A null cipher
is an ancient form of encryption
where the plaintext
is mixed with a large amount of non-cipher material. It would today be regarded as a simple form of steganography
. Null ciphers can also be used to hide ciphertext, as part of a more complex system.
In classical cryptography a null is intended to confuse the cryptanalyst. Typically, a null will be a character which decrypts to obvious nonsense at the end of an otherwise intelligible phrase. In a null cipher, most of the characters may be nulls.
An example follows (Kipper 9):
- News Eight Weather: Tonight increasing snow. Unexpected precipitation smothers eastern towns. Be extremely cautious and use snowtires especially heading east. The [highway is not] knowingly slippery. Highway evacuation is suspected. Police report emergency situations in downtown ending near Tuesday.
Taking the first letter in each word successively yields the real message: "Newt is upset because he thinks he is President."
In modern cryptology, null cipher is also defined as choosing not to use encryption in a system where various encryption options are offered, such as for testing/debugging, or authentication-only communication.
- Kipper, Gregory Investigator’s guide to steganography 2004 CRC Press LLC