High Quality Content by WIKIPEDIA articles! In cryptography, to guarantee the integrity of a message, one can use either public key digital signatures or use a Message Authentication Code (MAC). A MAC is one of the possible authentication techniques involving the use of a secret key to generate a small fixed-size block of data. The basic setting of MAC is as follows. Two parties A and B want to communicate by sending a message . They share a secret key . When A sends a message to B then A calculates the MAC as a function of the message and the key . The message and the key are sent to B. Then B uses the same secret key and calculates the MAC on the received message. The received MAC is compared to the new MAC. When it matches then the message is authentic because only the receiver and the sender know the secret key.