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rfc:rfc2404

Network Working Group C. Madson Request for Comments: 2404 Cisco Systems Inc. Category: Standards Track R. Glenn

                                                                  NIST
                                                         November 1998
             The Use of HMAC-SHA-1-96 within ESP and AH

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 (1998).  All Rights Reserved.

Abstract

 This memo describes the use of the HMAC algorithm [RFC-2104] in
 conjunction with the SHA-1 algorithm [FIPS-180-1] as an
 authentication mechanism within the revised IPSEC Encapsulating
 Security Payload [ESP] and the revised IPSEC Authentication Header
 [AH]. HMAC with SHA-1 provides data origin authentication and
 integrity protection.
 Further information on the other components necessary for ESP and AH
 implementations is provided by [Thayer97a].

1. Introduction

 This memo specifies the use of SHA-1 [FIPS-180-1] combined with HMAC
 [RFC-2104] as a keyed authentication mechanism within the context of
 the Encapsulating Security Payload and the Authentication Header.
 The goal of HMAC-SHA-1-96 is to ensure that the packet is authentic
 and cannot be modified in transit.
 HMAC is a secret key authentication algorithm. Data integrity and
 data origin authentication as provided by HMAC are dependent upon the
 scope of the distribution of the secret key. If only the source and
 destination know the HMAC key, this provides both data origin

Madson & Glenn Standards Track [Page 1] RFC 2404 The Use of HMAC-SHA-1-96 within ESP and AH November 1998

 authentication and data integrity for packets sent between the two
 parties; if the HMAC is correct, this proves that it must have been
 added by the source.
 In this memo, HMAC-SHA-1-96 is used within the context of ESP and AH.
 For further information on how the various pieces of ESP - including
 the confidentiality mechanism -- fit together to provide security
 services, refer to [ESP] and [Thayer97a]. For further information on
 AH, refer to [AH] and [Thayer97a].
 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 [RFC 2119].

2. Algorithm and Mode

 [FIPS-180-1] describes the underlying SHA-1 algorithm, while [RFC-
 2104] describes the HMAC algorithm. The HMAC algorithm provides a
 framework for inserting various hashing algorithms such as SHA-1.
 HMAC-SHA-1-96 operates on 64-byte blocks of data.  Padding
 requirements are specified in [FIPS-180-1] and are part of the SHA-1
 algorithm.  If you build SHA-1 according to [FIPS-180-1] you do not
 need to add any additional padding as far as HMAC-SHA-1-96 is
 concerned.  With regard to "implicit packet padding" as defined in
 [AH] no implicit packet padding is required.
 HMAC-SHA-1-96 produces a 160-bit authenticator value.  This 160-bit
 value can be truncated as described in RFC2104.  For use with either
 ESP or AH, a truncated value using the first 96 bits MUST be
 supported.  Upon sending, the truncated value is stored within the
 authenticator field.  Upon receipt, the entire 160-bit value is
 computed and the first 96 bits are compared to the value stored in
 the authenticator field.  No other authenticator value lengths are
 supported by HMAC-SHA-1-96.
 The length of 96 bits was selected because it is the default
 authenticator length as specified in [AH] and meets the security
 requirements described in [RFC-2104].

2.1 Performance

 [Bellare96a] states that "(HMAC) performance is essentially that of
 the underlying hash function".  As of this writing no detailed
 performance analysis has been done of SHA-1, HMAC or HMAC combined
 with SHA-1.

Madson & Glenn Standards Track [Page 2] RFC 2404 The Use of HMAC-SHA-1-96 within ESP and AH November 1998

 [RFC-2104] outlines an implementation modification which can improve
 per-packet performance without affecting interoperability.

3. Keying Material

 HMAC-SHA-1-96 is a secret key algorithm. While no fixed key length is
 specified in [RFC-2104], for use with either ESP or AH a fixed key
 length of 160-bits MUST be supported.  Key lengths other than 160-
 bits MUST NOT be supported (i.e. only 160-bit keys are to be used by
 HMAC-SHA-1-96).  A key length of 160-bits was chosen based on the
 recommendations in [RFC-2104] (i.e. key lengths less than the
 authenticator length decrease security strength and keys longer than
 the authenticator length do not significantly increase security
 strength).
 [RFC-2104] discusses requirements for key material, which includes a
 discussion on requirements for strong randomness.  A strong pseudo-
 random function MUST be used to generate the required 160-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
 replacement 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
 communication.
 [RFC-2104] makes the following recommendation with regard to
 rekeying.  Current attacks do not indicate a specific recommended
 frequency for key changes as these attacks are practically
 infeasible.  However, periodic key refreshment is a fundamental
 security practice that helps against potential weaknesses of the
 function and keys, reduces the information avaliable to a
 cryptanalyst, and limits the damage of an exposed key.

4. Interaction with the ESP Cipher Mechanism

 As of this writing, there are no known issues which preclude the use
 of the HMAC-SHA-1-96 algorithm with any specific cipher algorithm.

Madson & Glenn Standards Track [Page 3] RFC 2404 The Use of HMAC-SHA-1-96 within ESP and AH November 1998

5. Security Considerations

 The security provided by HMAC-SHA-1-96 is based upon the strength of
 HMAC, and to a lesser degree, the strength of SHA-1.  At the time of
 this writing there are no practical cryptographic attacks against
 HMAC-SHA-1-96.
 [RFC-2104] states that for "minimally reasonable hash functions" the
 "birthday attack" is impractical.  For a 64-byte block hash such as
 HMAC-SHA-1-96, an attack involving the successful processing of 2**80
 blocks would be infeasible unless it were discovered that the
 underlying hash had collisions after processing 2**30 blocks.  A hash
 with such weak collision-resistance characteristics would generally
 be considered to be unusable.
 It is also important to consider that while SHA-1 was never developed
 to be used as a keyed hash algorithm, HMAC had that criteria from the
 onset.
 [RFC-2104] also 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.
 As [RFC-2104] provides a framework for incorporating various hash
 algorithms with HMAC, it is possible to replace SHA-1 with other
 algorithms such as MD5. [RFC-2104] contains a detailed discussion on
 the strengths and weaknesses of HMAC algorithms.
 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
 implementation in all of the participating systems.  [RFC-2202]
 contains test vectors and example code to assist in verifying the
 correctness of HMAC-SHA-1-96 code.

6. Acknowledgments

 This document is derived in part from previous works by Jim Hughes,
 those people that worked with Jim on the combined DES/CBC+HMAC-MD5
 ESP transforms, the ANX bakeoff participants, and the members of the
 IPsec working group.
 We would also like to thank Hugo Krawczyk for his comments and
 recommendations regarding some of the cryptographic specific text in
 this document.

Madson & Glenn Standards Track [Page 4] RFC 2404 The Use of HMAC-SHA-1-96 within ESP and AH November 1998

7. References

 [FIPS-180-1] NIST, FIPS PUB 180-1: Secure Hash Standard,
              April 1995.
              http://csrc.nist.gov/fips/fip180-1.txt (ascii)
              http://csrc.nist.gov/fips/fip180-1.ps  (postscript)
 [RFC-2104]   Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.
 [Bellare96a] Bellare, M., Canetti, R., and H. Krawczyk, "Keying Hash
              Functions for Message Authentication", Advances in
              Cryptography, Crypto96 Proceeding, June 1996.
 [ARCH]       Kent, S., and R. Atkinson, "Security Architecture for
              the Internet Protocol", RFC 2401, November 1998.
 [ESP]        Kent, S., and R. Atkinson, "IP Encapsulating Security
              Payload", RFC 2406, November 1998.
 [AH]         Kent, S., and R. Atkinson, "IP Authentication Header",
              RFC 2402, November 1998.
 [Thayer97a]  Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
              Document Roadmap", RFC 2411, November 1998.
 [RFC-2202]   Cheng, P., and R. Glenn, "Test Cases for HMAC-MD5 and
              HMAC-SHA-1", RFC 2202, March 1997.
 [RFC-2119]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8. Editors' Address

 Cheryl Madson
 Cisco Systems, Inc.
 EMail: cmadson@cisco.com
 Rob Glenn
 NIST
 EMail: rob.glenn@nist.gov

Madson & Glenn Standards Track [Page 5] RFC 2404 The Use of HMAC-SHA-1-96 within ESP and AH November 1998

The IPsec working group can be contacted through the chairs:

 Robert Moskowitz
 ICSA
 EMail: rgm@icsa.net
 Ted T'so
 Massachusetts Institute of Technology
 EMail: tytso@mit.edu

Madson & Glenn Standards Track [Page 6] RFC 2404 The Use of HMAC-SHA-1-96 within ESP and AH November 1998

9. 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
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required 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
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Madson & Glenn Standards Track [Page 7]

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