A Generic Authentication Architecture bootstrapping procedure is performed between a mobile terminal and a bootstrapping server function resulting in the mobile terminal and the bootstrapping server function each acquiring at least a bootstrapping transaction Identifier associated with the mobile terminal and a corresponding shared key. The mobile terminal derives a network application function specific key based on at least the acquired shared key and an identifier of said network application function. The bootstrapping transaction identifier and the network application function specific key are sent from the mobile terminal to the proxy mobile node. A request message for Mobile Internet Protocol registration is sent from the proxy mobile node to a home agent on behalf of the mobile terminal, the request message including the bootstrapping transaction identifier and an identifier of the proxy mobile node. The registration message is verified in the home agent with the use of a network application function specific key obtained from the bootstrapping server function or a network application function. The request message for Mobile Internet Protocol registration is authenticated with the proxy mobile node acting on behalf of the mobile terminal by verifying the message authentication code with the obtained network application function specific key.