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Internet Draft                                       IPsec Working Group
November 2001                                           S. Frankel, NIST
Expiration Date: May 2002                            S. Kelly, SonicWALL


          The HMAC-SHA-256-96 Algorithm and Its Use With IPsec
                 <draft-ietf-ipsec-ciph-sha-256-00.txt>




Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of Internet-Drafts Shadow Directories can be accessed at
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   This document is a submission to the IETF Internet Protocol Security
   (IPsec) Working Group. Comments are solicited and should be addressed
   to the working group mailing list (ipsec@lists.tislabs.com) or to the
   editors.

   Distribution of this memo is unlimited.





















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INTERNET DRAFT   <draft-ietf-ipsec-ciph-sha-256-00.txt>    November 2001


                             Table of Contents



 1. Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
 2. Specification of Requirements  . . . . . . . . . . . . . . . . .   3
 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .   3
 4. The HMAC-SHA-256-96 Algorithm  . . . . . . . . . . . . . . . . .   3
    4.1 Keying Material  . . . . . . . . . . . . . . . . . . . . . .   4
    4.2 Padding  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
    4.3 Truncation . . . . . . . . . . . . . . . . . . . . . . . . .   4
    4.4 Interaction with the ESP Cipher Mechanism  . . . . . . . . .   5
    4.5 Performance  . . . . . . . . . . . . . . . . . . . . . . . .   5
    4.6 Test Vectors . . . . . . . . . . . . . . . . . . . . . . . .   5
 5. IKE Interactions . . . . . . . . . . . . . . . . . . . . . . . .   5
    5.1 Phase 1 Identifier . . . . . . . . . . . . . . . . . . . . .   5
    5.2 Phase 2 Identifier . . . . . . . . . . . . . . . . . . . . .   5
 6. Security Considerations  . . . . . . . . . . . . . . . . . . . .   6
 7. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . .   6
 8. Intellectual Property Rights Statement . . . . . . . . . . . . .   6
 9. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .   6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .   7
12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . .   8

































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INTERNET DRAFT   <draft-ietf-ipsec-ciph-sha-256-00.txt>    November 2001


1. Abstract

   Ths document describes the use of the HMAC algorithm in conjunction
   with the SHA-256 algorithm as an authentication mechanism within the
   context of the IPsec Authentication Header and the IPsec Encapsulat-
   ing Security Payload. HMAC with SHA-256 provides data origin authen-
   tication and integrity protection. This version of the HMAC-SHA-256
   authenticator specifies truncation to 96 bits, and is therefore named
   HMAC-SHA-256-96.

2. Specification of Requirements

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" that
   appear in this document are to be interpreted as described in
   [RFC-2119].

3. Introduction

   This document specifies the use of SHA-256 [SHA2-1] combined with
   HMAC [HMAC] as a keyed authentication mechanism within the context of
   the Encapsulating Security Payload [ESP] and the Authentication Head-
   er [AH].  This algorithm is named HMAC-SHA-256-96.  For further in-
   formation on ESP, refer to [ESP] and [ROADMAP].  For further informa-
   tion on AH, refer to [AH] and [ROADMAP].

   Using the SHA-256 block cipher, with its increased block size (512
   bits) and increased hash length (256 bits), provides the new algo-
   rithm with the ability to withstand continuing advances in crypto-an-
   alytic techniques and computational capability.  It also allows less
   frequent re-keying, which is useful for high-speed networks and high-
   volume applications.

   The goal of HMAC-SHA-256-96 is to ensure that the packet is authentic
   and cannot be modified in transit.  Data integrity and data origin
   authentication as provided by HMAC-SHA-256-96 are dependent upon the
   scope of the distribution of the secret key. If the key is known only
   by the source and destination, this algorithm will provide both data
   origin authentication and data integrity for packets sent between the
   two parties.  In addition, only a party with the identical key can
   verify the hash.

4. The HMAC-SHA-256-96 Algorithm

   [SHA2-1] and [SHA2-2] describe the underlying SHA-256 algorithm,
   while [HMAC] describes the HMAC algorithm. The HMAC algorithm pro-
   vides a framework for inserting various hashing algorithms such as
   SHA-256.

   The following sections contain descriptions of the various character-
   istics and requirements of the HMAC-SHA-256-96 algorithm.






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4.1 Keying Material

   HMAC-SHA-256-96 is a secret key algorithm. While no fixed key length
   is specified in [HMAC], for use with either ESP or AH a fixed key
   length of 256-bits MUST be supported.  Key lengths other than 256-
   bits MUST NOT be supported (i.e. only 256-bit keys are to be used by
   HMAC-SHA-256-96).  A key length of 256-bits was chosen based on the
   recommendations in [HMAC] (i.e. key lengths less than the authentica-
   tor length decrease security strength and keys longer than the au-
   thenticator length do not significantly increase security strength).

   [HMAC] discusses requirements for key material, which includes a dis-
   cussion on requirements for strong randomness. A strong pseudo-random
   function MUST be used to generate the required 256-bit key.

   At the time of this writing there are no specified weak keys for use
   with HMAC. This does not mean to imply that weak keys do not exist.
   If, at some point, a set of weak keys for HMAC are identified, the
   use of these weak keys MUST be rejected followed by a request for re-
   placement keys or a newly negotiated Security Association.

   [ARCH] describes the general mechanism for obtaining keying material
   when multiple keys are required for a single SA (e.g. when an ESP SA
   requires a key for confidentiality and a key for authentication).

   In order to provide data origin authentication, the key distribution
   mechanism must ensure that unique keys are allocated and that they
   are distributed only to the parties participating in the communica-
   tion.

   [HMAC] makes the following recommendation with regard to rekeying.
   Current attacks do not necessitate a specific recommended frequency
   for key changes. However, periodic key refreshment is a fundamental
   security practice that helps against potential weaknesses of the
   function and the keys, reduces the information available to a crypt-
   analyst, and limits the damage resulting from a compromised key.

4.2 Padding

   HMAC-SHA-256-96 operates on 512-bit blocks of data. Padding require-
   ments are specified in [SHA2-1] and are part of the SHA-256 algo-
   rithm. If you build SHA-256 according to [SHA2-1] you do not need to
   add any additional padding as far as HMAC-SHA-256-96 is concerned.
   With regard to "implicit packet padding" as defined in [AH], no im-
   plicit packet padding is required.


4.3 Truncation

   HMAC-SHA-256-96 produces a 256-bit authenticator value. This 256-bit
   value can be truncated as described in [HMAC].  For use with either
   ESP or AH, a truncated value using the first 96 bits MUST be support-
   ed.  Upon sending, the truncated value is stored within the authenti-
   cator field. Upon receipt, the entire 256-bit value is computed and



Frankel,Kelly                                                   [Page 4]


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   the first 96 bits are compared to the value stored in the authentica-
   tor field. No other authenticator value lengths are supported by
   HMAC-SHA-256-96.

   The length of 96 bits was selected because it is the default authen-
   ticator length as specified in [AH] and meets the security require-
   ments described in [HMAC]. [HMAC] discusses the potential additional
   security which is provided by the truncation of the resulting hash.
   Specifications which include HMAC are strongly encouraged to perform
   this hash truncation.

4.4 Interaction with the ESP Cipher Mechanism

   As of this writing, there are no known issues which preclude the use
   of the HMAC-SHA-256-96 with any specific cipher algorithm.

4.5 Performance

   [HASH] states that "(HMAC) performance is essentially that of the un-
   derlying hash function".  As of this writing no detailed performance
   analysis has been done of SHA-256, HMAC or HMAC combined with
   SHA-256.

   [HMAC] outlines an implementation modification which can improve per-
   packet performance without affecting interoperability.

4.6 Test Vectors

   TBD

5. IKE Interactions

5.1 Phase 1 Identifier

   For Phase 1 negotiations, IANA has assigned a Hash Algorithm ID of 4
   for SHA2-256.

   For further information on the use of Hash Algorithm IDs within IKE,
   see [IKE].

5.2 Phase 2 Identifier

   For Phase 2 negotiations, IANA has assigned an AH Transform Identifi-
   er of 5 for AH_SHA2-256.

   For Phase 2 negotiations, IANA has assigned an AH/ESP Authentication
   Attribute Value of 5 for HMAC-SHA2-256.

   For further information on the use of Transform Identifiers and At-
   tributes Values within IKE, see [IKE] and [DOI].







Frankel,Kelly                                                   [Page 5]


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6. Security Considerations

   The security provided by HMAC-SHA-256-96 is based upon the strength
   of HMAC and, to a lesser degree, the strength of SHA-256.  At the
   time of this writing there are no practical cryptographic attacks
   against HMAC-SHA-256-96.

   As is true with any cryptographic algorithm, part of its strength
   lies in the correctness of the algorithm implementation, the security
   of the key management mechanism and its implementation, the strength
   of the associated secret key, and upon the correctness of the imple-
   mentation in all of the participating systems.  This draft contains
   test vectors to assist in verifying the correctness of HMAC-
   SHA-256-96 code.

7. IANA Considerations

   IANA has assigned Hash Algorithm ID 4 to SHA2-256.
   IANA has assigned AH Transform Identifier 5 to AH_SHA2-256.
   IANA has assigned AH/ESP Authentication Attribute Value 5 to HMAC-
   SHA2-256.


8. Intellectual Property Rights Statement


   Pursuant to the provisions of [RFC-2026], the authors represent that
   they have disclosed the existence of any proprietary or intellectual
   property rights in the contribution that are reasonably and personal-
   ly known to the authors.  The authors do not represent that they per-
   sonally know of all potentially pertinent proprietary and intellectu-
   al property rights owned or claimed by the organizations they repre-
   sent or third parties.

   The IETF takes no position regarding the validity or scope of any in-
   tellectual property or other rights that might be claimed to pertain
   to the implementation or use of the technology described in this doc-
   ument or the extent to which any license under such rights might or
   might not be available; neither does it represent that it has made
   any effort to identify any such rights.  Information on the IETF's
   procedures with respect to rights in standards-track and standards-
   related documentation can be found in BCP-11.  Copies of claims of
   rights made available for publication and any assurances of licenses
   to be made available, or the result of an attempt made to obtain a
   general license or permission for the use of such proprietary rights
   by implementers or users of this specification can be obtained from
   the IETF Secretariat.

9. Acknowledgments

   Portions of this text were unabashedly borrowed from [HMAC-SHA].






Frankel,Kelly                                                   [Page 6]


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10. References


     [AH]        Kent, S. and R. Atkinson, "IP Authentication Header",
                 RFC 2402, November 1998.

     [ARCH]      Kent, S. and R. Atkinson, "Security Architecture for
                 the Internet Protocol", RFC 2401, November 1998.

     [DOI]       Piper, D., "The Internet IP Security Domain of
                 Interpretation for ISAKMP,"

     [ESP]       Kent, S. and R. Atkinson, "IP Encapsulating Security
                 Payload (ESP)", RFC 2406, November 1998.

     [HASH]      Bellare, M., R. Canetti and H. Krawczyk, "Keying Hash
                 Functions for Message Authentication," Advances in
                 Cryptography, Crypto96 Proceedings, June 1996.

     [HMAC]      Krawczyk, H., M. Bellare and R. Canetti, "HMAC: Keyed-
                 Hashing for Message Authentication," RFC 2104, February
                 1997.

     [HMAC-SHA]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
                 within ESP and AH," RFC 2404, November 1998.

     [IKE]       Harkins, D. and D. Carrel, "The Internet Key Exchange
                 (IKE)", RFC 2409, November 1998.

     [RFC-2026]  Bradner, S., "The Internet Standards Process --
                 Revision 3", RFC2026, October 1996.

     [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", RFC-2119, March 1997.

     [ROADMAP]   Thayer, R., N. Doraswamy, and R. Glenn, "IP Security
                 Document Roadmap", RFC 2411, November 1998.

     [SHA2-1]    "Specifications for the Secure Hash Standard," Draft
                 FIPS 180-2, May 2001.
                 http://csrc.nist.gov/encryption/shs/dfips-180-2.pdf.

     [SHA2-2]    "Descriptions of SHA-256, SHA-384, and SHA-512."
                 http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf.


11. Authors' Addresses


        Sheila Frankel
        NIST
        820 West Diamond Ave.
        Room 680
        Gaithersburg, MD 20899



Frankel,Kelly                                                   [Page 7]


INTERNET DRAFT   <draft-ietf-ipsec-ciph-sha-256-00.txt>    November 2001


        Phone: +1 (301) 975-3297
        Email: sheila.frankel@nist.gov

        Scott Kelly
        SonicWALL, Inc.
        1160 Bordeaux Dr.
        Sunnyvale, CA 94089
        Phone: +1 (408) 745-9600
        Email: skelly@sonicwall.com

   The IPsec working group can be contacted through the chairs:

        Barbara Fraser
        Cisco Systems Inc.
        Email: byfraser@cisco.com

        Theodore T'so
        Massachusetts Institute of Technology
        Email: tytso@mit.edu

12. Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this doc-
   ument itself may not be modified in any way, such as by removing the
   copyright notice or references to the Internet Society or other In-
   ternet organizations, except as needed for the purpose of developing
   Internet standards in which case the procedures for copyrights de-
   fined in the Internet Standards process must be followed, or as re-
   quired to translate it into languages other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HERE-
   IN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
   CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.











Frankel,Kelly                                                   [Page 8]


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