hashthenMAC

Hash-then-MAC is a cryptographic construction used to authenticate messages by applying a hash function to the message and then computing a MAC (message authentication code) over that hash value using a secret key. The resulting tag can be transmitted with the message to provide integrity and authenticity.

In typical use, a message m is first processed by a hash function H to produce a fixed-length digest h = H(m). A MAC algorithm with a secret key k then computes t = MAC_k(h). To verify, the recipient recomputes h = H(m) and t' = MAC_k(h) and checks that t' matches the received tag t in constant time.

Rationale and security considerations: Hash-then-MAC aims to combine the benefits of hashing with a secret-key MAC, and can help mitigate certain attack vectors that arise when authenticating long or variable-length messages. The security of the construction relies on the underlying MAC’s security (typically EUF-CMA) and on the hash function not introducing weaknesses in the MAC process. In practice, hash-then-MAC is one approach among several for providing message integrity and authentication, but it is not universally preferred over other paradigms such as HMAC directly on the message or the use of authenticated encryption schemes.

Variants and practical notes: A common practical interpretation is to apply a MAC to the hash output, i.e., t = MAC_k(H(m)). This differs from a MAC applied directly to the message (as in HMAC) and from encryption-then-MAC approaches. Hash-then-MAC can introduce extra computation due to the additional hash step, and its security guarantees depend on the chosen hash and MAC algorithms. In modern protocols, many implementations favor directly macing the message with a robust MAC or using authenticated encryption for combined confidentiality and integrity.

See also: HMAC, MAC, hash function, authenticated encryption.