Public key encryption is an extraordinary recent development that has made internet commerce possible. It allows people to encrypt and decrpyt messages without having to share a password to unlock them. It is hard to believe that such a system could exist, yet this mode of encryption is widely used because it is so easy to set up.
This activity works with two people, but it's most exciting with a whole class, with everybody else trying to intercept a message sent between two students.
This activity is fairly demanding, and requires students to be careful in the way the encode messages. Also, the samples provided here are aimed at junior high students; some younger students may find it too difficult, and more capable or older students are likely to need more complex "maps" to make the encryption convincing, since the one provided can be solved relatively easily. Balancing the amount of tedious effort required for large maps against the insecurity of smaller maps needs to be done by the teacher, although this issue can be a discussion point for the class, since the goal is to understand the issues around encryption.
Auburn Robo Camp student Taylor Macowksky has implemented this activity in Alice programming language. See the Video: Unplugged Public Key Encryption Implemented in Alice by Taylor Mackowsky
Misha Leder, a Software Engineer at Google has an activity called Privacy and Encryption which can be a nice extension activity. You need to send a secret message to your friend via Internet. But there are very curious guys who want to read your message. How can you deliver the message safely? How can you confuse the very curious guys?
Frank Tapson - Centre for Innovation in Mathematics Teaching has an informative booklet on Public Key Cryptography developed as a resource for Modulo Arithmetic Maths Curriculum.
Centre for Innovation in Mathematics Teaching has the following teaching package in Modern Encryption developed to teach Codes and Ciphers in their Maths Curriculum: Modern Encryption: Teacher Guide, Student Guide, OHP Slides, Lesson Plans.
Try Engineering has an activity called Hand Biometrics Technology which explores how engineers incorporate biometrics technologies into products, but also explores the challenges of engineers who must weigh privacy, security and other issues when designing a system. Students explore different biometrics techniques, find their own hand geometry biometrics, then work in teams of "engineers" to design a high-tech security system for a museum. Download Full Lesson Plan and Student Worsksheets.
Dr. Lisa N. Michaud has the following introductory handouts in Private Key Encryption and Public Key Encryption. See also Making Huffman Codes Handout to go with Making Huffman Codes Activity, and a Cryptography Lab Session
nrich Maths has the following activities with notes and solutions provided: