Null cipher
A null cipher conceals a message inside innocent-looking cover text by selecting characters according to a hidden rule.
Definition
A null cipher hides meaningful letters among ordinary-looking words. A rule might select every nth character, initial letters, or letters at agreed positions; the remaining cover text acts as null material.
Concealment and security
The technique hides the existence or location of the message rather than transforming it with strong cryptography. Forced wording and suspicious patterns can reveal the rule, so null ciphers are historical steganographic methods, not modern protection.
Not necessarily. The extracted letters may be plain text unless a separate cipher is applied.
It refers to cover or padding characters that carry no part of the extracted message.