1. A method for establishing an authenticated and secure communication session for transactions between a client and a server in a wireless data network, the client remotely located with respect to the server, the method comprising: the client sending a session-request signal to the server for creating the session therebetween, the session-request signal comprising at least one client message encrypted according to a shared secret encrypt key previously residing on both the client and the server; the server conducting a first client authentication by decrypting the encrypted client message according to the shared secret encrypt key upon receiving the session-request signal; the server generating a session key for the session in creation, a first derivative from the decrypted client message, and a server message; the server sending a session-reply signal comprising the session key, the first derivative and the server message, with the session key, the first derivative and the server message being encrypted according to the shared secret encrypt key; the client conducting a first server authentication by decrypting the first derivative and the server message being encrypted according to the shared secret encrypt key; and the client conducting a second server authentication by validating the first derivative with the client message.