The Arnold Cipher was a book cipher used by

John André John André (2 May 1750/1751''Gravesite–Memorial''
Westmi ...

and

Benedict Arnold Benedict Arnold ( Brandt (1994), p. 4June 14, 1801) was an American military officer who served during the Revolutionary War. He fought with distinction for the American Continental Army and rose to the rank of major general before defect ...

during the negotiations that led to Arnold's failed attempt to surrender West Point to the British in 1780.

Background

In May 1779,

Continental Army The Continental Army was the army of the United Colonies (the Thirteen Colonies) in the Revolutionary-era United States. It was formed by the Second Continental Congress after the outbreak of the American Revolutionary War, and was establis ...

Major General

initiated what became a series of communications with British Army Major

John André John André (2 May 1750/1751''Gravesite–Memorial''
Westmi ...

, the adjutant and spy chief to Lieutenant General Sir Henry Clinton, the commander-in-chief of British forces in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. 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 The ''Commentaries on the Laws of England'' are an influential 18th-century treatise on the common law of England by Sir William Blackstone, originally published by the Clarendon Press at Oxford, 1765–1770. The work is divided into four volume ...

'' by William Blackstone or '' Nathan Bailey'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, 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, 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