Exercise 4 (2 pts). In the Elgamal cryptosystem, Bobs public key is (p = 23, g = 7, gx = 4). His private key is x = 6. Alice uses Bobs public key to encrypt a message M for him. The resulting ciphertext is (d = 11, k = 21). Recover the message M. (You can use online modular calculators available at the Web.)