Book cipher

A book cipher is a cipher in which the key is some aspect of a book or other piece of text; books being common and widely available in modern times, users of book ciphers take the position that the details of the key are sufficiently well hidden from attackers in practice. It is typically essential that both correspondents not only have the same book, but the same edition.

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

This can have problems; if a word appears in the plaintext but not in the book, it cannot be encoded. An alternative approach which gets around this problem is to replace individual letters rather than words. One such method, used in the second Beale cipher, substitutes the first letter of a word in the book with that word's position. In this case, the book cipher is properly a cipher — specifically, a homophonic substitution cipher. However, if used often, this technique has the side effect of creating a larger ciphertext (typically 4 to 6 digits being required to encipher each letter or syllable) and increases the time and effort required to decode the message.

The main strength of a book cipher is the key. The sender and receiver of encoded messages can agree to use any book or other publication available to both of them as the key to their cipher. Someone intercepting the message and attempting to decode it, unless they are a skilled cryptographer (see Security below), must somehow identify the key from a huge number of possibilities available. In the context of espionage, a book cipher has a considerable advantage for a spy in enemy territory. A conventional codebook, if discovered by the local authorities, instantly incriminates the holder as a spy and gives the authorities the chance of deciphering the code and sending false messages impersonating the agent. On the other hand a book, if chosen carefully to fit with the spy's cover story, would seem entirely innocuous. The drawback to a book cipher is that both parties have to possess an identical copy of the key. The book must not be of the sort that would look out of place in the possession of those using it and it must be of a type likely to contain any words required. Thus, for example, a spy wishing to send information about troop movements and numbers of armaments would be unlikely to find a cookery book or a romantic novel useful keys.

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