The Ottendorf Cypher, or Book Cypher, is a cyphertext is made up of numbers in groups of 3, and these numbers correspond to positions in a book (or other type of text).  Most often the numbers refer to Line, Word, and Letter.  To encode a message, the encoder needs only to find the word at the prescribed location within the book and record its coordinates.  Typically, the code is three numbers following the sequence:  Page number, line number on the page, and word number on that line.  The decoder needs to then know the ‘Key’ which is the book from which the original code is constructed.  The advantage of the Ottendorf Cypher is that there are literally millions of potential books to use as the key and since books are widely distributed, the sender never needs to give the key to the recipient. 

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.

One example of the use of an Ottendorf cypher is the communication between John Andre and Benedict Arnold.  Benedict Arnold used a book cypher, which used Sir William Blackstone’s Commentaries on the Laws of England as a key text. 

Example of a Book Cypher (using 1733 Poor Richard’s Almanack) is:

21 24 4 19 22 4 2 24 7 5 27 7 1 25 1 11 2 4 22 32 5 12 8 6 14 1 3 17 32 1

Translates as follows:

21 24 419 22 42 24 75 27 71 25 1
11 2 422 32 512 8 614 1 317 32 1
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