HMAC

Hash-Based Message Authentication Code (HMAC) is a type of MAC that can, at least in theory, be constructed from any cryptographic hash function. That’s why it’scommon to see HMAC to be more precisely referred to as, for example, HMAC-MD5, where the suffix identifies the name of the underlying hash function.

The design of HMAC is very simple. Let $h$ be a hash function, and let $K_1$ and $K_2$ be two symmetric keys. Then the MAC on message $M$ is computed as follows: