solution.py

python/solution.py

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+def create_playfair_matrix(key):
+    """
+    Creates a 5x5 Playfair matrix using the provided key.
+    """
+    alphabet = "ABCDEFGHIKLMNOPQRSTUVWXYZ"
+    matrix = []
+    key_unique = []
+
+    # Add key characters to the matrix
+    for char in key.upper():
+        if char not in key_unique and char in alphabet:
+            key_unique.append(char)
+
+    # Add remaining alphabet characters to the matrix
+    for char in alphabet:
+        if char not in key_unique:
+            key_unique.append(char)
+
+    # Create 5x5 matrix
+    for i in range(0, 25, 5):
+        matrix.append(key_unique[i:i+5])
+    return matrix
+
+def preprocess_text(text):
+    """
+    Prepares the text for decryption:
+    - Converts text to uppercase.
+    - Replaces 'J' with 'I'.
+    - Splits text into digraphs, inserting 'X' between duplicate letters and at the end if necessary.
+    """
+    text = text.upper().replace("J", "I")
+    processed = ""
+    i = 0
+
+    while i < len(text):
+        processed += text[i]
+        if i+1 < len(text) and text[i] == text[i+1]:
+            processed += "X"
+        elif i+1 < len(text):
+            processed += text[i+1]
+            i += 1
+        i += 1
+
+    if len(processed) % 2 != 0:
+        processed += "X"
+
+    return processed
+
+def find_position(matrix, char):
+    """
+    Finds the row and column of a character in the Playfair matrix.
+    """
+    for row in range(5):
+        for col in range(5):
+            if matrix[row][col] == char:
+                return row, col
+    return None
+
+def decrypt_digraph(matrix, digraph):
+    """
+    Decrypts a pair of characters (digraph) using Playfair cipher rules.
+    """
+    row1, col1 = find_position(matrix, digraph[0])
+    row2, col2 = find_position(matrix, digraph[1])
+
+    if row1 == row2:
+        return matrix[row1][(col1-1) % 5] + matrix[row2][(col2-1) % 5]
+    elif col1 == col2:
+        return matrix[(row1-1) % 5][col1] + matrix[(row2-1) % 5][col2]
+    else:
+        return matrix[row1][col2] + matrix[row2][col1]
+
+def decrypt_playfair_cipher(cipher_text, key):
+    """
+    Decrypts a Playfair cipher text using the provided key.
+    """
+    matrix = create_playfair_matrix(key)
+    cipher_text = preprocess_text(cipher_text)
+    decrypted_text = ""
+
+    for i in range(0, len(cipher_text), 2):
+        decrypted_text += decrypt_digraph(matrix, cipher_text[i:i+2])
+
+    return decrypted_text.replace("X", "")
+
+if __name__ == "__main__":
+    key = "SUPERSPY"
+    cipher_text = "IKEWENENXLNQLPZSLERUMRHEERYBOFNEINCHCV"
+    decrypted_text = decrypt_playfair_cipher(cipher_text, key)
+    print(decrypted_text)