A null cipher, also known as concealment cipher, is an ancient form of encryption where the plaintext is mixed with a large amount of non-cipher material. Today it is regarded as a simple form of steganography, which can be used to hide ciphertext.
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Classical cryptography
In classical cryptography, a null is intended to confuse the cryptanalyst. In a null cipher, the plaintext is included within the ciphertext and one needs to discard certain characters in order to decrypt the message. Most characters in such a cryptogram are nulls, only some are significant, and some others can be used as pointers to the significant ones.
Examples of messages containing null ciphers:
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."
Other options include positioning of the significant letters next to or at certain intervals from punctuation marks or particular characters. Historically, users of concealment ciphers often used substitution and transposition ciphers on the data prior to concealment. For example, Cardinal Richelieu is said to have used a grille to write secret messages, after which the blank spaces were filled out with extraneous matter to create the impression of a continuous text.
Usage
It is difficult and time-consuming to produce encapsulating messages that feel natural and wouldn't raise suspicions. The message may read clumsily, and suspected messages can be detected by mail filters. More importantly, the security of the message relies entirely on the secrecy of the concealment method. Null ciphers are no longer in serious use.