# Non CPA on custom MAC

While preparing for a crypto exam there is a task we don't really understand:

Let $E_K$ be a permutation $\{0,1\}^b \rightarrow \{0,1\}^b$ defined by a blockcipher with the key $K$. Let $M = (M_1,...,M_n) \in *(\{0,1\}^b)^n$ be an nb-bit message.

We interpret the output of $E_K$ as an integer $E_K(\cdot) \in \mathbb{Z}_{2^b}$ and define the MAC as the following:

we should now provide an attack which is always successful. The restriction is that we are not allowed to perform a CPA. So we search for the a pair $(M',A')$ with $A' = MAC_K(M')$