Watermarking attack

In cryptography, a watermarking attack is an attack on disk encryption methods where the presence of a specially crafted piece of data can be detected by an attacker without knowing the encryption key.

Problem description

Disk encryption suites generally operate on data in 512-byte

The problem is analogous to that of using block ciphers in the

Alternatively, one can use modes of operation specifically designed for disk encryption (see disk encryption theory). This weakness affected many disk encryption programs, including older versions of BestCrypt^[2] as well as the now-deprecated cryptoloop.^[3]

To carry out the attack, a specially crafted plaintext file is created for encryption in the system under attack, to "NOP-out" the IV^[4] such that the first ciphertext block in two or more sectors is identical. This requires that the input to the cipher (plaintext, ${\displaystyle \scriptstyle P}$,

${\displaystyle \scriptstyle IV}$

${\displaystyle \scriptstyle P_{1}\,\oplus \,IV_{1}\;=\;P_{2}\,\oplus \,IV_{2}}$

${\displaystyle \scriptstyle P_{1},\,P_{2}}$

${\displaystyle \scriptstyle P_{1}\,\oplus \,P_{2}\;=\;IV_{1}\,\oplus \,IV_{2}}$

The ciphertext block patterns generated in this way give away the existence of the file, without any need for the disk to be decrypted first.

See also

• Disk encryption theory
• Initialization vector
• Block cipher modes of operation
• Watermark

References