RFC 1828 (rfc1828) - Page 2 of 5

IP Authentication using Keyed MD5

RFC 1828                         AH MD5                      August 1995


2.  Calculation

   The 128-bit digest is calculated as described in [RFC-1321].  The
   specification of MD5 includes a portable 'C' programming language
   description of the MD5 algorithm.

   The form of the authenticated message is

            key, keyfill, datagram, key, MD5fill

   First, the variable length secret authentication key is filled to the
   next 512-bit boundary, using the same pad with length technique
   defined for MD5.

   Then, the filled key is concatenated with (immediately followed by)
   the invariant fields of the entire IP datagram (variant fields are
   zeroed), concatenated with (immediately followed by) the original
   variable length key again.

   A trailing pad with length to the next 512-bit boundary for the
   entire message is added by MD5 itself.  The 128-bit MD5 digest is
   calculated, and the result is inserted into the Authentication Data
   field.

   Discussion:
      When the implementation adds the keys and padding in place before
      and after the IP datagram, care must be taken that the keys and/or
      padding are not sent over the link by the link driver.



Security Considerations

   Users need to understand that the quality of the security provided by
   this specification depends completely on the strength of the MD5 hash
   function, the correctness of that algorithm's implementation, the
   security of the key management mechanism and its implementation, the
   strength of the key [CN94], and upon the correctness of the
   implementations in all of the participating nodes.

   At the time of writing of this document, it is known to be possible
   to produce collisions in the compression function of MD5 [dBB93].
   There is not yet a known method to exploit these collisions to attack
   MD5 in practice, but this fact is disturbing to some authors
   [Schneier94].

   It has also recently been determined [vOW94] that it is possible to
   build a machine for $10 Million that could find two chosen text



Metzger & Simpson             Standards Track