Hash-based Message Authentication Code - Definition (from RFC 2104)

Definition (from RFC 2104)

HMAC (K,m) = H ((K ⊕ opad) ∥ H ((K ⊕ ipad) ∥ m))

where

H is a cryptographic hash function,

K is a secret key padded to the right with extra zeros to the input block size of the hash function, or the hash of the original key if it's longer than that block size,

m is the message to be authenticated,

∥ denotes concatenation,

⊕ denotes exclusive or (XOR),

opad is the outer padding (0x5c5c5c…5c5c, one-block-long hexadecimal constant),

and ipad is the inner padding (0x363636…3636, one-block-long hexadecimal constant).