CMAC

CMAC stands for Cipher-based Message Authentication Code. It is a method for producing a fixed-size authentication tag from a message and a secret key, using a block cipher as the underlying primitive. CMAC is designed to provide data integrity and authenticity, ensuring that a message has not been altered and that it originates with the holder of the secret key.

Conceptually, CMAC operates by processing the message as fixed-size blocks with a block cipher in CBC mode, starting with an all-zero initialization vector. Before processing, two subkeys, K1 and K2, are derived from the encryption of a zero block and a finite-field doubling operation. If the final message block is complete (full block), it is XORed with K1; if it is partial, the block is padded with zeros and XORed with K2. The result is then fed through the CBC chain, and the final block’s output is taken as the CMAC tag. For a block cipher with 128-bit blocks (such as AES), the default tag length is 128 bits, but truncation to shorter lengths is common in practice.

CMAC is designed to be secure when used with a secret key that is never reused for another CMAC with the same key. It is related to CBC-MAC but fixes certain weaknesses through the subkey construction. It is standardized in NIST SP 800-38B and is applicable to any block cipher with a suitable block size.

See also: HMAC, CBC-MAC, block ciphers, AES, cryptographic standards.