HMAC

HMAC (Hash-based Message Authentication Code) is a symmetric MAC construction defined in RFC 2104 and FIPS 198-1 that combines a secret key K with a cryptographic hash function H to produce a tag: HMAC(K,m) = H((K' XOR opad) || H((K' XOR ipad) || m)). The nested structure makes HMAC secure against length-extension attacks even when used with Merkle-Damgård hashes like SHA-256, and its security is reducible to weak collision resistance and the pseudorandomness of the compression function. Common instantiations are HMAC-SHA-256 and HMAC-SHA-512, used in TLS record protection, JWT signing, OAuth, IPsec, and as the PRF inside PBKDF2 and HKDF. HMAC remains a strong default choice for symmetric message authentication.