Book cipher

A book cipher is a cipher in which the key is the identity of a book.

Traditionally book ciphers work by replacing words in the plaintext of a message with the location of words from a book. In this mode, book cyphers are more properly called codess.

This can have problems as if a word appears in the plaintext that doesn't appear in the book then it can't be encoded. An alternative approach which gets around this problem is to replace individual letters rather than words, in which case the book cypher is properly a cypher. However, if needed often, this has the side effect of creating a larger cipherext.

Perhaps the most famous use of a book cipher is in the Beale ciphers.

See also: Running key cipher

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Classical cryptography