RFC 2403 (rfc2403) - Page 2 of 7

The Use of HMAC-MD5-96 within ESP and AH

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RFC 2403        The Use of HMAC-MD5-96 within ESP and AH   November 1998


   In this memo, HMAC-MD5-96 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

   [RFC-1321] describes the underlying MD5 algorithm, while [RFC-2104]
   describes the HMAC algorithm. The HMAC algorithm provides a framework
   for inserting various hashing algorithms such as MD5.

   HMAC-MD5-96 operates on 64-byte blocks of data.  Padding requirements
   are specified in [RFC-1321] and are part of the MD5 algorithm.  If
   MD5 is built according to [RFC-1321], there is no need to add any
   additional padding as far as HMAC-MD5-96 is concerned.  With regard
   to "implicit packet padding" as defined in [AH], no implicit packet
   padding is required.

   HMAC-MD5-96 produces a 128-bit authenticator value.  This 128-bit
   value can be truncated as described in RFC 2104.  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 128-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-MD5-96.

   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".  [RFC-1810] provides some performance
   analysis and recommendations of the use of MD5 with Internet
   protocols.  As of this writing no performance analysis has been done
   of HMAC or HMAC combined with MD5.

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





Madson & Glenn              Standards Track
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