draft-ietf-ipsec-auth-hmac-ripemd-160-96


Network Working Group                                Angelos D. Keromytis
INTERNET DRAFT                                               Niels Provos
Expire in six months                                        February 1999




	   The Use of HMAC-RIPEMD-160-96 within ESP and AH
	  <draft-ietf-ipsec-auth-hmac-ripemd-160-96-03.txt>




Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026. 

   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@tis.com) or to the editor.

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   Distribution of this memo is unlimited.

Abstract

   This draft 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].




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1.  Introduction

   This draft 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 draft, 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.




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   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.

   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


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   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 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.






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6.  Acknowledgments

   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]  Dobbertin, H., Bosselaers A., and Preneel, B.
                 "RIPEMD-160: A Strengthened Version of RIPEMD"
                 April 1996, ftp.esat.kuleuven.ac.be:
		 /pub/COSIC/bosselae/ripemd/ripemd160.ps.gz

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

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


   [ESP]         Kent, S., Atkinson, R., "IP Encapsulating Security
                 Payload", draft-ietf-ipsec-esp-v2-01.txt,
                 work in progress, October, 1997

   [AH]          Kent, S., Atkinson, R., "IP Authentication Header",
                 draft-ietf-ipsec-auth-header-02.txt, work in progress,
                 October 1997

   [Thayer97a]   Thayer, R., Doraswamy, N., Glenn, R., "IP Security
                 Document Framework",
                 draft-ietf-ipsec-doc-framework-01.txt, work in
                 progress, July 1997.

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

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










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8.  Authors' Address

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

	Niels Provos
	Center for Information Technology Integration
	University of Michigan
	519 W. William
	Ann Arbor, Michigan 48103 USA
	provos@citi.umich.edu

   The IPsec working group can be contacted through the chairs:

        Robert Moskowitz
        rgm@icsa.net
        International Computer Security Association

        Ted T'so
        tytso@mit.edu
        Massachusetts Institute of Technology

























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