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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 (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.
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).