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heimdal/doc/standardisation/draft-zhu-kerb-enctype-nego-03.txt
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NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) K. Jaganathan
Expires: January 20, 2006 Microsoft Corporation
July 19, 2005
Kerberos Cryptosystem Negotiation Extension
draft-zhu-kerb-enctype-nego-03
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 20, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies an extension to the Kerberos protocol where
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.
Zhu, et al. Expires January 20, 2006 [Page 1]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Negotiation Extension . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . 7
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1. Introduction
Under the current mechanism [RFC4120], the 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 can not 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 a different and possible
stronger cryptosystem to be used 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 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 sends the list of enctypes it supports
(including the one selected by the KDC) in decreasing preference
order.
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
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 [X690] encoding of the
following ASN.1 type:
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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.
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.
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: Luke Howard, Tom Yu, Love Hornquist
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Astrand, Sam Harman, Ken Raeburn and Martin Rex.
6. IANA Considerations
No IANA actions are required for this document.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[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.
[X690] ASN.1 encoding rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER), ITU-T Recommendation X.690 (1997) |
ISO/IEC International Standard 8825-1:1998.
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
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Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: karthikj@microsoft.com
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Intellectual Property Statement
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Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
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Disclaimer of Validity
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Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Zhu, et al. Expires January 20, 2006 [Page 7]
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