Patent ID: 7447908

Claim:
A method of using a mobile device to authenticate a print medium offline before completing printing onto the print medium, the mobile device including processing means, a printhead and a sensor, the print medium comprising a laminar substrate, the method comprising steps of: sensing coded data provided on a surface of the laminar substrate as the print medium moves past the sensor whilst moving along a print path defined in the mobile device; using the processing means: determining, from the sensed coded data: an identity of the print medium; a plurality of parts of a signature, the signature being a digital signature of at least part of the identity; and a part identifier of each of the parts of the signature; determining, using the plurality of parts of the signature and the part identifiers, a determined signature; generating, using the determined signature and a public key stored in the mobile device, a generated identity; comparing the identity of the print medium to the generated identity; and authenticating the print medium using the results of the comparison; and in the event the authentication step is successful, using the printhead to print onto the print medium.