HMAC Construction & Security Proof

HMAC is the standard keyed MAC construction. Its double-hash design prevents length-extension attacks and has a formal security proof reducing HMAC security to the underlying hash function.

HMAC(K, M) = H((K' ⊕ opad) || H((K' ⊕ ipad) || M)) Where: - K' = K zero-padded to block size B (or H(K) if |K|>B) - ipad = 0x36 repeated B times - opad = 0x5C repeated B times