Codes and Concealment

The lengths to which Revolutionary spymasters such as Benjamin Tallmadge and John André went to conceal their activities and that of their spy networks were extraordinary. They employed a variety of different methods of clandestine communication that sometimes were so elaborate that they confused even themselves (as with the complex cipher created by the Committee for Secret Correspondence's James Lovell, which befuddled even the erudite John Adams).

The most tried and true method of acquiring intelligence was the job of so-called "Black Chambers" — those agents responsible for capturing and deciphering the diplomatic and military correspondence of the other side. The development of codes and other ways to hide the actual meaning of letters was a matter of great importance. Simple codes of longstanding use in merchant communication, such as the popular one in which letters of the alphabet were simply shifted to correspond with other letters (for example, if a = e, then "art" would be written as "evx"), were jettisoned early in the war as far too easy to break. Instead, such codes were combined with more particular systems that depended on previously agreed upon forms between individual agents. Benedict Arnold and André used an especially effective one based on an edition of a book that both owned, William Blackstone's Commentaries on the Laws of England, that connected coded words to a page number, the line on the page on which the word appeared, and the number of the word in that line. For instance, to decipher a word written "120.9.7," a reader would take his or her copy of Blackstone, go to page 120, read down to the ninth line, and then trace over to the seventh word to fine "General" (a reference to Washington). Unless one owned that edition and knew that it was the basis of the code, the letter would be virtually impossible to decipher. Tallmadge's code was more individual as it blended a number of forms, including the commercial code, the book code, and a set of numbers he created to represent individual names and places (e.g., artillery was "46" and Virginia was "739"). The Marquis de Lafayette employed a simple numerical code while American patriot-turned-British-spy Benjamin Church used a complex cipher that combined the English alphabet with Greek and Latin symbols.

Other methods went beyond codes to more innovative forms of hiding information such as invisible "ink," masks, and physical concealment of messages. Invisible ink was fairly easy to come by as lemon juice, milk, or vinegar could be used to write a secret message between the lines of an otherwise innocuous letter. Once the letter was exposed to some heat, such as candlelight, the secret would be revealed. A favorite technique of Tallmadge's, but a quite complicated one, was the use of a particular compound of acids to write and reveal clandestine news. A favorite British practice that Americans never seem to have used was the much simpler "Cardan Grille" (named for a 16th-century Italian code-maker). The grille (or mask) was a piece of paper out of which any form could be cut (British commander-in-chief Sir Henry Clinton used an hourglass, others used spectacles, some used a grid) and simply placed over a common letter to reveal the real message hidden within, which was usually written first within the forms and then other words were filled in around them to comprise the body of the letter. Some spies came up with other ways to hide their messages, such as writing them on long, narrow strips of paper that could be inserted into hollowed-out quills of large feathers or on small pieces of parchment that could fit inside a ball resembling a rifle bullet (and which could be swallowed if necessary).