Assume you can give Bob two files, containing whatever data you want them to contain, but with the restriction that they must be the same length. Bob will then pick one of them, encrypt it with a secret key, and then give you the resulting ciphertext. You have to guess which file he encrypted!

Bob uses AES in ECB mode. How can you play this game so that you can win? Be very specific, including a clear explanation of why your strategy allows you to win.