An ANSI standard in cryptography for a short piece of information used to authenticate a message based on DES. A message authentication code involves an algorithm (often a one-way hash function or a block cipher) that accepts a secret key and a message as input; it then produces a MAC (sometimes known as a tag). This process provides both an integrity check (by ensuring that a different MAC will result if the message has been altered) and an authenticity check (because only the person knowing the secret key could have produced a MAC). GNU Free Documentation License. Message Authentication Code (MAC). [Online, April 21, 2005.] GNU Free Documentation License Website. http://en.wikipedia.org/ wiki/Message_authentication_code.