Imagine you have a string of plaintext (
a), IV (
ai) and the MAC (
am) for the plaintext (
How can you generate a MAC (
bm) for a different string of plaintext (
b) - using an IV (
ib) of your choice, using the same key - which you don't know and can't retrieve.
You can also assume both plaintexts are the same length.
Edit: I have found this question which I think provides a clue: CBC-MAC insecure with random IV
But I'm not sure if I understand correctly. You can also assume your plaintexts are one-block plaintexts.
This should prove that cbc-mac with random IV is insecure.