Secret Prefix MAC

A method of implementing MAC with $\text{MAC}(\text{message},\text{key})=\text{Hash}(\text{key}||\text{message})$

Formal Definition

$m=h(k||x)=h(k|||x_1||\dots ||x_n)$

• In case of message $x$ being larger than blocksize (like 512 for SHA)

Vulnerability

• Secret Prefix Length Extension Attack