Create spiral_transposition.py

ciphers/spiral_transposition.py

@@ -0,0 +1,95 @@
+# spiral_transposition.py
+
+from __future__ import annotations
+import math
+
+
+def encrypt(plaintext: str) -> str:
+    """
+    Encrypts text by writing it in a square matrix
+    and reading characters in spiral order.
+    """
+    text = "".join(ch for ch in plaintext.upper() if ch.isalpha())
+    n = math.ceil(math.sqrt(len(text)))
+    matrix = [["X"] * n for _ in range(n)]
+
+    idx = 0
+    for r in range(n):
+        for c in range(n):
+            if idx < len(text):
+                matrix[r][c] = text[idx]
+                idx += 1
+
+    result = []
+    top, left, bottom, right = 0, 0, n - 1, n - 1
+    while top <= bottom and left <= right:
+        for c in range(left, right + 1):
+            result.append(matrix[top][c])
+        top += 1
+        for r in range(top, bottom + 1):
+            result.append(matrix[r][right])
+        right -= 1
+        if top <= bottom:
+            for c in range(right, left - 1, -1):
+                result.append(matrix[bottom][c])
+            bottom -= 1
+        if left <= right:
+            for r in range(bottom, top - 1, -1):
+                result.append(matrix[r][left])
+            left += 1
+    return "".join(result)
+
+
+def decrypt(cipher_text: str) -> str:
+    """
+    Attempts to reconstruct the original message by reversing the spiral order.
+    """
+    L = len(cipher_text)
+    n = math.ceil(math.sqrt(L))
+    matrix = [[None] * n for _ in range(n)]
+
+    top, left, bottom, right = 0, 0, n - 1, n - 1
+    idx = 0
+    while top <= bottom and left <= right:
+        for c in range(left, right + 1):
+            matrix[top][c] = cipher_text[idx]
+            idx += 1
+        top += 1
+        for r in range(top, bottom + 1):
+            matrix[r][right] = cipher_text[idx]
+            idx += 1
+        right -= 1
+        if top <= bottom:
+            for c in range(right, left - 1, -1):
+                matrix[bottom][c] = cipher_text[idx]
+                idx += 1
+            bottom -= 1
+        if left <= right:
+            for r in range(bottom, top - 1, -1):
+                matrix[r][left] = cipher_text[idx]
+                idx += 1
+            left += 1
+
+    result = []
+    for row in matrix:
+        for ch in row:
+            if ch:
+                result.append(ch)
+    return "".join(result)
+
+
+if __name__ == "__main__":
+    while True:
+        print("\n" + "-" * 10 + "\nSpiral Transposition Cipher\n" + "-" * 10)
+        print("1. Encrypt\n2. Decrypt\n3. Quit")
+        choice = input("Choice: ").strip()
+        if choice == "1":
+            pt = input("Plaintext: ")
+            print("Ciphertext:", encrypt(pt))
+        elif choice == "2":
+            ct = input("Ciphertext: ")
+            print("Recovered:", decrypt(ct))
+        elif choice == "3":
+            break
+        else:
+            print("Invalid option.")