High Quality Content by WIKIPEDIA articles! In cryptography, a watermarking attack is an attack on disk encryption methods where the presence of a specially crafted piece of data (e.G., a decoy file) can be detected by an attacker without knowing the encryption key. DIsk encryption suites generally operate on data in 512-byte sectors which are individually encrypted and decrypted. THese 512-byte sectors alone can use any block cipher mode of operation (typically CBC), but since arbitrary sectors in the middle of the disk need to be accessible individually, they cannot depend on the contents of their preceding/succeeding sectors. THus, with CBC, each sector alone has to use an initialization vector (IV). IF these IVs are predictable by an attacker, then a specially crafted file can be generated to "NOP-out" the IV, causing different blocks on the encrypted disk to have identical sectors, or at least the first block in a number of sectors to be identical.