<p>neat little fact about CRCs and stream ciphers: if you append a CRC to a message, you always get the same CRC.<br />if you encrypt the resulting message with a stream cipher, the CRC of the message is cancelled out so you essentially get the CRC of the keystream. as a result, if you encrypt 2 different files of same length with CRC appended with the same (unsalted) key, you get the same CRC on the encrypted file</p>