The Caesar cipher is one of the oldest and simplest methods of encryption. It’s a type of substitution cipher, meaning each letter in the original message (plaintext) is replaced with a different letter from the alphabet. This replacement follows a fixed rule: shifting each letter a certain number of positions down the alphabet.

How It Works

  • Shift (or Key): The core parameter of a Caesar cipher is the shift or key. This is a number that determines how many places each letter will be moved.
  • Encryption: To encrypt a message, each letter is shifted down the alphabet by the key value. For example, with a shift of 3:
    • ‘A’ becomes ‘D’
    • ‘B’ becomes ‘E’
    • ‘Z’ becomes ‘C’ (wrapping around from the end)
  • Decryption: The decryption process is simply the reverse. Each letter is shifted backwards by the key value.

Implementation 

  def caesar_cipher(text, shift, mode='encrypt'):
    result = ""
    for char in text:
        if char.isalpha():  # Only shift letters
            base = ord('a') if char.islower() else ord('A')
            result += chr((ord(char) - base + shift) % 26 + base)
        else:
            result += char  # Keep non-letters as they are
    return result