The Arnold Cipher was a

book cipher A book cipher, or Ottendorf 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, are more convenient for this use than objects made specifically for crypto ...

used by John André and Benedict Arnold during the negotiations that led to Arnold's failed attempt to surrender

West Point The United States Military Academy (USMA), also known Metonymy, metonymically as West Point or simply as Army, is a United States service academies, United States service academy in West Point, New York. It was originally established as a f ...

to the British in 1780.

Background

In May 1779, Continental Army Major General Benedict Arnold initiated what became a series of communications with

British Army The British Army is the principal land warfare force of the United Kingdom, a part of the British Armed Forces along with the Royal Navy and the Royal Air Force. , the British Army comprises 79,380 regular full-time personnel, 4,090 Gurk ...

Major John André, the adjutant and spy chief to Lieutenant General Sir Henry Clinton, the commander-in-chief of British forces in North America. In these communications, which were at first mediated by Joseph Stansbury, a Philadelphia merchant, Arnold offered his services to the British. André responded to this offer with a letter dated May 10, 1779, in which he described the types of services Arnold might provide, and described a code which they should use to obscure their communications. The book used as a key to the cipher was either '' Commentaries on the Laws of England'' by

William Blackstone Sir William Blackstone (10 July 1723 – 14 February 1780) was an English jurist, judge and Tory politician of the eighteenth century. He is most noted for writing the ''Commentaries on the Laws of England''. Born into a middle-class family ...

or ''

Nathan Bailey Nathan Bailey (died 27 June 1742), was an English philologist and lexicographer. He was the author of several dictionaries, including his '' Universal Etymological Dictionary'', which appeared in some 30 editions between 1721 and 1802. Bailey's ...

's Dictionary''. The cipher consisted of a series of three numbers separated by periods. These numbers represented a page number of the agreed book, a line number on that page, and a word number in that line. Arnold added missing letters or suffixes where he could not find a match in one of the books. For example, 120.9.7 would refer to the 120th page, the 9th line on that page, and the seventh word in that line, which, in the following example is decoded as "general". The actual communications were often disguised by embedding it in a letter written by Arnold's wife Peggy, where the cipher would be written in

invisible ink Invisible ink, also known as security ink or sympathetic ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means, such as heat or ultraviolet light. Invisible ...

, but might also have been disguised as what appeared to be routine business communications.

Coded example

This code was generated by Arnold for a message to André dated July 12, 1780:

Decoded example

Here is how

Jonathan Odell Jonathan Odell (25 September 1737 – 25 November 1818) was a Loyalist poet who lived during the American Revolution. Early life and career Odell was born in Newark, New Jersey, in 1737 to John and Temperance Odell. He graduated from Prince ...

, André's assistant, decoded the message:Arnold to André, July 12, 1780 (Decoded)

References

* * * * {{cite web, url=http://www.clements.umich.edu/Spies/letter-1779may10.html , title=André to Stansbury, May 10, 1779 , publisher=Clements Library, University of Michigan , accessdate=2010-03-12 , url-status=dead , archiveurl=https://web.archive.org/web/20100608115252/http://www.clements.umich.edu/Spies/letter-1779may10.html , archivedate=June 8, 2010 Classical ciphers Benedict Arnold