The disclosure relates to a method of operating a system, the system comprising a near field communication, NFC, tag, an NFC device associated with a user of the system and a computer, the method comprising: the NFC device requesting a message from the NFC tag; the NFC tag generating a message comprising a representation of a counter value; the NFC tag sending the message to the NFC device; the NFC device generating a request comprising the representation of the counter value in response to receiving the message; the NFC device sending the request to a computer; the NFC device submitting user authentication data to the computer; and the computer executing an operation on verification of the counter value and the authentication data.