After a radio link is established between a mobile subscriber terminal and an access network, to authenticate the subscriber an authentication proxy server of an intermediate network forwards at least one authentication message containing a subscriber identification between the access network and a home network of the subscriber. If authentication is given by an authentication server of the home network, the authentication proxy server of the intermediate network stores the subscriber identification. The home agent receives a registration request message originating from the subscriber terminal and containing a subscriber identification; the home agent transmits a key request message, containing the subscriber identification, for a mobile key to the relevant authentication proxy server. The authentication proxy server provides a mobile key for the home agent, if the subscriber identification contained in the key request message matches one of the subscriber identifications that has been stored by the authentication proxy server.