So hypothetically I have a an arbitrary block cipher operating in CBC-MAC mode that makes use of a public and static $IV$ as well as a static key $K$.
I want to be able to that this won't be preimage resistant using a proof for an arbitrary single-block message $P$ that hashes to a given digest $T$. So far I've gotten as far as $$T = E(P \oplus IV, K)$$ $$D(T, K) = P \oplus IV$$ $$P = D(T, K) \oplus IV$$ I can't for the life of me figure out how to move from this to something that will show that the operation is not preimage resistant. As far as I understand if $K$ and $IV$ are fixed it should be computationally infeasible to find another $T_x$ such that $$D(T,K) \oplus IV = D(T_x, K) \oplus IV$$
Ideally I'd want to be able to extend this to show that for $P$ of any length I could find a $T_x$.