Modified Lab 1 IS

IS/Lab/Lab1/q6_affine_bf.py

@@ -0,0 +1,47 @@
+def main():
+    # Affine cipher: E(x) = (ax + b) mod 26
+    # Given: "ab" -> "GL"
+    # a=0, b=1 -> G=6, L=11
+    # So: 6 = (a*0 + b) mod 26 -> b = 6
+    # And: 11 = (a*1 + b) mod 26 -> 11 = (a + 6) mod 26 -> a = 5
+
+    ciphertext = "XPALASXYFGFUKPXUSOGEUTKCDGEXANMGNVS"
+
+    # Try all possible values of a and b for affine cipher
+    for a in range(1, 26):
+        # a must be coprime to 26
+        if gcd(a, 26) != 1:
+            continue
+
+        for b in range(26):
+            # Check if this key produces "ab" -> "GL"
+            if (a * 0 + b) % 26 == 6 and (a * 1 + b) % 26 == 11:
+                # Found the key, now decrypt the message
+                a_inv = mod_inverse(a, 26)
+                decrypted = ""
+                for char in ciphertext:
+                    if char.isalpha():
+                        y = ord(char.upper()) - ord('A')
+                        x = (a_inv * (y - b)) % 26
+                        decrypted += chr(x + ord('A'))
+                    else:
+                        decrypted += char
+
+                print(f"Key found: a={a}, b={b}")
+                print(f"Ciphertext: {ciphertext}")
+                print(f"Decrypted: {decrypted}")
+                return
+
+def gcd(a, b):
+    while b:
+        a, b = b, a % b
+    return a
+
+def mod_inverse(a, m):
+    for i in range(1, m):
+        if (a * i) % m == 1:
+            return i
+    return None
+
+if __name__ == '__main__':
+    main()