Book cipher

For example, using the same War of the Worlds book as the key, the message "no ammo" could be encoded as "12 10 / 50 31 59 34" since the words with those positions in the novel are "nineteenth", "of", "almost", "mortal", "might", and "own".

Therefore, in practice, the key has usually been a codebook created for the purpose: a simple dictionary-like listing of all the words that might be needed to form a message, each with the respective code number(s).

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.

Thus, a spy wishing to send information about troop movements and numbers of armaments would be unlikely to find a cookbook or romance novel useful keys.

The Bible is a widely available book that is almost always printed with chapter and verse markings making it easy to find a specific string of text within it, making it particularly useful for this purpose; the widespread availability of concordances can ease the encoding process as well.

It is still susceptible to other methods of cryptanalysis, and as such is quite easily broken, even without sophisticated means, without the cryptanalyst having any idea to what book the cipher is keyed.