Null cipher

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."

Identity function encryption

In modern cryptology, null cipher (or NONE 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. Thus the text is the same before and after encryption. In mathematics such a function is known as the identity function. Examples of this are the "eNull", "aNull", "Null" and "aDH" cipher suites in OpenSSL and the "NULL Encryption Algorithm" in IPSec.

Decoy Cypher

The weak link in decryption is the human in the loop. Human computation is slow and expensive. Whenever a cypher needs to be sent to a human for semantic processing, this substantially increases the cost of decryption.

A decoy cypher can take the form of noise - sending copious messages of encrypted garbage plaintext. This decreases the signal-to-noise ratio for humans trying to interpret decrypted "plaintext" messages.

A decoy cypher can also take the form of misleading information - for example, in an onion cypher, most of the layers may contain information that when decrypted will produce a message that directly misleads the person reading it - often resulting in them taking actions against their interest - such as signalling that they are evesdropping by responding to a specific false signal, false flag attacks, or causing them to suspect the wrong parties. The actual message can still be contained at some level of the onion - but preferably not the lowest level - which may include an innocuous message so that if all layers are decrypted the core seems innocent. (see noise decoy cypher).

References

• Kipper, Gregory Investigator’s guide to steganography 2004 CRC Press LLC
• High Performance Enabled SSH/SCP (Pittsburgh Supercomputing Center) Retrieved 16-12-2008.