Whether you’re a student passing notes in class or a spy trying to send a foreign communiqué, the ability to send a secret note is sometimes critical. But how are such secret messages created, and how can they be decoded when they reach their destination?

In this lesson, students learn about encrypting and decrypting messages using ciphers. After encrypting messages both graphically and algebraically, students try to decrypt some messages too. In the end, they’ll learn what makes for a useful cipher, and what makes a cipher impossible to decode.

### Students will

• Encode messages using different types of ciphers
• Decode messages using different types of ciphers
• Encode and decode messages using graphical representations of ciphers
• Determine what features of a cipher make it a good or bad candidate for encrypting messages

### Before you begin

Students should be comfortable evaluating linear and quadratic functions, and should have experience interpreting graphs of functions. This lesson can be used to introduce some concepts from the study of inverse functions; prior exposure to inverse functions isn’t required, though it may help enrich some of the mathematical conversation.