cipher: add columnar transposition cipher

Author: Marzhal

ciphers/columnar_transposition_cipher.py

@@ -0,0 +1,73 @@
+import math
+
+def encrypt_columnar_cipher(message: str, key: str) -> str:
+    """
+    Encrypts a message using the Columnar Transposition Cipher.
+    
+    :param message: Text to encrypt.
+    :param key: String key used to define column order.
+    :return: Encrypted message.
+    """
+    # Remove spaces and calculate dimensions
+    message = message.replace(" ", "")
+    num_cols = len(key)
+    num_rows = math.ceil(len(message) / num_cols)
+    
+    # Fill the grid with characters
+    grid = [''] * num_cols
+    for i, char in enumerate(message):
+        grid[i % num_cols] += char
+
+    # Sort columns based on the key order
+    sorted_key_indices = sorted(range(len(key)), key=lambda k: key[k])
+    ciphertext = ''.join([grid[i] for i in sorted_key_indices])
+
+    return ciphertext
+
+
+def decrypt_columnar_cipher(ciphertext: str, key: str) -> str:
+    """
+    Decrypts a message encrypted with the Columnar Transposition Cipher.
+    
+    :param ciphertext: Encrypted text.
+    :param key: String key used to define column order.
+    :return: Decrypted message.
+    """
+    num_cols = len(key)
+    num_rows = math.ceil(len(ciphertext) / num_cols)
+    num_shaded_boxes = (num_cols * num_rows) - len(ciphertext)
+
+    # Sort columns based on the key order
+    sorted_key_indices = sorted(range(len(key)), key=lambda k: key[k])
+    col_lengths = [num_rows] * num_cols
+    for i in range(num_shaded_boxes):
+        col_lengths[sorted_key_indices[-(i+1)]] -= 1
+
+    # Distribute ciphertext into columns based on the sorted key
+    grid = []
+    start = 0
+    for col_length in col_lengths:
+        grid.append(ciphertext[start:start + col_length])
+        start += col_length
+
+    # Rebuild plaintext row by row
+    plaintext = ''
+    for i in range(num_rows):
+        for j in range(num_cols):
+            if i < len(grid[sorted_key_indices[j]]):
+                plaintext += grid[sorted_key_indices[j]][i]
+
+    return plaintext
+
+
+# Example usage
+message = "HELLO WORLD FROM COLUMNAR"
+key = "3412"
+
+# Encrypt the message
+encrypted = encrypt_columnar_cipher(message, key)
+print("Encrypted:", encrypted)
+
+# Decrypt the message
+decrypted = decrypt_columnar_cipher(encrypted, key)
+print("Decrypted:", decrypted)