Method of using a mobile device to determine a position of a print medium configured to be printed on by the mobile device

A method of using a mobile device to determine a position of an authorized print medium, the print medium configured to be printed on by the mobile device in a print direction, the mobile device including a printer, a first sensor and processing means, the print medium comprising a substrate defining first and second opposite faces, the first face having coded tags printed in substantially invisible ink substantially over the entire surface of the first face, each coded tag encoding data indicative of a location of that coded tag on the first face, an identifier of the print medium which identifies authorization of the print medium for use with the mobile device and a unique digital signature associated with the identifier, the method comprising the steps of:

• (a) using the first sensor to capture a first image of at least one of the coded tags when the print medium is at a position within a media feed path within the mobile device;
• (b) using the processing means to decode the data encoded by said at least one sensed coded tag, thereby to determine the location of the sensed coded tag and the identifier and digital signature encoded thereby;
• and
• (c) verifying the digital signature to authorize use of the print medium by determining the position of the print medium based on the location determined in step (b) so that the printer can print information in visible ink in registration with said at least one sensed coded tag.