Data transmission system including a proxy connected to the Internet network and at least a content server to which a user can gain access by the intermediary of the proxy, the proxy being associated with authentication means adapted to perform form-based authentication of the user when receiving a user request for Internet resources therefrom and wherein the proxy transmits the user request to the content server which sends back a response to the proxy. The authentication means comprise a Single Sign-On (SSO) server adapted to obtain a login form from the content server when receiving the user request from the proxy, to fill the login form by using the credentials associated with the user and to send back the filled login form thereby, playing the role of the user regarding the content server, so that the content server can provide the requested information after authentication of the user.