Added Lab2/Lab3 IS

IS/Lab/Lab3/ElGamal.py

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+import random
+from Crypto.Util.number import getPrime
+
+def elgamal_keygen(bits=512):
+    p = getPrime(bits)
+    g = random.randint(2, p-1)
+    x = random.randint(1, p-2)  # private key
+    h = pow(g, x, p)  # public key component
+    return (p, g, h), x
+
+def elgamal_encrypt(message, pub_key):
+    p, g, h = pub_key
+    m = int.from_bytes(message.encode('utf-8'), 'big')
+    if m >= p:
+        raise ValueError("Message too large for key size")
+    
+    k = random.randint(1, p-2)
+    c1 = pow(g, k, p)
+    c2 = (m * pow(h, k, p)) % p
+    return (c1, c2)
+
+def elgamal_decrypt(ciphertext, priv_key, pub_key):
+    c1, c2 = ciphertext
+    p, g, h = pub_key
+    x = priv_key
+    
+    s = pow(c1, x, p)
+    s_inv = pow(s, p-2, p)  # modular inverse
+    m = (c2 * s_inv) % p
+    
+    return m.to_bytes((m.bit_length() + 7) // 8, 'big').decode('utf-8')
+
+def main():
+    print("Welcome to ElGamal")
+    ptext = input("Enter plaintext: ")
+    
+    # Generate keys
+    pub_key, priv_key = elgamal_keygen()
+    p, g, h = pub_key
+    
+    print(f"\nPublic Key (p, g, h):")
+    print(f"p = {hex(p)}")
+    print(f"g = {g}")
+    print(f"h = {hex(h)}")
+    
+    # Encrypt
+    ctext = elgamal_encrypt(ptext, pub_key)
+    print(f"\nCiphertext (c1, c2):")
+    print(f"c1 = {hex(ctext[0])}")
+    print(f"c2 = {hex(ctext[1])}")
+    
+    # Decrypt
+    decrypted = elgamal_decrypt(ctext, priv_key, pub_key)
+    print(f"Decrypted: {decrypted}")
+
+if __name__ == '__main__':
+    main()