Book cipher

A book cipher is a cipher in which each word or letter of the plaintext is replaced by a code that locates it in another text, the key. In the simplest version the key is a specific book and each word is replaced by the number of its position in that book; a variant gives a page, line and word triplet to avoid error-prone counting, or chapter, verse and word for the Bible. Both sender and receiver must hold exactly the same edition of the key book. A word-based scheme fails when the message uses a word missing from the text, so a letter-based variant replaces each letter by a position where it occurs, which is properly a homophonic substitution cipher and was used in one of the Beale ciphers. The main strength is the key itself: an interceptor must identify the book out of a huge number of possibilities, and for a spy a book looks more innocent than a conventional codebook. In practice the key has often been a purpose-made codebook, a version properly called a code, used from the fifteenth century until the Second World War.