In cryptography, a message authentication code (MAC) is a short piece of information used to authenticate a message. A MAC algorithm (sometimes termed a keyed hash function) accepts as input a secret key as well as the message, and produces a MAC (sometimes known as a tag). The MAC protects both a message's integrity—by ensuring that a different MAC will be produced if the message has changed—as well as its authenticity—because only someone who knows the secret key could have generated a valid MAC.
MAC algorithms can be constructed from other cryptographic primitives, such as cryptographic hash functions (as in the case of HMAC) or from block cipher algorithms (OMAC and PMAC).
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