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Assignment_2- Affine cipher.py

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+import math
+
+def gcd(a, b):
+    while b != 0:
+        a, b = b, a % b
+    return a
+
+def mod_inverse(a, m):
+    a = a % m
+    for x in range(1, m):
+        if (a * x) % m == 1:
+            return x
+    return None  # No inverse exists
+
+def affine_encrypt(text, a, b):
+    result = []
+    for char in text:
+        if char.isalpha():
+            base = ord('A') if char.isupper() else ord('a')
+            x = ord(char) - base
+            y = (a * x + b) % 26
+            result.append(chr(y + base))
+        else:
+            result.append(char)  # Non-alphabetic characters unchanged
+    return ''.join(result)
+
+def affine_decrypt(text, a, b):
+    a_inv = mod_inverse(a, 26)
+    if a_inv is None:
+        raise ValueError("Key 'a' has no modular inverse. Choose a different key.")
+
+    result = []
+    for char in text:
+        if char.isalpha():
+            base = ord('A') if char.isupper() else ord('a')
+            y = ord(char) - base
+            x = (a_inv * (y - b)) % 26
+            result.append(chr(x + base))
+        else:
+            result.append(char)  # Non-alphabetic characters unchanged
+    return ''.join(result)
+
+def main():
+    print("Affine Cipher")
+    choice = input("1. Encrypt\n2. Decrypt\nEnter choice (1/2): ")
+    text = input("Enter text: ")
+    a = int(input("Enter key 'a' (must be coprime with 26): "))
+    b = int(input("Enter key 'b': "))
+
+    if gcd(a, 26) != 1:
+        print("Error: 'a' must be coprime with 26 (valid keys: 1,3,5,7,9,11,15,17,19,21,23,25).")
+        return
+
+    if choice == '1':
+        ciphertext = affine_encrypt(text, a, b)
+        print(f"Encrypted: {ciphertext}")
+    elif choice == '2':
+        try:
+            plaintext = affine_decrypt(text, a, b)
+            print(f"Decrypted: {plaintext}")
+        except ValueError as e:
+            print(e)
+    else:
+        print("Invalid choice.")
+
+if __name__ == "__main__":
+    main()