The connection between the master node and slave nodes are encrypted by a symmetric cryptographic algorithm, which is determined by the slave node, while the master node is noticed by encrypting the message with the public key MN_Pub of the master node. It is already pointed out that an attacker cannot have any knowledge about the symmetric cryptographic algorithm SCA and the corresponding key SC_Key. In case of an MITM attack, an attacker may modify the encryption infor- mation {SCA, SC_Key} from the slave node by using the public key of the master node to encrypt the encryption information that is generated by the attacker. But with the hash value SCA_Check, the master node can verify the validity and integrity of the encryption information. Since the hash value SCA_Check is encrypted with the private key SN_Prv of the salve node, it is impossible for the attacker to encrypt the encryption information correctly. The attack is hence avoided.