Added ciphers/permutation_cipher.py.

ciphers/permutation_cipher.py

@@ -0,0 +1,142 @@
+"""
+The permutation cipher, also called the transposition cipher, is a simple encryption
+technique that rearranges the characters in a message based on a secret key. It
+divides the message into blocks and applies a permutation to the characters within
+each block according to the key. The key is a sequence of unique integers that
+determine the order of character rearrangement.
+
+For more info: https://www.nku.edu/~christensen/1402%20permutation%20ciphers.pdf
+"""
+import random
+
+
+def generate_valid_block_size(message_length: int) -> int:
+    """
+    Generate a valid block size that is a factor of the message length.
+
+    Args:
+        message_length (int): The length of the message.
+
+    Returns:
+        int: A valid block size.
+
+    Example:
+        >>> random.seed(1)
+        >>> generate_valid_block_size(12)
+        3
+    """
+    block_sizes = [
+        block_size
+        for block_size in range(2, message_length + 1)
+        if message_length % block_size == 0
+    ]
+    return random.choice(block_sizes)
+
+
+def generate_permutation_key(block_size: int) -> list[int]:
+    """
+    Generate a random permutation key of a specified block size.
+
+    Args:
+        block_size (int): The size of each permutation block.
+
+    Returns:
+        list[int]: A list containing a random permutation of digits.
+
+    Example:
+        >>> random.seed(0)
+        >>> generate_permutation_key(4)
+        [2, 0, 1, 3]
+    """
+    digits = list(range(block_size))
+    random.shuffle(digits)
+    return digits
+
+
+def encrypt(
+    message: str, key: list[int] | None = None, block_size: int | None = None
+) -> tuple[str, list[int]]:
+    """
+    Encrypt a message using a permutation cipher with block rearrangement using a key.
+
+    Args:
+        message (str): The plaintext message to be encrypted.
+        key (list[int]): The permutation key for decryption.
+        block_size (int): The size of each permutation block.
+
+    Returns:
+        tuple: A tuple containing the encrypted message and the encryption key.
+
+    Example:
+        >>> encrypted_message, key = encrypt("HELLO WORLD")
+        >>> decrypted_message = decrypt(encrypted_message, key)
+        >>> decrypted_message
+        'HELLO WORLD'
+    """
+    message = message.upper()
+    message_length = len(message)
+
+    if key is None or block_size is None:
+        block_size = generate_valid_block_size(message_length)
+        key = generate_permutation_key(block_size)
+
+    encrypted_message = ""
+
+    for i in range(0, message_length, block_size):
+        block = message[i : i + block_size]
+        rearranged_block = [block[digit] for digit in key]
+        encrypted_message += "".join(rearranged_block)
+
+    return encrypted_message, key
+
+
+def decrypt(encrypted_message: str, key: list[int]) -> str:
+    """
+    Decrypt an encrypted message using a permutation cipher with block rearrangement.
+
+    Args:
+        encrypted_message (str): The encrypted message.
+        key (list[int]): The permutation key for decryption.
+
+    Returns:
+        str: The decrypted plaintext message.
+
+    Example:
+        >>> encrypted_message, key = encrypt("HELLO WORLD")
+        >>> decrypted_message = decrypt(encrypted_message, key)
+        >>> decrypted_message
+        'HELLO WORLD'
+    """
+    key_length = len(key)
+    decrypted_message = ""
+
+    for i in range(0, len(encrypted_message), key_length):
+        block = encrypted_message[i : i + key_length]
+        original_block = [""] * key_length
+        for j, digit in enumerate(key):
+            original_block[digit] = block[j]
+        decrypted_message += "".join(original_block)
+
+    return decrypted_message
+
+
+def main() -> None:
+    """
+    Driver function to pass message to get encrypted, then decrypted.
+
+    Example:
+    >>> main()
+    Decrypted message: HELLO WORLD
+    """
+    message = "HELLO WORLD"
+    encrypted_message, key = encrypt(message)
+
+    decrypted_message = decrypt(encrypted_message, key)
+    print(f"Decrypted message: {decrypted_message}")
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+    main()