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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.
+
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+Zhu, et al. Expires January 20, 2006 [Page 1]
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+Internet-Draft Enctype Negotiation July 2005
+
+
+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
+
+ 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
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+Disclaimer of Validity
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+ This document and the information contained herein are provided on an
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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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+Zhu, et al. Expires January 20, 2006 [Page 7]
+\f
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