invisible ink: During the Revolutionary War invisible ink usually consisted of a mixture of ferrous sulfate and water.
secret codes: Ciphers and secret codes were used to ensure that the contents of a letter could not be understood if correspondence was captured.
Mask letters: Search Results
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A "Mask Letter" is the name given to a method that the British used during the Revolutionary War to conceal a secret message. This method allowed the secret message to be hidden in plain sight. ... To discover the hidden message, the reader would place a "mask" on top of the letter to reveal the secret message