A method, apparatus, system and computer program where an apparatus stores user specific credentials, receives a certificate via the communication interface from a certificate authority and stores the certificate in the memory. The apparatus further stores a private key and a public key in the memory and attempts authenticating of the apparatus to a ticket reader for accessing a service, by transmission of one or more messages, wherein the messages contain an authenticator that has at least one of the following: the certificate or its cryptographic derivative; one or more data items contained by the certificate or a cryptographic derivative thereof. The messages are prepared such that the public key is not recoverable from outside of the authenticator.