What is Message Authentication Code (MAC) or the Financial Institution Message Authentication Standard (FIMAS)

In order to protect against fraud in electronic fund transfers, the Message Authentication Code (MAC), ANSI X9.9, was developed. The MAC is a check value derived from the contents of the message itself that is sensitive to the bit changes in a message. It is similar to a Cyclic Redundancy Check (CRC). A MAC is appended to the message before it is transmitted. At the receiving end, a MAC is generated from the received message and is compared to the MAC of an original message. A match indicates that the message was received without any modification occurring while en route.

To strengthen the MAC algorithm, a keyed MAC can be generated using a symmetric key encryption, such as DES. Typically, the Exclusive Or function of the DES key with a message is performed on the sequential, 8-byte blocks of the message to generate the MAC. As with all symmetric key applications, the key must be distributed securely so that sender and receiver have the same key.