Cracking Code with a Caesar Cipher

A Caesar Cipher hides a message by shifting every letter of the alphabet by the same amount — shift by 3, and A becomes D, B becomes E, and so on. That shift is the key, and it is what keeps the message secret.

What is a Caesar Cipher?

Imagine the alphabet as a circle. A Caesar Cipher shifts each letter the same number of places around it.

Encoding a Message

Let's encode the message "HELLO" with a shift of 3.

So "HELLO" becomes "KHOOR".

Decoding a Message

Now, let's decode the message "Lipps Asvph" that was encoded with a shift of 4.

So "Lipps Asvph" becomes "Hello World".

What happens at the end of the alphabet?

Look at the shifted alphabet again: DEFGHIJKLMNOPQRSTUVWXYZABC. The last three letters have wrapped round to the front, which is what "imagine the alphabet as a circle" really means.

Try it yourself

Here is a Caesar cipher encoder and decoder written in Python.

def caesar_cipher(text, shift, mode):
    result = ''
    for char in text:
        if char.isalpha():
            start = ord('a') if char.islower() else ord('A')
            shifted_char = chr((ord(char) - start + shift) % 26 + start) if mode == 'encode' else chr((ord(char) - start - shift) % 26 + start)
        elif char.isdigit():
            shifted_char = str((int(char) + shift) % 10) if mode == 'encode' else str((int(char) - shift) % 10)
        else:
            shifted_char = char
        result += shifted_char
    return result

# Example usage
message = "Hello, World! 123"
key = 3

encoded_message = caesar_cipher(message, key, 'encode')
print(f"Encoded: {encoded_message}") # Output: Khoor, Zruog! 456

decoded_message = caesar_cipher(encoded_message, key, 'decode')
print(f"Decoded: {decoded_message}") # Output: Hello, World! 123

Troubleshooting:

• If you get strange characters, double-check your shift value and make sure you are using the correct encoding/decoding mode.
• Make sure your Python environment is set up correctly.

The shift number is the whole secret. Anyone without it has to try all 25 possible shifts before they find your message.