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http://www.womenhistoryblog.com/2009/01/women-spies-of-revolution.html This is a secondary source for the silver balls or silver bullets.
1.) They were small enough to be swallowed if the messenger was caught
2.) They ripped the paper into slivers in order to fit it in the ball.
3.) The paper slivers can be put together in order to reveal the secret message

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The book George Washington Spymaster is a primary source for silver balls or bullets

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https://www.ducksters.com/history/american_revolution/spies.php This is a secondary source on mask letters and invisible ink.
1.) American invisible ink was called "stain".
2.) Masks were used on what seemed like ordinary normal letters.

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http://history.org/foundation/journal/summer04/spies.cfm this is a a primary source on mask letters and invisible ink.

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http://www.drdobbs.com/security/the-book-cipher-algorithm/210603676 this is a secondary source for the code book.
1.)Book ciphers use words to encode messages

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https://indiana.pbslearningmedia.org/resource/mv15-soc-ush-culpcode/culper-code-book/#.wqbQ_6inHrc
here is a primary source of the cupler spy code book.

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https://www.encyclopedia.com/social-sciences-and-law/law/crime-and-law-enforcement/codes-and-ciphers here is a secondary source on codes such as encryption.
1.) Codes and cyphers are a type of cryptography.
2.) With out code books no one would know that for example, "OAKEN 7890 SPINDRIFT" could be a disguise "Kill the king."
3.) Ciphers are things used to decode messages such as encryption.

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https://www.brainpickings.org/2012/02/28/invisible-ink/ here is a primary source on encryption.

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https://sites.google.com/site/revolutionarywarspyquest/mask-letter this is a secondary source on mask letters
1.) Name given by British meaning to conceal a message
2.) Their would be a paper mask that would cover the main paper to reveal a secret message

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https://www.artofmanliness.com/2011/09/09/man-knowledge-the-history-of-invisible-ink/ this is a a primary source that goes with my invisible ink.

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http://www.amc.com/shows/turn/exclusives/spy-tools-of-the-american-revolution this is a secondary source. For my invisible egg recipe.
1.) Invisible ink was used in many different ways including egg invisible ink
2.) Signal to warn others would be as simple as a black dress on a clothesline surrounded by white quilt sheets

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http://clements.umich.edu/exhibits/online/spies/index-gallery.html hidden letters from revolutionary time. This is a primary source.