Lesson Plan: Creating a Caesar Cipher

Lesson Overview

In this lesson, students will learn about the Caesar cipher, a simple encryption technique. They will then implement their own Caesar cipher encoder and decoder using Python.

Learning Objectives

Understand how the Caesar cipher works.
Implement a basic Caesar cipher in Python.
Encrypt and decrypt messages using their implementation.
Modify the cipher to handle edge cases (e.g., preserving spaces, punctuation, and case sensitivity).

Lesson Structure

1. Introduction to Cryptography (10 min)

- Brief history of cryptography.
- Overview of substitution ciphers.
- Explanation of the Caesar cipher:

Each letter is shifted by a fixed number of positions.
Example: Shift of 3 → A → D, B → E, C → F, etc.
Decryption reverses the shift.

2. Pseudocode for Caesar Cipher (10 min)

Discuss the logic:

Choose a shift value.
Convert each letter to its ASCII value.
Shift the value and convert it back to a letter.
Handle uppercase and lowercase letters separately.
Preserve non-alphabetic characters (spaces, punctuation).

3. Implementing the Caesar Cipher in Python (20 min)

Step 1: Writing the encryption function


        def caesar_cipher_encrypt(text, shift):
            encrypted_text = ""
            for char in text:
                if char.isalpha():
                    shift_amount = shift % 26
                    new_char = chr(((ord(char.lower()) - ord('a') + shift_amount) % 26) + ord('a'))
                    encrypted_text += new_char.upper() if char.isupper() else new_char
                else:
                    encrypted_text += char  # Preserve spaces and punctuation
            return encrypted_text

Step 2: Writing the decryption function


        def caesar_cipher_decrypt(text, shift):
            return caesar_cipher_encrypt(text, -shift)

Step 3: Testing the functions


        message = "Hello, World!"
        shift_value = 3
        encrypted_message = caesar_cipher_encrypt(message, shift_value)
        decrypted_message = caesar_cipher_decrypt(encrypted_message, shift_value)

        print(f"Original: {message}")
        print(f"Encrypted: {encrypted_message}")
        print(f"Decrypted: {decrypted_message}")

4. Enhancements & Challenges (15 min)

Modify the function to allow for non-alphabetic character shifts.
Add user input functionality.
Create a function that brute-force decrypts a message (trying all 25 shifts).
Implement a Caesar cipher using list comprehensions.

5. Q&A and Wrap-up (5 min)

Review key concepts.
Discuss real-world applications of simple ciphers.
Homework: Research frequency analysis and how it can be used to break the Caesar cipher.

Assessment

Students submit their working code.
Students encrypt and decrypt messages and verify correctness.
Discussion on edge cases and improvements.