feat(ciphers): add scytale (skytale) transposition cipher with doctests

Author: Saiprashanth Pulisetti

ciphers/skytale_cipher.py

@@ -0,0 +1,104 @@
+"""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
+"""
+from __future__ import annotations
+
+from typing import List
+
+
+def 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
+    """
+    if key <= 0:
+        raise ValueError("Key must be a positive integer")
+    if key == 1 or len(plaintext) <= key:
+        return plaintext
+
+    # Read every key-th character starting from each row offset
+    return "".join(plaintext[row::key] for row in range(key))
+
+
+def 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
+    """
+    if key <= 0:
+        raise ValueError("Key must be a positive integer")
+    if key == 1 or len(ciphertext) <= key:
+        return ciphertext
+
+    length = len(ciphertext)
+    base = length // key
+    extra = length % key
+
+    # Determine each row length
+    row_lengths: List[int] = [base + (1 if r < extra else 0) for r in range(key)]
+
+    # Slice ciphertext into rows
+    rows: List[str] = []
+    idx = 0
+    for r_len in row_lengths:
+        rows.append(ciphertext[idx : idx + r_len])
+        idx += r_len
+
+    # Pointers to current index in each row
+    pointers = [0] * key
+
+    # Reconstruct by taking characters column-wise across rows
+    result_chars: List[str] = []
+    for i in range(length):
+        r = i % key
+        if pointers[r] < len(rows[r]):
+            result_chars.append(rows[r][pointers[r]])
+            pointers[r] += 1
+    return "".join(result_chars)
+
+
+if __name__ == "__main__":  # pragma: no cover
+    import doctest
+
+    doctest.testmod()