module for creating standard message integrity checks

 HMAC is used for message integrity checks between two parties
 that share a secret key, and works in combination with some other
 Digest algorithm, usually MD5 or SHA-1. The HMAC mechanism
 is described in RFC 2104.
 .
 The Digest::HMAC module follow the common Digest:: interface,
 but the constructor takes the secret key and the name of some
 other simple Digest:: module as argument.