HMAC
       keyed-Hashing for Message AuthentiCation (cryptography, RFC 2104)
       

Keyed-Hashing Message Authentication
HMAC

    (HMAC) A mechanism for message
   authentication using cryptographic hash functions.  HMAC
   can be used with any iterative cryptographic hash function,
   e.g., MD5, SHA-1, in combination with a secret shared key.
   The cryptographic strength of HMAC depends on the properties
   of the underlying hash function.

   [RFC 2104].

   (1997-05-10)