Today we learned that the Germans used a device called the Engima machine to encrypt their messages during WWII. They thought it was completely secure, but the Allies broke the code!
We watched this video about how the Engima machine works. The general idea is that when you encode a letter, it goes through three different substitution ciphers, bounces off a "reflector," and then goes back through those same three substitution ciphers again. Each substitution cipher is on a rotor. Each time you encrypt a new letter, at least one rotor moves, so you end up using a different set of substitution ciphers for each letter!
We made our own Enigma machines out of paper and tape. Here's the Enigma machine construction in progress:
Using the engima machine to decode a message:
There's a nice article here about how the Engima machine works and about some of the things that helped the Allies figure out what was going on.
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