A null cipher is an ancient form of encryption where the plaintext is mixed with a large amount of non-cypher 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 modern cryptology, the term null cipher is also used to choosing not to use encryption at all in a system where various encryption options are offered. This is completely different from the sense above.
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