A method is provided for authenticating a user's communications terminal with an authentication server connected to a gateway terminal by using a communications network. The method includes: obtaining a piece of data representing an identity of the user from the gateway terminal; configuring, by the authentication server, a data transmission link between the authentication server and the terminal, using a predefined data transmission interface of the gateway terminal and as a function of the piece of data representing the identity of the user; transmitting, by the authentication server, to the terminal, a piece of encrypted data for checking authentication, using the data transmission link; receiving, by the authentication user, coming from the terminal, a piece of encrypted data for counter-checking authentication; issuing an assertion of authentication of the user when the piece of data for the counter-checking of authentication corresponds to the piece of data for checking authentication.