Python Exercise

Substitution Cipher

Overview

Students will use a substitution cipher to encrypt a message and then to decrypt a message. In a substitution cipher each letter of the alphabet is swapped with a different letter.

Purpose

Substitution ciphers are significantly more secure than a Caesar cipher but require a more complicated key. Look at the differences in complexity and portability of different ciphers.

Objectives

Students will be able to:

• Encode and decode messages using a substitution cipher.
• Identify the mathematical advantage of substitution over shift cipher.

Preparation

• Print the Substitution Cipher Activity Worksheet
• Arduino Student Inventor Kit (Extended Activity)

Links

• For the Teacher

• For the Students

  • https://www.guardsupport.com/crypto/index.asp
  • http://www.cryptoclub.org/games/rogue_computer.php
  • http://www.cryptograms.org/play.php

Vocabulary

Teaching Guide

Getting Started

Introduce the idea of a substitution cipher

• Each letter of our alphabet is swapped for that of a jumbled alphabet
• There are 26! different possible alphabets that could be used.

Activity

• Substitution Cipher Activity Worksheet

• Students will encrypt and decrypt several messages using a substitution cipher.

• They will then explore a keyword cipher and finally encrypt a message with a keyword to swap with someone else in the class.

Activity

• Python Programming: There are multiple ways to generate a “key” alphabet in the cipher.
  • SubstitutionCipher.py
• Python Programming: Program to analyze the letter frequency of a message.
  • LetterFrequency.py
  • Output of this is a CSV that can be graphed in Excel

Wrap-up

Discussion

1. How is this type of cipher more secure than a shift cipher like Caesar?
2. What are the weaknesses of this cipher?
3. How does the random substitution compare to a keyword cipher?
   1. What are strengths/weaknesses of each?

Assessment Questions

• How would frequency analysis be used to decrypt a message encrypted with a substitution cipher?
• Is there are way to change this encryption so it is not vulnerable to frequency analysis?

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Source: Derek Babb, https://derekbabb.github.io/CyberSecurity/Classic_Cryptography/Substitution_Cipher.html
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License.