After a radio link is established between a mobile subscriber terminal and an access network, the subscriber is authenticated by a proxy server of an intermediate network forwarding, from the access network to a home network of the subscriber, authentication message(s) containing a subscriber identification. If the subscriber is authenticated and the subscriber identification is already stored in the proxy server, the proxy server assigns a group-specific mobile key to the subscriber identification. When the home agent receives a registration request message originating from a subscriber terminal and containing a subscriber identification and transmits a key request message, containing the subscriber identification, for a mobile key to the proxy server, if the subscriber identification in the key request message matches a subscriber identification stored by the proxy server, a mobile key for cryptographic protection of mobile signalling messages is provided to the home agent by the proxy server.