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. 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, however this has the side effect of creating a much larger ciphertext.

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