The Substitute Cipher
The substitute cipher is one of the oldest ciphers spies have ever used. Believe it or not, it was invented by Julius Caesar. In this cipher, one letter is substituted for another letter.
When the key code to a substitute cipher is unknown, people called cryptanalysts work to figure out these ciphers. They often start by counting letters. In the English language, "E" is the letter used most often. They would look at the message and figure out which letter is used the most. This is the letter that probably stands for "E." It can take a very long time to figure out this cipher if you don't have the key code. In the 17th century, it took a man sixteen years to figure out a message in Morocco.
Here is a chart showing an example of a substitute cipher. On the top row, (red) are the original letters, and on the bottom row (blue) is the translation.
You and your friends can have fun creating your own substitute ciphers. Just make sure you keep your key codes in a safe place.
Try to decode this message!
L KRSH BRX OLNHG PB SDJH!
Hill, Laban Carrick. Spy's Survival Handbook. New York, New York: Tangerine Press, 2003.
Platt, Richard. Eyewitness Books Spy. New York, New York: Alfred A. Knopf, Inc., 1996.
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