Create two_square_cipher.py

ciphers/two_square_cipher.py

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+def generate_square(keyword: str) -> list:
+    """
+    Generate a 5x5 Playfair square (I/J combined) from a keyword.
+    """
+    keyword = keyword.upper().replace("J", "I")
+    seen = set()
+    square = []
+    for char in keyword:
+        if char.isalpha() and char not in seen:
+            seen.add(char)
+            square.append(char)
+    for char in "ABCDEFGHIKLMNOPQRSTUVWXYZ":
+        if char not in seen:
+            seen.add(char)
+            square.append(char)
+    return [square[i : i + 5] for i in range(0, 25, 5)]
+
+
+def find_position(square, char):
+    """
+    Return (row, col) of char in given 5x5 square.
+    """
+    if char == "J":
+        char = "I"
+    for r in range(5):
+        for c in range(5):
+            if square[r][c] == char:
+                return r, c
+    return None
+
+
+def two_square_encrypt(plaintext: str, key1: str, key2: str) -> str:
+    """
+    Encrypt plaintext using the Two-Square cipher.
+
+    >>> two_square_encrypt("HELLO", "KEYWORD", "EXAMPLE")
+    'BFCCLN'
+    """
+    plaintext = plaintext.upper().replace("J", "I")
+    plaintext = "".join([c for c in plaintext if c.isalpha()])
+
+    # split into digraphs
+    pairs = []
+    i = 0
+    while i < len(plaintext):
+        a = plaintext[i]
+        b = plaintext[i + 1] if i + 1 < len(plaintext) else "X"
+        if a == b:
+            b = "X"
+            i += 1
+        else:
+            i += 2
+        pairs.append((a, b))
+
+    square1 = generate_square(key1)
+    square2 = generate_square(key2)
+
+    ciphertext = ""
+    for a, b in pairs:
+        r1, c1 = find_position(square1, a)
+        r2, c2 = find_position(square2, b)
+        ciphertext += square2[r1][c2]
+        ciphertext += square1[r2][c1]
+    return ciphertext
+
+
+def two_square_decrypt(ciphertext: str, key1: str, key2: str) -> str:
+    """
+    Decrypt ciphertext using the Two-Square cipher.
+
+    >>> two_square_decrypt("BFCCLN", "KEYWORD", "EXAMPLE")
+    'HELLOX'
+    """
+    ciphertext = ciphertext.upper().replace("J", "I")
+    ciphertext = "".join([c for c in ciphertext if c.isalpha()])
+
+    if len(ciphertext) % 2 != 0:
+        raise ValueError("Ciphertext must have even length")
+
+    square1 = generate_square(key1)
+    square2 = generate_square(key2)
+
+    plaintext = ""
+    for i in range(0, len(ciphertext), 2):
+        a, b = ciphertext[i], ciphertext[i + 1]
+        r1, c1 = find_position(square2, a)
+        r2, c2 = find_position(square1, b)
+        plaintext += square1[r1][c2]
+        plaintext += square2[r2][c1]
+    return plaintext
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
+
+    # Example usage
+    plaintext = "HELLO"
+    key1 = "KEYWORD"
+    key2 = "EXAMPLE"
+    encrypted = two_square_encrypt(plaintext, key1, key2)
+    decrypted = two_square_decrypt(encrypted, key1, key2)
+
+    print("\n\n")
+    print("Plaintext:", plaintext)
+    print("Encrypted:", encrypted)
+    print("Decrypted:", decrypted)