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NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Expires: October 2, 2005 K. Jaganathan
Microsoft Corporation
March 31, 2005
Kerberos Cryptosystem Negotiation Extension
draft-zhu-kerb-enctype-nego-01
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with
RFC 3668.
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 October 2, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies an extension by Kerberos to negotiate new
encryption types between the client-server peers.
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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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . . 4
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 5
A. Leveraging this Enctype Negotiation in Windows SPNEGO
Implementations . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . 7
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1. Introduction
Under the current mechanism [CLAR], the KDC must limit the ticket
session key 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 Kerberos 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 [CLAR]. 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 [CLAR].
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 MUST send 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 [CLAR]. A new authorization data element
type AD-ETYPE-NEGOTIATION (129) is defined. 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 [CLAR]. The value of this authorization element
contains the DER [X60] 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 [CLAR].
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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 MUST be
used for subsequent communication.
This negotiation extension MUST NOT be used when the client does not
expect the subkey in the AP-REP message from the server.
Note that to preserve the quality of randomness provided by the KDC,
implementations of this extension SHOULD consider using 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 policy by which the client or the server chooses an enctype
(i.e., how the preference order for the supported enctypes is
selected) is an implementation-specific 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.
In all cases, the communicating peers are exposed to the denial of
service threat.
5. IANA Considerations
No IANA actions are required for this document.
6. Normative References
[CLAR] RFC-Editor: To be replaced by RFC number for draft-ietf-
krb-wg-kerberos-clarifications. Work in Progress.
[GSS-CFX] RFC-Editor: To be replaced by RFC number for draft-ietf-
krb-wg-gssapi-cfx. Work in Progress.
[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.
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[SPNEGOBIS]
RFC-Editor: To be replaced by RFC number for draft-ietf-
kitten-2478bis. Work in progress.
[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
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: karthikj@microsoft.com
Appendix A. Leveraging this Enctype Negotiation in Windows SPNEGO
Implementations
The SPNEGO implementations in Windows 2000, Windows XP and Windows
2003 do not generate or verify the mechlistMIC field when it is
required [SPNEGOBIS].
When the SPNEGO implementations that are updated according to
[SPNEGOBIS], an SSPI initiator or acceptor needs to determine if the
peer is updated, so that it can generate the mechlistMIC token when
the peer can process it. With the bidirectional negotiation, the
updated SPNEGO implementation can achieve the following two goals:
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o It can remain backward compatible with legacy implementations, if
local policy allows unsafe and unprotected negotiation with
downlevel implementations when the mechlistMIC token exchange
would otherwise be required by [SPNEGOBIS].
o The mechanism negotiation is protected according to [SPNEGOBIS]
when both peers are updated.
However, the updated SPNEGO implementation itself can not securely
inform the peer whether the local implementation is updated, thus it
has to obtain such information from the negotiated mechanism.
For Windows SPNEGO implementations, both the initiator and the
acceptor are assumed to have been updated if a "newer" [CLAR] or
different enctype is negotiated for use by the Kerberos GSS-API
mechanism.
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Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
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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on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
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The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
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Disclaimer of Validity
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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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.
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