Systems and methods for characterizing a client apparatus on at least one server apparatus are provided. A first certificate is received in the event of a first request for a connection set-up from a server apparatus in a client apparatus. One or more predefined certificate parameters of the first certificate are stored as a set of characterization parameters in the client apparatus. Each further certificate from a server apparatus is checked that is received in the client apparatus in the event of a request for a further connection set-up, against the stored characterization parameter set. A request for a further connection set-up is accepted only if all of the predefined certificate parameters of the further certificate match all characterization parameters of the characterization parameter set.