During the Revolutionary War spies had to devise different ways of hiding correspondence in case it would get captured by their enemies. They used ciphers, secret codes, invisible ink, hidden letters, mask letters and other methods to conceal messages.
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In ciphers, letter were replaced by other letters and a special key was used to read the correspondence. Some ciphers included letters and numbers to create a secret code.
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Another method of secret writing was using invisible ink. The secret writing was written in between the lines of a regular letter and would become visible after heat or a special chemical substance was applied.
![Picture](/uploads/9/8/2/9/98295840/published/9.jpg?1486596671)
Hidden letters was also used to hide messages and correspondence during the Revolutionary War. Spies would roll notes or letters very tightly on a thin strip of paper and place them into different objects such as buttons, hollow silver balls and even quills of large feathers.
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Mask letters would hide the content of a hidden message within a regular letter. To view it the recipient had to apply a template over a full letter in order to reveal the secret message. In this case the messanger had to make two deliveries, one to deliver the letter and another to deliver the template. This precaution was used in case one of the objects was captured.