Caesar Cipher

Overview

Students will explore encoding/decoding messages using the Caesar Cipher. Using a cipher wheel, messages will be encoded and decoded.

Purpose

The Caesar Cipher is one of the best known forms of encryption. This signifies a change from encoding where a message was hidden with a set of substitutions for each letter to one where the only knowledge needed is a key to “unlock” the message.

Objectives

Students will be able to:

• Encode/Decode a message using a Caesar Cipher and a known key
• Begin to decode messages where the key is unknown using cryptanalysis

Preparation

• Print the Caesar Cipher wheel on cardstock
• Get brad fasteners
• Print Caesar Cipher Worksheet
• Download CaesarCipher.py python file.

Links

• For the Teacher
  • https://en.wikipedia.org/wiki/Caesar_cipher
  • http://practicalcryptography.com/ciphers/caesar-cipher/
• For the Students
  • http://www.cryptoclub.org/games/desert_oasis.php

Vocabulary

Plaintext - a message or text that can be read normally Ciphertext - a message that has been encrypted so it is not readable in current form. Encode - the act of converting plaintext to ciphertext Decode - the act of converting ciphertext to plaintext

Teaching Guide

Getting Started

• Overview of Caesar Cipher (video)
  • https://www.youtube.com/watch?v=sMOZf4GN3oc

Activity:

Paper craft Caesar Cipher wheel

• Have students cut out and assemble the paper cipher wheel.
• Students will work through the encryption process on the worksheet.
• Students can engage with the CryptoClub Desert Oasis game.

Activity (coding):

Look at the CaesarCipher.py Python file.

• The program is setup to accept a message
• The message can be encoded using a Caesar Shift
• Have students finish the decode method to convert ciphertext to plaintext.

Wrap-up

Discussion:

1. How many possible keys are there in a Caesar Cipher?

2. What level of security does this provide us? How difficult is it to break a message encrypted using the Caesar Cipher?

3. TED Talk: Why Privacy Matters

Assessment Questions

• How have computers changed the security of a cipher like this one?
• Create a Caesar Cipher where the key changes for each word or letter.
  • What would the rules of this cipher be?
  • How would you encode / decode this message?
  • What would you need to know to decode this message?
  • How does this change the security of the cipher?

Extended Learning

3D Printing: Print the Caesar Cipher ring to use instead of the paper wheel.

• https://www.thingiverse.com/thing:18315 3D Printing: Print a Cipher Disk
• https://www.thingiverse.com/thing:3404817 3D Printing: Print a Braille Disk
• https://www.thingiverse.com/thing:3701018 Programming: Bruteforce Caesar Decode
• Using the decode method written in the programming activity, create a program that decodes using all 25 possible Caesar shifts.

Standards Alignment

License

Cyber Security Curriculum is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.