Fun With Python: Intro To Cryptography - Caesar and ROT13

Hey Everyone,

Sorry for not posting yesterday. I was planning to, but I got stuck down a deep rabbit hole last night. I wanted to start a new branch of the "Fun with Python" series that could also serve as an introduction to cryptography, which is what today's post is all about.

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Intro to Cryptography: The Caesar Cipher

Probably one of the most well-known and simplest cryptography algorithms is the Caesar cipher. It's a type of substitution cipher where each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a shift of 3, 'A' would become 'D', 'B' would become 'E', and so on.

Its more modern and well-known relative is ROT13, which is just a Caesar cipher with a fixed shift of 13. The fun thing about ROT13 is that it's reciprocal; applying the same operation twice gets you back to your original text.

Here are a couple of simple Python scripts to demonstrate.

caesar.py

This script can handle any shift value, for both encrypting and decrypting.

#!/usr/bin/env python3
import argparse


def caesar_cipher(text, shift, decrypt=False):
    """
    Applies the Caesar cipher to a given text with a given shift.
    Handles both uppercase and lowercase letters, preserving case.
    Non-letter characters are left unchanged.
    :param text: The text to be processed.
    :param shift: The shift amount (integer).
    :param decrypt: If True, shifts backward for decryption.
    :return: The processed text.
    """
    if not text:
        raise ValueError("Text cannot be empty.")

    result = []
    effective_shift = shift if not decrypt else -shift

    for char in text:
        if char.isupper():
            base = ord("A")
            new_char = chr((ord(char) - base + effective_shift) % 26 + base)
            result.append(new_char)
        elif char.islower():
            base = ord("a")
            new_char = chr((ord(char) - base + effective_shift) % 26 + base)
            result.append(new_char)
        else:
            result.append(char)

    return "".join(result)


def main():
    parser = argparse.ArgumentParser(description="Caesar Cipher")
    parser.add_argument("text", help="Text to encrypt/decrypt")
    parser.add_argument("shift", type=int, help="Shift amount")
    parser.add_argument(
        "--decrypt", action="store_true", help="Decrypt instead of encrypt"
    )
    args = parser.parse_args()

    try:
        result = caesar_cipher(args.text, args.shift, args.decrypt)
        print(f"Result: {result}")
    except ValueError as e:
        print(f"Error: {e}")


if __name__ == "__main__":
    main()

rot13.py

And here is a dedicated script just for ROT13.

#!/usr/bin/env python3
import argparse


def rot13_cipher(text):
    """
    Applies the ROT13 cipher to a given text.
    ROT13 is a Caesar cipher with a fixed shift of 13.
    It is reciprocal: applying it again decrypts the text.
    Handles both uppercase and lowercase letters, preserving case.
    Non-letter characters are left unchanged.
    :param text: The text to be processed.
    :return: The processed text.
    """
    if not text:
        raise ValueError("Text cannot be empty.")

    result = []
    shift = 13

    for char in text:
        if char.isupper():
            base = ord("A")
            new_char = chr((ord(char) - base + shift) % 26 + base)
            result.append(new_char)
        elif char.islower():
            base = ord("a")
            new_char = chr((ord(char) - base + shift) % 26 + base)
            result.append(new_char)
        else:
            result.append(char)

    return "".join(result)


def main():
    parser = argparse.ArgumentParser(description="ROT13 Cipher")
    parser.add_argument("text", help="Text to process with ROT13")
    args = parser.parse_args()

    try:
        result = rot13_cipher(args.text)
        print(f"Result: {result}")
    except ValueError as e:
        print(f"Error: {e}")


if __name__ == "__main__":
    main()

As you can see, running ROT13 is the same as running the Caesar cipher with a shift of 13.

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About That Rabbit Hole...

So, what sent me down this rabbit hole? While researching different ciphers we could play with, I got curious: "I've always wondered how the Enigma Machine works. How hard could it be? It was made in the 1930s, right?"

Oof. Boy, do I have some serious respect for the people who created that thing, and for the codebreakers who broke it during WWII. It was a bit of a journey, but the end result was worth it.

Let's just say I have a fully historically accurate recreation of the Enigma I machine... but that's going to have to be a post for another day.

As always,
Michael Garcia a.k.a. TheCrazyGM