cipher.py

import random
import re
from collections import Counter


def encrypt(message, shift):

    result = ""

    shift = shift % 26

    for char in message:

        if char.isalpha():

            base = ord('A') if char.isupper() else ord('a')

            result += chr((ord(char) - base + shift) % 26 + base)

        else:
            result += char

    return result


def decrypt(message, shift):

    return encrypt(message, -shift)


def vigenere_encrypt(text, key):

    result = ""

    key = key.lower()

    if not key or not key.isalpha():

        print("Key must contain letters only.")

        return text

    key_index = 0

    for char in text:

        if char.isalpha():

            shift = ord(key[key_index % len(key)]) - ord('a')

            base = ord('A') if char.isupper() else ord('a')

            result += chr((ord(char) - base + shift) % 26 + base)

            key_index += 1

        else:
            result += char

    return result


def vigenere_decrypt(text, key):

    result = ""

    key = key.lower()

    if not key or not key.isalpha():

        print("Key must contain letters only.")

        return text

    key_index = 0

    for char in text:

        if char.isalpha():

            shift = ord(key[key_index % len(key)]) - ord('a')

            base = ord('A') if char.isupper() else ord('a')

            result += chr((ord(char) - base - shift) % 26 + base)

            key_index += 1

        else:
            result += char

    return result


def crack(ciphertext):

    print("\nTrying all 25 shifts...\n")

    common = {
        "the", "and", "is", "to", "a",
        "hello", "hi", "you", "at",
        "we", "he", "she", "it",
        "of", "in", "that", "are"
    }

    best_score = 0
    best_shift = 1
    best_guess = ""

    for shift in range(1, 26):

        guess = decrypt(ciphertext, shift)

        words = re.findall(r"[a-z]+", guess.lower())

        score = sum(word in common for word in words)

        if score > best_score:

            best_score = score
            best_shift = shift
            best_guess = guess

        readable = " ← readable?" if score > 0 else ""

        print(f"[{shift:>2}] {guess}{readable}")

    print(f"\nBest Guess (shift={best_shift}):")

    print(best_guess)


def frequency_analysis(text):

    letters = [c.lower() for c in text if c.isalpha()]

    if not letters:

        print("No letters found.")

        return

    counts = Counter(letters)

    total = sum(counts.values())

    print("\nLetter Frequency:\n")

    for char, count in sorted(counts.items()):

        percent = (count / total) * 100

        bar = "*" * int(percent)

        print(f"{char}: {count:>3} ({percent:5.1f}%) {bar}")


def strength_report(message, shift):

    letters = sum(1 for c in message if c.isalpha())

    score = 0

    tips = []

    if len(message) >= 15:

        score += 1

    else:
        tips.append("Longer messages are harder to analyse.")

    if shift not in [1, 3, 13]:

        score += 1

    else:
        tips.append("Common shifts are easy to guess.")

    has_upper = any(c.isupper() for c in message if c.isalpha())

    has_lower = any(c.islower() for c in message if c.isalpha())

    if has_upper and has_lower:

        score += 1

    else:
        tips.append("Mixed case slightly improves resistance.")

    labels = {
        0: "Terrible",
        1: "Weak",
        2: "Fair",
        3: "Okay"
    }

    print("\n===== Strength Report =====")

    print(f"Rating      : {labels[score]}")
    print(f"Letters     : {letters}")
    print(f"Shift       : {shift}")
    print(f"Key Space   : 25")
    print(f"Crack Time  : < 0.001 sec")

    print("\nTips:")

    if tips:

        for tip in tips:

            print(f" - {tip}")

    else:

        print(" - Best possible Caesar usage.")

    print("\nCaesar Cipher is NOT secure for real-world encryption.")


def save_to_file(text):

    filename = input("Filename to save: ").strip()

    if not filename:
        filename = "output.txt"

    try:

        with open(filename, "w", encoding="utf-8") as f:

            f.write(text)

        print("\nSaved successfully!\n")

    except Exception as e:

        print(f"\nCould not save file: {e}\n")


def read_from_file():

    filename = input("Filename to read: ").strip()

    if not filename:

        print("No filename entered.")

        return ""

    try:

        with open(filename, "r", encoding="utf-8") as f:

            data = f.read()

        print("\nFile loaded successfully!\n")

        return data

    except Exception as e:

        print(f"\nCould not read file: {e}\n")

        return ""


def random_shift():

    return random.randint(1, 25)


def rot13(text):

    return encrypt(text, 13)


def compare_shifts(message):

    print("\nCaesar Cipher Comparison:\n")

    for i in range(1, 26):

        print(f"{i:>2}: {encrypt(message, i)}")


def about():

    print("""
==============================
        ABOUT CIPHER
==============================

Caesar Cipher:
- Monoalphabetic substitution cipher
- Invented by Julius Caesar
- Uses alphabet shifting
- Key space = 25
- Vulnerable to brute force
- Broken using frequency analysis

Vigenere Cipher:
- Polyalphabetic substitution cipher
- Uses multiple shifts via a keyword
- Stronger than Caesar but breakable

Real-world encryption:
- AES-256 for encryption
- bcrypt / Argon2 for passwords

==============================
""")


def show_menu():

    print("""
╔══════════════════════════════════╗
║         CIPHER TOOLKIT          ║
╠══════════════════════════════════╣
║ 1  · Encrypt Caesar             ║
║ 2  · Decrypt Caesar             ║
║ 3  · Brute Force Attack         ║
║ 4  · Strength Report            ║
║ 5  · Frequency Analysis         ║
║ 6  · Random Key Encrypt         ║
║ 7  · ROT13                      ║
║ 8  · Compare All Shifts         ║
║ 9  · Vigenere Encrypt           ║
║ 10 · Vigenere Decrypt           ║
║ 11 · Encrypt File               ║
║ 12 · About Cipher               ║
║ q  · Quit                       ║
╚══════════════════════════════════╝
""")


def get_shift():

    try:

        s = int(input("Shift (1-25): ").strip())

        if 1 <= s <= 25:

            return s

        print("Shift out of range. Using default shift: 3")

        return 3

    except ValueError:

        print("Invalid input. Using default shift: 3")

        return 3


def main():

    while True:

        show_menu()

        choice = input("→ ").strip().lower()

        if choice == "1":

            msg = input("Message: ")

            shift = get_shift()

            result = encrypt(msg, shift)

            print(f"\nEncrypted:\n{result}\n")

            save = input("Save to file? (y/n): ").lower()

            if save == "y":

                save_to_file(result)

        elif choice == "2":

            msg = input("Ciphertext: ")

            shift = get_shift()

            print(f"\nDecrypted:\n{decrypt(msg, shift)}\n")

        elif choice == "3":

            msg = input("Ciphertext to crack: ")

            crack(msg)

        elif choice == "4":

            msg = input("Message: ")

            shift = get_shift()

            strength_report(msg, shift)

        elif choice == "5":

            msg = input("Text: ")

            frequency_analysis(msg)

        elif choice == "6":

            msg = input("Message: ")

            shift = random_shift()

            result = encrypt(msg, shift)

            print(f"\nRandom Shift Used: {shift}")

            print(result)

        elif choice == "7":

            msg = input("Message: ")

            print(f"\nROT13:\n{rot13(msg)}\n")

        elif choice == "8":

            msg = input("Message: ")

            compare_shifts(msg)

        elif choice == "9":

            msg = input("Message: ")

            key = input("Key (letters only): ")

            print(f"\nEncrypted:\n{vigenere_encrypt(msg, key)}\n")

        elif choice == "10":

            msg = input("Ciphertext: ")

            key = input("Key (letters only): ")

            print(f"\nDecrypted:\n{vigenere_decrypt(msg, key)}\n")

        elif choice == "11":

            data = read_from_file()

            if data:

                shift = get_shift()

                encrypted = encrypt(data, shift)

                print("\nEncrypted File Content:\n")

                print(encrypted)

                save_to_file(encrypted)

        elif choice == "12":

            about()

        elif choice == "q":

            print("\nBye 👋\n")

            break

        else:

            print("Invalid option.\n")


if __name__ == "__main__":

    main()