A communication system which includes a terminal, a telecommunications network server able to provide a network service to the terminal; and an application server able to provide application services to the terminal via the network and the network server. The terminal and the network server share and store a same network root key kept secret from the application server and are configured to generate, on the basis of this network root key, a network session key used to sign and verify the integrity of messages exchanged on the network between the terminal and the network server. The terminal and the application server share and store a same application root key kept secret from the network server, and configured to generate, on the basis of this application root key, an application session key used to encrypt and decrypt messages exchanged between the terminal and this application server via the network.