Method for providing at least one mobile key for the cryptographic protection of mobile signalling messages for a home agent, with the following steps: (a) Establishment of a radio connection between a mobile subscriber terminal and an access network, wherein 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, for the authentication of the subscriber and, if the authentication by an authentication server of the home network is successful, assigns a group-specific mobile key to the subscriber identification if the subscriber identification contained in the authentication message is already stored in the authentication proxy server; (b) Receipt by the home agent of a registration request message, that contains a subscriber identification, originating from a subscriber terminal; (c) Transmission of a key request message for a mobile key from the home agent to the associated authentication proxy server; wherein the key request message contains the subscriber identification contained in the registration request message; and (d) Provision of a mobile key by the authentication proxy server for the home agent, - if it is identified that the authentication server of the home network does not provide a mobile key, and - if the subscriber identification contained in the key request message agrees with one of the subscriber identifications stored by the authentication proxy server. (a) Establishment of a radio connection between a mobile subscriber terminal and an access network, wherein 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, for the authentication of the subscriber and, if the authentication by an authentication server of the home network is successful, assigns a group-specific mobile key to the subscriber identification if the subscriber identification contained in the authentication message is already stored in the authentication proxy server; (b) Receipt by the home agent of a registration request message, that contains a subscriber identification, originating from a subscriber terminal; (c) Transmission of a key request message for a mobile key from the home agent to the associated authentication proxy server; wherein the key request message contains the subscriber identification contained in the registration request message; and (d) Provision of a mobile key by the authentication proxy server for the home agent, - if it is identified that the authentication server of the home network does not provide a mobile key, and - if the subscriber identification contained in the key request message agrees with one of the subscriber identifications stored by the authentication proxy server. - if it is identified that the authentication server of the home network does not provide a mobile key, and - if the subscriber identification contained in the key request message agrees with one of the subscriber identifications stored by the authentication proxy server.