A method of authenticating the identity of an item bearing or associated with a printed security marker, the method comprising: a. printing the security marker with a pattern that encodes at least one of a unique product identifier, a batch identifier, and positional information about a portion of the marker; b. scanning and storing an image of the security marker as printed, said image being taken at sufficiently high resolution to capture artefacts of the printing process; and authentication steps comprising: c. imaging a portion of the marker at sufficiently high resolution to capture the artefacts, and d. querying a data store by providing the image of the portion of the marker or a compressed or parameterised version of the image and data derived from the pattern.