HMAC – a hash-based MAC

Hash-based Message Authentication Code (HMAC) is a MAC that is generated by the HMAC function.

The HMAC function uses a cryptographic hash function and a secret key. The function is not overly complex and can be understood by many non-cryptographers. It is defined as follows:

HMAC(K, message) = H(K' XOR opad ‖ H(K' XOR ipad ‖ message))

Here, the following can be understood:

• Message: This is the message to be authenticated.
• H: This is the hash function, for example, SHA3-256.
• K: This is the secret key.
• K': This is a block-sized key derived from K depending on the hash function’s internal block size, B.
• ipad: This is the inner padding, consisting of byte 0x36, which is repeated B times.
• opad: This is the outer padding, consisting of byte 0x5C, which is repeated B times.
• ‖: This refers to a concatenation.

Note that K' is derived from K, as follows:

• If...