Caesar Cipher

A simple Python implementation of the Caesar Cipher — one of the oldest encryption techniques — that lets you encrypt and decrypt messages using a shift key.

How It Works

Each letter in your message is shifted forward (encryption) or backward (decryption) by a fixed number of positions in the alphabet. Non-alphabetic characters (numbers, punctuation, spaces) are left unchanged.

Example with shift = 3:

Plaintext:  Hello, World!
Ciphertext: Khoor, Zruog!

Usage

Run the program:

python Caesar_Cipher.py

You'll be presented with a menu:

1. Encrypt a message
2. Decrypt a message
3. Run test cases
4. Exit

Enter your message and a shift key between 0 and 25 when prompted.

Features

• Encrypts and decrypts any text message
• Preserves letter case (upper/lowercase)
• Handles large shift values via modulo 26 normalisation
• Includes automated test cases covering edge cases and round-trips