03:39:48 AM Feb 13th 2011
Should there be a Real Life trope for "Plaintext Attack", its not really a fiction trope as its too complex for most authors but the essence is that there are three things, the ciphertext, the key and the plaintext. Know two and you can determine the third. The hole occurs when an already known file gets encrypted, the classic example being the Enigma crib where someone's full title was included at the head of the message. ZIP archives are extremely vulnerable to this as the password is used seperately on each file in an archive in turn, if one file is known already then the password can be recovered.