The purpose of encrypting the user session is to make sure that the user session, received by the slave node, is the same one that the master node has sent. At the same time, the slave node selects a symmetric cryptographic algorithm SCA, generates a corresponding key SC_Key for this algorithm, and saves the pair {SCA, SC_key} together with the user session. Another copy of the name of the chosen cryptographic algorithm and its key SC_Key is encrypted with the public key MN_Pub of the master node. This algorithm and the corresponding key SC_Key will be used to encrypt all the data transmissions between the master node and this slave node after this phase. Here, it must be noticed that the key to encrypt the random message CA_Rand as well as the user session is the private key SN_Prv of the slave node, while the algorithm’s name SCA and the key SC_Key is encrypted with the public key MN_Pub of the master node. To avoid modification of the encryption information {SCA, SC_key} by an attacker, the hash value of this encryption information is computed by the slave node. The hash value is encrypted with the private key of the slave node and sent then to the master node: