Network Working Group L. Zhu
Request for Comments: 4537 P. Leach
Updates: 4120 K. Jaganathan
Category: Standards Track Microsoft Corporation
June 2006
Kerberos Cryptosystem Negotiation Extension
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies an extension to the Kerberos protocol as
defined in RFC 4120, in which the client can send a list of supported
encryption types in decreasing preference order, and the server then
selects an encryption type that is supported by both the client and
the server.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . . 2
3. Negotiation Extension . . . . . . . . . . . . . . . . . . . . . 2
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . . 4
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RFC 4537 Enctype Negotiation June 2006
1. Introduction
Under the current mechanism [RFC4120], the Kerberos Distribution
Center (KDC) must limit the ticket session key encryption type
(enctype) chosen for a given server to one it believes is supported
by both the client and the server. If both the client and server
understand a stronger enctype than the one selected by the KDC, they
cannot negotiate it. As the result, the protection of application
traffic is often weaker than necessary when the server can support
different sets of enctypes depending on the server application
software being used.
This document specifies an extension to the Kerberos protocol to
allow clients and servers to negotiate use of a different and
possibly stronger cryptosystem in subsequent communication.
This extension utilizes an authorization data element in the
authenticator of the AP-REQ message [RFC4120]. The client sends the
list of enctypes that it supports to the server; the server then
informs the client of its choice. The negotiated subkey is sent in
the AP-REP message [RFC4120].
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Negotiation Extension
If the client prefers an enctype over that of the service ticket
session key, then it SHOULD send a list of enctypes in decreasing
preference order to the server. Based on local policy, the client
selects enctypes out of all the enctypes available locally to be
included in this list, and it SHOULD NOT include enctypes that are
less preferable than that of the ticket session key in the service
ticket. In addition, the client SHOULD NOT include negative (local-
use) enctype numbers unless it knows a priori that the server has
been configured to use the same negative enctype numbers for the same
enctypes.
The client sends the enctype list via the authorization-data of the
authenticator in the AP-REQ [RFC4120]. A new authorization data
element type AD-ETYPE-NEGOTIATION is defined.
AD-ETYPE-NEGOTIATION 129
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RFC 4537 Enctype Negotiation June 2006
This authorization data element itself is enclosed in the AD-IF-
RELEVANT container; thus, a correctly implemented server that does
not understand this element should ignore it [RFC4120]. The value of
this authorization element contains the DER [X680] [X690] encoding of
the following ASN.1 type:
EtypeList ::= SEQUENCE OF Int32
-- Specifies the enctypes supported by the client.
-- This enctype list is in decreasing preference order
-- (favorite choice first).
-- Int32 is defined in [RFC4120].
If the EtypeList is present and the server prefers an enctype from
the client's enctype list over that of the AP-REQ authenticator
subkey (if that is present) or the service ticket session key, the
server MUST create a subkey using that enctype. This negotiated
subkey is sent in the subkey field of AP-REP message, and it is then
used as the protocol key or base key [RFC3961] for subsequent
communication.
If the enctype of the ticket session key is included in the enctype
list sent by the client, it SHOULD be the last on the list;
otherwise, this enctype MUST NOT be negotiated if it was not included
in the list.
This negotiation extension SHOULD NOT be used when the client does
not expect the subkey in the AP-REP message from the server.
A note on key generation: The KDC has a strong Pseudo-Random Number
Generator (PRNG); as such, the client can take advantage of the
randomness provided by the KDC by reusing the KDC key data when
generating keys. Implementations SHOULD use the service ticket
session key value as a source of additional entropy when generating
the negotiated subkey. If the AP-REQ authenticator subkey is
present, it MAY also be used as a source of entropy.
The server MAY ignore the preference order indicated by the client.
The policy by which the client or the server chooses an enctype
(i.e., how the preference order for the supported enctypes is
selected) is a local matter.
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RFC 4537 Enctype Negotiation June 2006
4. Security Considerations
The client's enctype list and the server's reply enctype are part of
encrypted data; thus, the security considerations are the same as
those of the Kerberos encrypted data.
Both the EtypeList and the server's sub-session key are protected by
the session key or sub-session key used for the AP-REQ, and as a
result, if a key for a stronger enctype is negotiated underneath a
key for a weaker enctype, an attacker capable of breaking the weaker
enctype can also discover the key for the stronger enctype. The
advantage of this extension is to minimize the amount of cipher text
encrypted under a weak enctype to which an attacker has access.
5. Acknowledgements
The authors would like to thank the following individuals for their
comments and suggestions: Ken Raeburn, Luke Howard, Tom Yu, Love
Hornquist Astrand, Sam Hartman, and Martin Rex.
6. 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.
[X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
Zhu, et al. Standards Track [Page 4]
RFC 4537 Enctype Negotiation June 2006
Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: lzhu@microsoft.com
Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: paulle@microsoft.com
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: karthikj@microsoft.com
Zhu, et al. Standards Track [Page 5]
RFC 4537 Enctype Negotiation June 2006
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Zhu, et al. Standards Track [Page 6]