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CVS shishi/doc/specifications
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shishi-commit |
Subject: |
CVS shishi/doc/specifications |
Date: |
Tue, 25 Oct 2005 01:02:11 +0200 |
Update of /home/cvs/shishi/doc/specifications
In directory dopio:/tmp/cvs-serv14605
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draft-zhu-kerb-enctype-nego-04.txt
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2005/10/24 23:02:10 1.1
NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) K. Jaganathan
Expires: April 24, 2006 Microsoft Corporation
October 21, 2005
Kerberos Cryptosystem Negotiation Extension
draft-zhu-kerb-enctype-nego-04
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 April 24, 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 April 24, 2006 [Page 1]
Internet-Draft Enctype Negotiation October 2005
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. Negotiation Extension . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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 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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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.
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.
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Internet-Draft Enctype Negotiation October 2005
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. Acknowledgments
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. 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.
[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).
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Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: address@hidden
Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: address@hidden
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: address@hidden
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Internet-Draft Enctype Negotiation October 2005
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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