically invalidate auditing information
or create a situation where the client and KDC disagree about what
ticket is issued. However, such attacks are unlikely to allow an
attacker who would not be able to authenticate as a principal to do
so. Even so, FAST is believed to defend against these attacks in
existing legacy mechanism. However, since there is no standard for
how legacy mechanisms bind the request to the pre-authentication or
provide integrity protection, security analysis can be difficult. In
some cases, FAST may significantly improve the integrity protection
of legacy mechanisms.
The security of the TGS exchange depends on authenticating the client
to the KDC. In the AS exchange, this is done using pre-
authentication data or FAST factors. In the TGS exchange, this is
done by presenting a TGT and by using the session (or sub-session)
key in constructing the request. Because FAST uses a request body in
the inner request, encrypted in the armor key, rather than the
request body in the outer request, it is critical that establishing
the armor key be tied to the authentication of the client to the KDC.
If this is not done, an attacker could manipulate the options
requested in the TGS request, for example, requesting a ticket with
different validity or addresses. The easiest way to bind the armor
key to the authentication of the client to the KDC is for the armor
key to depend on the sub-session key of the TGT. This is done with
the implicit TGS armor supported by this specification. Future armor
types designed for use with the TGS MUST either bind their armor keys
to the TGT or provide another mechanism to authenticate the client to
the KDC.
9. Acknowledgements
Sam Hartman would like to thank the MIT Kerberos Consortium for its
funding of his time on this project.
Several suggestions from Jeffrey Hutzelman based on early revisions
of this documents led to significant improvements of this document.
The proposal to ask one KDC to chase down the referrals and return
the final ticket is based on requirements in [CROSS].
Joel Weber had a proposal for a mechanism similar to FAST that
created a protected tunnel for Kerberos pre-authentication.
Hartman & Zhu Standards Track [Page 42]
RFC 6113 Kerberos Preauth Framework April 2011
Srinivas Cheruku and Greg Hudson provided valuable review comments.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications
for Kerberos 5", RFC 3961, February 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)",
RFC 4120, July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT)",
RFC 4556, June 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
[RFC6112] Zhu, L., Leach, P., and S. Hartman "Anonymity Support
for Kerberos", RFC 6112, April 2011.
10.2. Informative References
[ALG-AGILITY] Astrand, L. and L. Zhu, "PK-INIT algorithm agility",
Work in Progress, August 2008.
[CROSS] Sakane, S., Zrelli, S., and M. Ishiyama , "Problem
statement on the cross-realm operation of Kerberos in
a specific system", Work in Progress, July 2007.
[EKE] Bellovin, S. 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.
[HW-AUTH] Crawford, M., "Passwordless Initial Authentication to
Kerberos by Hardware Preauthentication", Work
in Progress, October 2006.
[IEEE1363.2] IEEE, "IEEE P1363.2: Password-Based Public-Key
Cryptography", 2004.
Hartman & Zhu Standards Track [Page 43]
RFC 6113 Kerberos Preauth Framework April 2011
[KRB-WG.SAM] Hornstein, K., Renard, K., Neuman, C., and G. Zorn,
"Integrating Single-use Authentication Mechanisms
with Kerberos", Work in Progress, July 2004.
[MS-KILE] Microsoft, "Kerberos Protocol Extensions", <http://
msdn.microsoft.com/en-us/library/cc206927.aspx>.
[OTP-PREAUTH] Richards, G., "OTP Pre-authentication", Work
in Progress, February 2011.
[PK-INIT-1999] Tung, B., Neuman, C., Hur, M., Medvinsky, A.,
Medvinsky, S., Wray, J., and J. Trostle, "Public Key
Cryptography for Initial Authentication in Kerberos",
Work in Progress, July 1999.
[PKU2U] Zhu, L., Altman, J., and N. Williams, "Public Key
Cryptography Based User-to-User Authentication -
(PKU2U)", Work in Progress, November 2008.
[REFERRALS] Hartman, S., Ed., Raeburn, K., and L. Zhu, "Kerberos
Principal Name Canonicalization and KDC-Generated
Cross-Realm Referrals", Work in Progress, March 2011.
[RFC4557] Zhu, L., Jaganathan, K., and N. Williams, "Online
Certificate Status Protocol (OCSP) Support for Public
Key Cryptography for Initial Authentication in
Kerberos (PKINIT)", RFC 4557, June 2006.
Hartman & Zhu Standards Track [Page 44]
RFC 6113 Kerberos Preauth Framework April 2011
Appendix A. Test Vectors for KRB-FX-CF2
This informative appendix presents test vectors for the KRB-FX-CF2
function. Test vectors are presented for several encryption types.
In all cases, the first key (k1) is the result of string-to-
key("key1", "key1", default_parameters) and the second key (k2) is
the result of string-to-key("key2", "key2", default_parameters).
Both keys are of the same enctype. The presented test vector is the
hexadecimal encoding of the key produced by KRB-FX-CF2(k1, k2, "a",
"b"). The peppers are one-octet ASCII strings.
In performing interoperability testing, there was significant
ambiguity surrounding [RFC3961] pseudo-random operations. These test
vectors assume that the AES pseudo-random operation is
aes-ecb(trunc128(sha-1(input))) where trunc128 truncates its input to
128 bits. The 3DES pseudo-random operation is assumed to be
des3-cbc(trunc128(sha-1(input))). The DES pseudo-random operation is
assumed to be des-cbc(md5(input)). As specified in RFC 4757, the RC4
pseudo-random operation is hmac-sha1(input).
Interoperability testing also demonstrated ambiguity surrounding the
DES random-to-key operation. The random-to-key operation is assumed
to be distribute 56 bits into high-7-bits of 8 octets and generate
parity.
These test vectors were produced with revision 22359 of the MIT
Kerberos sources. The AES 256 and AES 128 test vectors have been
confirmed by multiple other implementors. The RC4 test vectors have
been confirmed by one other implementor. The DES and triple DES test
vectors have not been confirmed.
aes 128 (enctype 17): 97df97e4b798b29eb31ed7280287a92a
AES256 (enctype 18): 4d6ca4e629785c1f01baf55e2e548566
b9617ae3a96868c337cb93b5e72b1c7b
DES (enctype 1): 43bae3738c9467e6
3DES (enctype 16): e58f9eb643862c13ad38e529313462a7f73e62834fe54a01
RC4 (enctype 23): 24d7f6b6bae4e5c00d2082c5ebab3672
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RFC 6113 Kerberos Preauth Framework April 2011
Appendix B. 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-ENC-TS-ENC, PA-DATA, KDC-REQ-BODY,
Microseconds, KerberosFlags, UInt32
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-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM
PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE {
pa-type [0] Int32,
-- same as padata-type.
pa-hint [1] OCTET STRING OPTIONAL,
pa-value [2] OCTET STRING OPTIONAL,
...
}
KrbFastArmor ::= SEQUENCE {
armor-type [0] Int32,
-- Type of the armor.
armor-value [1] OCTET STRING,
-- Value of the armor.
...
}
PA-FX-FAST-REQUEST ::= CHOICE {
armored-data [0] KrbFastArmoredReq,
...
}
KrbFastArmoredReq ::= SEQUENCE {
armor [0] KrbFastArmor OPTIONAL,
-- Contains the armor that identifies the armor key.
-- MUST be present in AS-REQ.
req-checksum [1] Checksum,
-- For AS, contains the checksum performed over the type
-- KDC-REQ-BODY for the req-body field of the KDC-REQ
-- structure;
-- For TGS, contains the checksum performed over the type
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RFC 6113 Kerberos Preauth Framework April 2011
-- AP-REQ in the PA-TGS-REQ padata.
-- 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_REQ_CHKSUM.
enc-fast-req [2] EncryptedData, -- KrbFastReq --
-- The encryption key is the armor key, and the key usage
-- number is KEY_USAGE_FAST_ENC.
...
}
KrbFastReq ::= SEQUENCE {
fast-options [0] FastOptions,
-- Additional options.
padata [1] SEQUENCE OF PA-DATA,
-- padata typed holes.
req-body [2] KDC-REQ-BODY,
-- Contains the KDC request body as defined in Section
-- 5.4.1 of [RFC4120].
-- This req-body field is preferred over the outer field
-- in the KDC request.
...
}
FastOptions ::= KerberosFlags
-- reserved(0),
-- hide-client-names(1),
-- kdc-follow-referrals(16)
PA-FX-FAST-REPLY ::= CHOICE {
armored-data [0] KrbFastArmoredRep,
...
}
KrbFastArmoredRep ::= SEQUENCE {
enc-fast-rep [0] EncryptedData, -- KrbFastResponse --
-- The encryption key is the armor key in the request, and
-- the key usage number is KEY_USAGE_FAST_REP.
...
}
KrbFastResponse ::= SEQUENCE {
padata [0] SEQUENCE OF PA-DATA,
-- padata typed holes.
strengthen-key [1] EncryptionKey OPTIONAL,
-- This, if present, strengthens the reply key for AS and
-- TGS. MUST be present for TGS
-- MUST be absent in KRB-ERROR.
Hartman & Zhu Standards Track [Page 47]
RFC 6113 Kerberos Preauth Framework April 2011
finished [2] KrbFastFinished OPTIONAL,
-- Present in AS or TGS reply; absent otherwise.
nonce [3] UInt32,
-- Nonce from the client request.
...
}
KrbFastFinished ::= SEQUENCE {
timestamp [0] KerberosTime,
usec [1] Microseconds,
-- timestamp and usec represent the time on the KDC when
-- the reply was generated.
crealm [2] Realm,
cname [3] PrincipalName,
-- Contains the client realm and the client name.
ticket-checksum [4] Checksum,
-- checksum of the ticket in the KDC-REP using the armor
-- and the key usage is KEY_USAGE_FAST_FINISH.
-- The checksum type is the required checksum type
-- of the armor key.
...
}
EncryptedChallenge ::= EncryptedData
-- Encrypted PA-ENC-TS-ENC, encrypted in the challenge key
-- using key usage KEY_USAGE_ENC_CHALLENGE_CLIENT for the
-- client and KEY_USAGE_ENC_CHALLENGE_KDC for the KDC.
END
Authors' Addresses
Sam Hartman
Painless Security
EMail: hartmans-ietf@mit.edu
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: larry.zhu@microsoft.com
Hartman & Zhu Standards Track [Page 48]
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