Arnold Cipher
Encyclopedia
Book cipher
A book 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, users of book ciphers take the position that the details of the key is sufficiently well hidden from attackers in practice. This is in some ways an...
used by John André
John André
John André was a British army officer hanged as a spy during the American War of Independence. This was due to an incident in which he attempted to assist Benedict Arnold's attempted surrender of the fort at West Point, New York to the British.-Early life:André was born on May 2, 1750 in London to...
and Benedict Arnold
Benedict Arnold
Benedict Arnold V was a general during the American Revolutionary War. He began the war in the Continental Army but later defected to the British Army. While a general on the American side, he obtained command of the fort at West Point, New York, and plotted to surrender it to the British forces...
during the negotiations that led to Arnold's failed attempt to surrender West Point to the British in 1780.
Background
In May 1779, Continental ArmyContinental Army
The Continental Army was formed after the outbreak of the American Revolutionary War by the colonies that became the United States of America. Established by a resolution of the Continental Congress on June 14, 1775, it was created to coordinate the military efforts of the Thirteen Colonies in...
Major General Benedict Arnold
Benedict Arnold
Benedict Arnold V was a general during the American Revolutionary War. He began the war in the Continental Army but later defected to the British Army. While a general on the American side, he obtained command of the fort at West Point, New York, and plotted to surrender it to the British forces...
initiated what became a series of communications with British Army
British Army
The British Army is the land warfare branch of Her Majesty's Armed Forces in the United Kingdom. It came into being with the unification of the Kingdom of England and Scotland into the Kingdom of Great Britain in 1707. The new British Army incorporated Regiments that had already existed in England...
Major John André
John André
John André was a British army officer hanged as a spy during the American War of Independence. This was due to an incident in which he attempted to assist Benedict Arnold's attempted surrender of the fort at West Point, New York to the British.-Early life:André was born on May 2, 1750 in London to...
, the adjutant and spy chief to Lieutenant General Sir Henry Clinton, the commander-in-chief of British forces in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. 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
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, 1765–1769...
by William Blackstone
William Blackstone
Sir William Blackstone KC SL 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 in London, Blackstone was educated at Charterhouse School before matriculating at Pembroke...
or Nathan Bailey
Nathan Bailey
Nathan Bailey was an English philologist and lexicographer.-Life:Bailey was a Seventh Day Baptist, admitted 1691 to a congregation in Whitechapel, London. He was probably excluded from the congregation by 1718. Later he had a school at Stepney...
'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 was often disguised by embedding it in a letter written by Arnold's wife Peggy
Peggy Shippen
Peggy Shippen, or Margaret Shippen , was the second wife of General Benedict Arnold...
, where the cipher would be written in invisible ink
Invisible ink
Invisible ink, also known as security ink, is a substance used for writing, which is invisible either on application or soon thereafter, and which later on can be made visible by some means. Invisible ink is one form of steganography, and it has been used in espionage...
, but might also be 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, 1780:120.9.7, W------- 105.9.5's on the .22.9.14.---- / of 163.8.19 F----- 172.8.7s to 56.9.8 |30.000| 172.8.70 to 11.94. in / 62.8.20. If 179.8.25, 84.8.9'd, 177.9.28. N---- is 111.9.27.'d on / 23.8.10. the 111.9.13, 180.9.19 if his 180.8.21 an .179.8.25., 255.8.17. for / that, 180.9.19, 44.8.9—a—is the 234.8.14 of 189.8.17. I --- / 44.8.9, 145.8.17, 294.9.12, in 266.8.17 as well as, 103.8.11, 184.9.15.---- / 80.4.20. ---- I149.8.7, 10.8.22'd the 57.9.71 at 288.9.9, 198.9.26, as, a / 100.4.18 in 189.8.19—I can 221.8.6 the 173.8.19, 102.8.26, 236.8.21's--- / and 289.8.17 will be in 175.9.7, 87.8.7--- the 166.8.11, of the .191.9.16 / are .129.19.21 'of --- 266.9.14 of the .286.8.20, and 291.8.27 to be an ---163.9.4 / 115.8.16 -'a .114.8.25ing --- 263.9.14. are 207.8.17ed, 125.8.15, 103.8.60--- / from this 294.8.50, 104.9.26—If 84.8.9ed—294.9.12, 129.8.7. only / to 193.8.3 and the 64.9.5, 290.9.20, 245.8.3 be at an, 99.8.14 . / the .204.8.2, 253.8.7s are 159.8.10 the 187.8.11 of a 94.9.9ing / 164.8.24, 279.8.16, but of a .238.8.25, 93.9.28.
Decoded example
Here is how Jonathan OdellJonathan Odell
Jonathan Odell was a Loyalist poet who lived during the American Revolution.Odell was born in Newark, New Jersey in 1737 to John and Temperance Odell. He graduated from Princeton University in 1754...
, André's assistant, decoded the message:
General W[ashington]--- expects on the arrival of the F[rench]--- Troops to collect / 30,000 Troops to act in conjunction; if not disappointed, N[ew]. York is fixed / on as the first Object, if his numbers are not sufficient for that Object, / Can-a- is the second; of which I can inform you in time, as well as of / every other design. I have accepted the command at W[est]. P[oint]. As a Post in which / I can render the most essential Services, and which will be in my disposal. / The mass of the People are heartily tired of the War, and wish to be on / their former footing - They are promised great events from this / year's exertion—If - disappointed - you have only to persevere / and the contest will soon be at an end. The present Struggles are / like the pangs of a dying man, violent but of a short duration---