The present invention provides means and method for Single Sign-On authentication of a user accessing a service network through an access network when the user has been already authenticated by a core network where the user holds a subscription. Therefore, a number of means are provided in different entities distributed between the core network and the service network, as well as in the user's equipment, for carrying out the proposed method. The Single Sign-On authentication takes place upon matching in the service network a shared key for the user submitted from the core network with another shared key for the user derived at the user's equipment.