The random message CA_Rand generated by the CA server is the second protection next to credentials. If an attacker has performed an MITM attack during the phase of applying and issuing the proxy credential or in the first handshaking step of Phase IV, the attacker may be able to have obtained the public key MN_Pub of the master node, the proxy credential, as well as the random message CA_Rand. However, the random message CA_Rand is encrypted with the private key MN_Prv of the master node. The attacker is unable to get the private key MN_Prv of the master node, which is randomly generated by the master node and never transmitted over the public networks. Therefore, the attacker cannot encrypt the random message CA_Rand and a failure will be caused during the authentication step by the slave nodes.