Understanding Encryption – Writing a Caesar Cipher in Python

Encryption is at the heart of modern cybersecurity — from messaging apps to online banking. Students often imagine encryption as something complex and mysterious, but many key ideas begin with surprisingly simple methods. One of the earliest examples is the Caesar cipher, used by Julius Caesar to send secure messages to his generals.

Writing a Caesar cipher in Python is an excellent introduction to encryption at GCSE and A Level Computing. It helps students understand substitution ciphers, modular arithmetic, character encoding, and the logic behind more advanced systems.

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What Is a Caesar Cipher?

A Caesar cipher shifts each letter in a message by a fixed number of positions in the alphabet.
For example, with a shift of 3:

• A → D

• B → E

• C → F

The message “HELLO” becomes “KHOOR”.

It’s simple, but it introduces students to two key ideas:

• Encryption (scrambling a message)

• Decryption (undoing the scrambling)

Modern encryption is vastly more complex — but the logic of substitution and key-based security begins here.

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Writing a Caesar Cipher in Python

Here is a simple encryption function:

def caesar_encrypt(text, shift):
    result = ""
    for char in text:
        if char.isalpha():
            base = ord('A') if char.isupper() else ord('a')
            result += chr((ord(char) - base + shift) % 26 + base)
        else:
            result += char
    return result

And a matching decryption function:

def caesar_decrypt(cipher, shift):
    return caesar_encrypt(cipher, -shift)

Students can test their program:

message = "Secret Message"
encrypted = caesar_encrypt(message, 4)
decrypted = caesar_decrypt(encrypted, 4)

This shows encryption and decryption clearly and logically.

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Extending the Task

More confident students can:

• Add support for punctuation and numbers

• Create a brute-force attack to test all 26 possible shifts

• Analyse letter frequencies to understand why the cipher is weak

• Link this to modern encryption and hashing algorithms

This builds understanding of cybersecurity, algorithm design, and ethical hacking.

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Why It Works in Teaching

Students gain experience in:

• String manipulation

• Loops and conditionals

• Character encoding (ASCII/Unicode)

• Modulus arithmetic

• Thinking like both a programmer and an attacker

Most importantly, they see that encryption is not magic — it’s a series of logical steps designed to hide information.