A method and authentication server provide a mobile key. According to the method, upon receipt of an authentication message (access authentication) that is transmitted when a subscriber logs on to the network, the authentication server extracts a subscriber identification contained in said message and generates a corresponding mobile key, which is stored together with the respective extracted subscriber identification. Upon subsequent receipt of a key request message (key request) that is transmitted when a subscriber registers, the authentication server extracts a mobile identification of the subscriber contained in said message and searches for an identical mobile identification, which can be derived in accordance with a configurable derivation function from a subscriber identification that is stored in the authentication server. Once a derived mobile identification that is identical or can be uniquely assigned to the extracted mobile identification has been found, the authentication server provides the stored corresponding mobile key that has been generated, to cryptographically protect the mobile signaling messages of the registered subscriber.