Ok, so you want to send a secret message to your friend but you don’t want anyone to know what you say. How are you going to do that?
This is a problem that people have been facing for thousands of years. In the 1st century BC, the famous emperor Julius Caesar used a special tool called a “Caesar Cipher” to accomplish this.
Let's see how it works. To encrypt a message, I first pick a secret “encryption key” that I share with my friend, and they use the key to figure out a “decryption key”. For a
Caesar Cipher, the encryption key is a number from 1 to 25 and the decryption key is the additive inverse (or minus) of that number.
So let’s say Alice wants to send Bob a secret message. The message is “CAT”. In order to encrypt the message, Alice first picks her encryption key — lets say it is the number 10 — and then shifts each letter 10 places down in the alphabet. So, C becomes M, A becomes K, and T becomes D (note that after Z we just start again at A). Alice sends Bob the message “MKD”. Bob then uses his decryption key (-10) and recovers the message “CAT”, while someone who doesn’t have the decryption key would only be able to read the message MKD.
In this project, you will make physical encryption and decryption wheels. You could then encrypt the message and have students try to decrypt the message.
Here are some interesting questions you might ask: