Add Playfair Cipher Decryption Solution

python/solution.py

@@ -0,0 +1,108 @@
+#!/usr/bin/env python3
+
+"""
+Playfair Cipher Decryption
+Author: Christopher Rossi
+Date: 2024-06-28
+
+This script decrypts a message encoded with the Playfair cipher using a given key.
+The Playfair cipher is a digraph substitution cipher that encrypts pairs of letters.
+
+Usage:
+    python playfair_decrypt.py
+
+Requirements:
+    Python 3.x
+"""
+
+def create_grid(key):
+    """
+    Create a 5x5 grid for the Playfair cipher using the provided key.
+    The grid will contain each letter of the alphabet once, with 'I' and 'J' treated as the same letter.
+
+    :param key: The keyword to create the grid.
+    :return: A 5x5 list of lists representing the grid.
+    """
+    alphabet = "ABCDEFGHIKLMNOPQRSTUVWXYZ"  # Note: 'J' is omitted as 'I' and 'J' are treated the same
+    key = "".join(dict.fromkeys(key))  # Remove duplicates while preserving order
+    key = key.replace("J", "")  # Remove 'J' as it's considered the same as 'I'
+    used_chars = set(key)  # Track characters already used in the key
+
+    # Append remaining letters of the alphabet to the key
+    for char in alphabet:
+        if char not in used_chars:
+            key += char
+
+    # Create a 5x5 grid from the key
+    return [list(key[i:i + 5]) for i in range(0, len(key), 5)]
+
+
+def find_position(grid, char):
+    """
+    Find the row and column of a character in the grid.
+
+    :param grid: The 5x5 grid.
+    :param char: The character to find.
+    :return: A tuple (row, col) indicating the position of the character.
+    """
+    for row in range(5):
+        for col in range(5):
+            if grid[row][col] == char:
+                return row, col
+    return None, None  # Character not found
+
+
+def decrypt_pair(grid, char1, char2):
+    """
+    Decrypt a pair of characters using the Playfair cipher rules.
+
+    :param grid: The 5x5 grid.
+    :param char1: The first character of the pair.
+    :param char2: The second character of the pair.
+    :return: The decrypted pair of characters.
+    """
+    row1, col1 = find_position(grid, char1)
+    row2, col2 = find_position(grid, char2)
+
+    if row1 == row2:
+        # Same row: replace with characters immediately to the left
+        return grid[row1][(col1 - 1) % 5] + grid[row2][(col2 - 1) % 5]
+    elif col1 == col2:
+        # Same column: replace with characters immediately above
+        return grid[(row1 - 1) % 5][col1] + grid[(row2 - 1) % 5][col2]
+    else:
+        # Rectangle: replace with characters on the same row but in opposite corners of the rectangle
+        return grid[row1][col2] + grid[row2][col1]
+
+
+def decrypt_playfair_cipher(ciphertext, key):
+    """
+    Decrypt a ciphertext encrypted with the Playfair cipher using the provided key.
+
+    :param ciphertext: The encrypted message.
+    :param key: The key to create the Playfair cipher grid.
+    :return: The decrypted plaintext message.
+    """
+    grid = create_grid(key)  # Create the grid from the key
+    plaintext = ""
+    i = 0
+
+    # Process the ciphertext in pairs of characters
+    while i < len(ciphertext):
+        char1 = ciphertext[i]
+        char2 = ciphertext[i + 1]
+
+        decrypted_pair = decrypt_pair(grid, char1, char2)
+        plaintext += decrypted_pair
+        i += 2
+
+    return plaintext.replace("X", "")  # Remove filler 'X' characters
+
+
+# Given ciphertext and key
+ciphertext = "IKEWENENXLNQLPZSLERUMRHEERYBOFNEINCHCV"
+key = "SUPERSPY"
+
+# Decrypting the ciphertext
+decrypted_message = decrypt_playfair_cipher(ciphertext, key)
+print(decrypted_message)