Revolutionary Ciphers

Eighteenth-century spies did all they could to conceal their activities and the information they carried. Spymasters such as Benjamin Tallmadge and John André created ciphers that were tough to break and put together teams of people whose whole job was to decipher the other side's secret messages. One common cipher, but one that was almost impossible to break, was the book cipher used between André and Arnold. The whole cipher was based on a book that both of them owned (William Blackstone's Commentaries on the Laws of England) that connected secret words to a page number, to the line on the page on which the word appeared, and to 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." Unless one owned that particular book, deciphering the word would be very difficult. Tallmadge's cipher system was just as tough to break because it blended together several different ciphers, including one based on switching around letters in the alphabet and a "numerical dictionary" cipher in which he chose numbers to represent words, names and places (for example, artillery was "46" and Virginia was "739"). Only those agents that had Tallmadge's cipherbook would ever know what the Continental Army's secret messages actually meant.

Other ways to conceal information included invisible ink and innovative ways to physically hide them. Invisible ink was a fairly easy way to write a secret message because lemon juice, milk, or vinegar could all be used as ink to write a secret message between the lines of a regular letter. To reveal the secret words, the letter only needed to be exposed to heat, such as that from a candle. A favorite British practice 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 grille and then other words were filled in around them to comprise the body of a normal 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 fit inside a ball resembling a rifle bullet (and which could be swallowed if necessary).