Network Working Group                                        A. Keromytis
Request for Comments: 2857                     University of Pennsylvania
Category: Standards Track                                       N. Provos
                            Center for Information Technology Integration
                                                                June 2000

The Use of HMAC-RIPEMD-160-96 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

Abstract

   This memo describes the use of the HMAC algorithm [RFC 2104] in
   conjunction with the RIPEMD-160 algorithm [RIPEMD-160] as an
   authentication mechanism within the revised IPSEC Encapsulating
   Security Payload [ESP] and the revised IPSEC Authentication Header
   [AH].  HMAC with RIPEMD-160 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 RIPEMD-160 [RIPEMD-160] 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-RIPEMD-160-96 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
   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-RIPEMD-160-96 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

   [RIPEMD-160] describes the underlying RIPEMD-160 algorithm, while
   [RFC 2104] describes the HMAC algorithm.  The HMAC algorithm provides
   a framework for inserting various hashing algorithms such as RIPEMD-
   160.

   HMAC-RIPEMD-160-96 operates on 64-byte blocks of data.  Padding
   requirements are specified in [RIPEMD-160] and are part of the
   RIPEMD-160 algorithm.  Padding bits are only necessary in computing
   the HMAC-RIPEMD-160 authenticator value and MUST NOT be included in
   the packet.

   HMAC-RIPEMD-160-96 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-RIPEMD-160-96.

   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".  [RIPEMD-160] provides some
   performance analysis.  As of this writing no detailed performance
   analysis has been done of HMAC or HMAC combined with RIPEMD-160.

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

3. Keying Material

   HMAC-RIPEMD-160-96 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 SHALL NOT be supported.  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.
   Implementors should refer to RFC 1750 for guidance on the
   requirements for such functions.

   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.

   [ESP] describes the general mechanism to obtain keying material for
   the ESP transform.  The derivation of the key from some amount of
   keying material does not differ between the manual and automatic key
   management mechanisms.

   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] states that for "minimally reasonable hash functions" the
   "birthday attack" is impractical.  For a 64-byte block hash such as
   HMAC-RIPEMD-160-96, an attack involving the successful processing of
   2**64 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.) No time-based attacks are
   discussed in the document.

   While it it still cryptographically prudent to perform frequent
   rekeying, current literature does not include any recommended key
   lifetimes for HMAC-RIPEMD.  When recommendations for HMAC-RIPEMD key
   lifetimes become available they will be included in a revised version
   of this document.

4. Interaction with the ESP Cipher Mechanism

   As of this writing, there are no known issues which preclude the use
   of the HMAC-RIPEMD-160-96 algorithm with any specific cipher
   algorithm.

5. Security Considerations

   The security provided by HMAC-RIPEMD-160-96 is based upon the
   strength of HMAC, and to a lesser degree, the strength of RIPEMD-160.
   At the time of this writing there are no known practical
   cryptographic attacks against RIPEMD-160.

   It is also important to consider that while RIPEMD-160 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 RIPEMD-160 with other
   algorithms such as SHA-1.  [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.  [Kapp97]
   contains test vectors and example code to assist in verifying the
   correctness of HMAC-RIPEMD-160-96 code.

6. Acknowledgements

   This document is derived from work by C. Madson and R. Glenn and 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.

7. References

   [RIPEMD-160]  3.ISO/IEC 10118-3:1998, "Information technology -
                 Security techniques - Hash-functions - Part 3:
                 Dedicated hash-functions," International Organization
                 for Standardization, Geneva, Switzerland, 1998.

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

   [Bellare96a]  Bellare, M., Canetti, R., Krawczyk, H., "Keying Hash
                 Functions for Message Authentication", Advances in
                 Cryptography, Crypto96 Proceeding, June 1996.

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

   [Kapp97]      Kapp, J., "Test Cases for HMAC-RIPEMD160 and HMAC-
                 RIPEMD128", RFC 2286, March 1998.

   [RFC 1750]    Eastlake 3rd, D., Crocker, S. and J. Schiller,
                 "Randomness Recommendations for Security", RFC 1750,
                 December 1994.

   [RFC 2119]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

8. Authors' Addresses

      Angelos D. Keromytis
      Distributed Systems Lab
      Computer and Information Science Department
      University of Pennsylvania
      200 S. 33rd Street
      Philadelphia, PA 19104 - 6389

      EMail: angelos@dsl.cis.upenn.edu

      Niels Provos
      Center for Information Technology Integration
      University of Michigan
      519 W. William
      Ann Arbor, Michigan 48103 USA

      EMail: provos@citi.umich.edu

   The IPsec working group can be contacted through the chairs:

      Robert Moskowitz
      International Computer Security Association

      EMail: rgm@icsa.net

      Ted T'so
      VA Linux Systems

      EMail: tytso@valinux.com

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
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