ciphers.skytale_cipher
======================

.. py:module:: ciphers.skytale_cipher

.. autoapi-nested-parse::

   Scytale (Skytale) transposition cipher.

   A classical transposition cipher used in ancient Greece. The sender wraps a
   strip of parchment around a rod (scytale) and writes the message along the rod.
   The recipient with a rod of the same diameter can read the message.

   Reference: https://en.wikipedia.org/wiki/Scytale

   Functions here keep characters as-is (including spaces). The key is a positive
   integer representing the circumference count (number of rows).

   >>> encrypt("WE ARE DISCOVERED FLEE AT ONCE", 3)
   'WA SVEFETNERDCEDL  C EIOR EAOE'
   >>> decrypt('WA SVEFETNERDCEDL  C EIOR EAOE', 3)
   'WE ARE DISCOVERED FLEE AT ONCE'

   Edge cases:
   >>> encrypt("HELLO", 1)
   'HELLO'
   >>> decrypt("HELLO", 1)
   'HELLO'
   >>> encrypt("HELLO", 5)  # key equals length
   'HELLO'
   >>> decrypt("HELLO", 5)
   'HELLO'
   >>> encrypt("HELLO", 0)
   Traceback (most recent call last):
       ...
   ValueError: Key must be a positive integer
   >>> decrypt("HELLO", -2)
   Traceback (most recent call last):
       ...
   ValueError: Key must be a positive integer



Functions
---------

.. autoapisummary::

   ciphers.skytale_cipher.decrypt
   ciphers.skytale_cipher.encrypt


Module Contents
---------------

.. py:function:: decrypt(ciphertext: str, key: int) -> str

   Decrypt Scytale ciphertext.

   Reconstruct rows by their lengths and interleave by columns.

   :param ciphertext: Encrypted string
   :param key: Positive integer number of rows
   :return: Decrypted plaintext
   :raises ValueError: if key <= 0


.. py:function:: encrypt(plaintext: str, key: int) -> str

   Encrypt plaintext using Scytale transposition.

   Write characters around a rod with `key` rows, then read off by rows.

   :param plaintext: Input message to encrypt
   :param key: Positive integer number of rows
   :return: Ciphertext string
   :raises ValueError: if key <= 0