A method of using a mobile device to authenticate a print medium online before completing printing onto the print medium, the mobile device including processing means, a printhead, a sensor, a transmitter and a receiver, the print medium comprising a laminar substrate, the method comprising the steps of: using the sensor to sense coded data provided on a surface of the substrate; using the processing means to determine, from the sensed coded data: an identity of the print medium; and at least part of a signature, the signature being a digital signature of at least part of the identity; using the transmitter to send first data to a remote computer system, the first data being indicative of the identity and the at least part of the signature; using the receiver to receive second data from the remote computer system in reply to the first data, the second data being indicative of whether the print medium is authentic based on the identity and the at least part of the signature; and in the event the print medium is authentic, using the printhead to print onto the print medium.