RFC 2104 (rfc2104) - Page 3 of 11

HMAC: Keyed-Hashing for Message Authentication

RFC 2104                          HMAC                     February 1997


2. Definition of HMAC

   The definition of HMAC requires a cryptographic hash function, which
   we denote by H, and a secret key K. We assume H to be a cryptographic
   hash function where data is hashed by iterating a basic compression
   function on blocks of data.   We denote by B the byte-length of such
   blocks (B=64 for all the above mentioned examples of hash functions),
   and by L the byte-length of hash outputs (L=16 for MD5, L=20 for
   SHA-1).  The authentication key K can be of any length up to B, the
   block length of the hash function.  Applications that use keys longer
   than B bytes will first hash the key using H and then use the
   resultant L byte string as the actual key to HMAC. In any case the
   minimal recommended length for K is L bytes (as the hash output
   length). See section 3 for more information on keys.

   We define two fixed and different strings ipad and opad as follows
   (the 'i' and 'o' are mnemonics for inner and outer):

                   ipad = the byte 0x36 repeated B times
                  opad = the byte 0x5C repeated B times.

   To compute HMAC over the data `text' we perform

                    H(K XOR opad, H(K XOR ipad, text))

   Namely,

    (1) append zeros to the end of K to create a B byte string
        (e.g., if K is of length 20 bytes and B=64, then K will be
         appended with 44 zero bytes 0x00)
    (2) XOR (bitwise exclusive-OR) the B byte string computed in step
        (1) with ipad
    (3) append the stream of data 'text' to the B byte string resulting
        from step (2)
    (4) apply H to the stream generated in step (3)
    (5) XOR (bitwise exclusive-OR) the B byte string computed in
        step (1) with opad
    (6) append the H result from step (4) to the B byte string
        resulting from step (5)
    (7) apply H to the stream generated in step (6) and output
        the result

   For illustration purposes, sample code based on MD5 is provided as an
   appendix.







Krawczyk, et. al.            Informational