In an authentication server, information representing a first part of a response to a challenge is received during the authentication preparation phase. The challenge and the first part of the response are stored for further use. The challenge is resent and information representing a second part of the response to the challenge is received during a modified authentication phase. The first and second parts of the response are checked against the challenge for authenticating the user. In a smartcard reader, the response received from the smartcard is sent to a computing device, when the smartcard reader received the challenge via an interface to the computing device during normal authentication. In response to the smartcard reader having received the challenge via the interface to the computing device during an authentication preparation phase, the smartcard reader sends the first part of the response to the computing device. In response to the smartcard reader having received the challenge via a user interface, it presents at least the second part of the response to a user via the user interface.