ciphers.columnar_transposition
==============================

.. py:module:: ciphers.columnar_transposition

.. autoapi-nested-parse::

   Columnar Transposition cipher.

   This classical cipher writes the plaintext in rows under a keyword and reads
   columns in the order of the alphabetical rank of the keyword letters.

   Reference: https://en.wikipedia.org/wiki/Transposition_cipher#Columnar_transposition

   We keep spaces and punctuation. Key must be alphabetic (case-insensitive).

   >>> pt = "WE ARE DISCOVERED. FLEE AT ONCE"
   >>> ct = encrypt(pt, "ZEBRAS")
   >>> decrypt(ct, "ZEBRAS") == pt
   True

   Edge cases:
   >>> encrypt("HELLO", "A")
   'HELLO'
   >>> decrypt("HELLO", "A")
   'HELLO'
   >>> encrypt("HELLO", "HELLO")
   'EHLLO'
   >>> decrypt("EHLLO", "HELLO")
   'HELLO'
   >>> encrypt("HELLO", "")
   Traceback (most recent call last):
       ...
   ValueError: Key must be a non-empty alphabetic string



Functions
---------

.. autoapisummary::

   ciphers.columnar_transposition._column_order
   ciphers.columnar_transposition._normalize_key
   ciphers.columnar_transposition.decrypt
   ciphers.columnar_transposition.encrypt


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

.. py:function:: _column_order(key: str) -> list[int]

.. py:function:: _normalize_key(key: str) -> str

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

   Decrypt columnar transposition ciphertext.

   :param ciphertext: Encrypted text
   :param key: Alphabetic keyword
   :return: Decrypted plaintext
   :raises ValueError: on invalid key


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

   Encrypt using columnar transposition.

   :param plaintext: Input text (any characters)
   :param key: Alphabetic keyword
   :return: Ciphertext
   :raises ValueError: on invalid key