In the late summer of 1781, General George Washington finally saw an opportunity to take New York City away from the British. Virtually from the beginning of the War for Independence six years earlier, the British held this key city and Washington long desired to take it into American hands. Washington laid siege to the town all summer. With the expected arrival of Admiral de Grasse and ships of the French fleet along with an additional 3,000 French soldiers, he believed he may finally have his chance. But on August 14th, he changed his mind and turned his eye to Yorktown, Virginia.Intelligence, gained partially through the decryption of captured British messages, gave Washington the assurance he needed to complete his move on Yorktown.Communication plays an important role in both a country's diplomacy and its wars. Even if that country doesn't yet exist. Keeping those communications secret, or the ability to understand the adversary's communications, can make the crucial difference in a leader's actions and abilities.At the time of the American Revolution, both the British and the American rebels practiced a variety of methods to keep their written communications secret. Both had networks of spies who needed to pass on their information right under the noses of their adversaries. Both turned to invisible inks, hidden messages, and secret writing in the form of ciphers and codes.Ciphers and codes, cryptography, change messages into something unintelligible by the use of keys and lists. Ciphers rearrange letters or change individual letters into a different letter, number, or symbol based on a prearranged setting known as a key. Codes change entire words or phrases into other words, number groups, or symbols based on a list or a book. To decrypt the secret messages, the receiver needs access to the original key. Theoretically, the adversary wouldn't have the key and therefore could not understand the message even if it was captured.Solving a message without having the key, cryptanalysis, has been a science employed by governments for as long as people have been using cryptography to make their messages secret. European governments have a long history of "Black Chambers" the offices where other countries' diplomatic mail was opened and read. If the message was encoded, a Black Chamber tried to solve the code and read the message.This is the story of revolutionary communications and cryptologic secrets and the role they played in America's war for independence.