A client 110 may be authenticated by transmitting or beaming a telecommunication network subscriber's authentication to the client from a device 120, over a wireless link. For example, a GSM telephone 120 may authenticate an electronic book 110 to a content providing service within the Internet. The service verifies the authentication using the subscriber's GSM network operator's Authentication Center 161 to generate an authenticator and the client correspondingly generates a local copy of the authenticator using a GSM SIM over the wireless local link. The authentication is then determined by checking that these authenticators match and thereafter the authenticator can be used as a session key to encrypt data in the service.