Keyed-Hashing for Message Authentication
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.
HMAC follow the common 'Digest::' interface, but the constructor takes the
secret key and the name of some other simple 'Digest::' as argument.