# Tag Info

There is a good reason to use zero-IV in CBC-MAC; if there is fixed no-Zero IV then an attacker can modify the $IV$ and $P_1$ so that the first block of the plaintext can become the advantage of the attacker. The first step of the CBC-MAC tag calculation is performed as $$C_1 = E_k(IV \oplus P_1)$$ Now let $P_1'$ be the target first block that the attacker ...