HMAC

HMAC stands for Keyed-Hash Message Authentication Code. It is a type of MAC that uses a cryptographic hash function and a secret key to provide data integrity and authentication. A recipient with the same key can verify that a message originated from the holder of the key and has not been altered.

Construction: Let H be a hash with block size b. If the key K is longer than b, first hash it to K' = H(K); if shorter, pad with zeros to b. Then inner = H((K' XOR ipad) || m) and HMAC = H((K' XOR opad) || inner), where ipad and opad are fixed byte pads (0x36 and 0x5c) repeated to length b.

Security: HMAC remains secure provided the underlying hash is cryptographically strong and the key stays secret. It resists length-extension attacks unlike plain hash-then-MAC schemes. In practice, security depends on the hash function and key management; keys should be random and of sufficient length, typically matching the hash function's block size.

Usage: HMAC is widely used to protect integrity and authenticity in protocols such as TLS and IPsec, and in signing tokens using HS256 (HMAC with SHA-256). The standard is defined in RFC 2104 (1997), with test vectors in RFC 4231. It is commonly instantiated with hash functions such as SHA-256, SHA-1, or SHA-3.

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