Editing Book cipher (section)

== Choosing the key ==
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 {{xref|(see: {{slink||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, 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.