1780: Treasonous Ciphered Letter
During the American Revolution, British Major John André joined with American General Benedict Arnold in a scheme to secure British control over the American fortification at West Point, New York. The reward - a large sum of money and a high position in the British army. Arnold was disgruntled with the Continental Army and corresponded with British general Sir Henry Clinton beginning in 1779. In July 1780, Arnold proposed the sale of West Point, a strategic Colonial stronghold, to the British. In a coded message he offered West Point, the garrison and supplies, for £20,000 sterling. Arnold used a dictionary code to create this message. Each word was represented by a three-digit code group. The numbers represented page, column, and word down that column. 12.8.16 represented the 16th word in the first column on page 12 of Nathan Bailey’s Dictionary. (Arnold added 7 to the column number to create 8 or 9 instead of 1 or 2)
Before it could be carried out, however, minutemen captured John André and informed General George Washington of the plot. Arnold managed to evade Washington's arrest warrant, but André was imprisoned at Tappan, New York, and on September 29, 1780, he was found guilty of being behind American lines "under a feigned name and in a disguised habit. He was executed as a spy by Washington's order October 2, 1780.
(Image and portions of text borrowed from www.mountvernon.org)