Arnold Cipher

The Arnold Cipher was a book cipher developed by Benedict Arnold, the infamous turncoat of the American Revolution. His cipher was used to communicate with his conspirator, John André. Arnold used a book as a key to the cipher, such as Commentaries on the Laws of England by William Blackstone or Nathan Bailey's Dictionary. He would send André a piece of paper on which was written a series of numbers (separated by periods); 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.

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 "general":

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.

The above translates into:

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---

-July 12, 1780 -- Letter from Benedict Arnold to John André (From the Collections of the Clements Library)