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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."
You can also choose to instead use the last letter of every word, or something like a pattern such as:
Susan sAys GaIl Lies. MAtt leTs Susan fEel joVial. Elated (or) aNgry?
Using the pattern (1,2,3,1,2,3 [each letter in each word]) gives the message: "Sail at seven."
- Kipper, Gregory Investigator’s guide to steganography 2004 CRC Press LLC
- High Performance Enabled SSH/SCP (Pittsburgh Supercomputing Center) Retrieved 16-12-2008.
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