An authentication server automatically selects one of plural authentications identified by authentication identifiers to authorize access by a user to a service dispensed by a service server of a provider identified by a provider identifier via a communication network. The server includes a module for selecting an authentication identifier in a memory as a function of the provider identifier and the type of the terminal and/or the network type of the communication network, and a module for authenticating the user by launching an authentication process associated with the authentication identifier.