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rfc:rfc4537

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

Zhu, et al. Standards Track [Page 1] 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

Zhu, et al. Standards Track [Page 2] 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.

Zhu, et al. Standards Track [Page 3] 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

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 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.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Zhu, et al. Standards Track [Page 6]

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