caesar_cipher.py

"""
Caesar Cipher — Basic Encryption & Decryption
-----------------------------------------------
Project 2 | Cyber Security Track | DecodeLabs Industrial Training Kit

Implements the classic Caesar (shift) cipher using the IPO model:
  INPUT   -> Plaintext
  PROCESS -> Algorithm (shift) + Key (n)
  OUTPUT  -> Ciphertext

Formulas:
  Encryption: E(x) = (x + n) % 26
  Decryption: D(x) = (x - n) % 26

Handles:
  - Uppercase and lowercase letters independently
  - Spaces, punctuation, and digits (left unchanged)
  - Negative / large shift keys via modulo wraparound
"""


def encrypt(text: str, shift: int) -> str:
    """Encrypt plaintext using a Caesar cipher with the given shift key."""
    result = []
    shift = shift % 26  # normalize shift so it always falls in [0, 25]

    for char in text:
        if char.isupper():
            result.append(chr((ord(char) - 65 + shift) % 26 + 65))
        elif char.islower():
            result.append(chr((ord(char) - 97 + shift) % 26 + 97))
        else:
            # Non-alphabetic characters (spaces, punctuation, digits) stay as-is
            result.append(char)

    return "".join(result)


def decrypt(text: str, shift: int) -> str:
    """Decrypt ciphertext using a Caesar cipher with the given shift key."""
    # Decryption is just encryption with the inverse shift
    return encrypt(text, -shift)


def brute_force(text: str) -> None:
    """Bonus: demonstrate why Caesar cipher is a lockbox, not a vault.
    Tries all 25 possible shifts (frequency-analysis style attack)."""
    print("\n--- Brute Force: All Possible Shifts ---")
    for key in range(1, 26):
        print(f"Shift {key:2d}: {decrypt(text, key)}")


def main():
    print("=" * 50)
    print("   CAESAR CIPHER — ENCRYPTION & DECRYPTION")
    print("=" * 50)

    message = input("\nEnter the text to encrypt: ")

    while True:
        try:
            shift_key = int(input("Enter the shift key (e.g. 3): "))
            break
        except ValueError:
            print("Please enter a valid integer for the shift key.")

    encrypted = encrypt(message, shift_key)
    decrypted = decrypt(encrypted, shift_key)

    print("\n--- Results ---")
    print(f"Plaintext : {message}")
    print(f"Shift Key : {shift_key}")
    print(f"Encrypted : {encrypted}")
    print(f"Decrypted : {decrypted}")

    verify = "PASSED ✅" if decrypted == message else "FAILED ❌"
    print(f"Round-trip Check: {verify}")

    show_bf = input("\nRun brute-force demo on the ciphertext? (y/n): ").strip().lower()
    if show_bf == "y":
        brute_force(encrypted)


if __name__ == "__main__":
    main()