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HMAC: Keyed-Hashing for Message Authentication

Definition: Mechanism for message authentification using cryptographic hash functions (any iterative cryptographic hash function: MD5, SHA-1...) in combination with a secret shared key. The cryptographic strength of HMAC depends on the properties of the underlying hash function.

Algorithm

Initialisation

  • Cryptographic hash function: H
  • Secret key: K
  • Byte-length of each block of H: B
  • Byte-length of hash outputs: L

ipad: the byte 0x36 repeated B times opad: the byte 0x5C repeated B times.

Note

(the 'i' and 'o' are mnemonics for inner and outer)

Compute

H(K XOR opad, H(K XOR ipad, text))

Full Algorithm

HMAC(Bytes M, Bytes K, Hash H) -> Hexa
    INT B = H.block_size
    Bytes ipad = "0x36" * B
    Bytes opad = "0x5c" * B

    IF key.length > B THEN
        K = H(K)
    ELSE IF key.length < B THEN
        K = Padding(K, B - K.length) // Add zero at the end of K
    
    RETURN H(K XOR opad, H(K XOR ipad, M))

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