1. An indicium for identifying a valid workpiece, the indicium comprising a set of one or more markings corresponding to a string that is based upon, at least in part, on recovered shapes of 3-D features of at least one portion of a surface of the workpiece, the recovered shapes being determined from one or more images of optical characteristics of the 3-D features produced when the at least one portion of the surface of the workpiece is illuminated with electromagnetic radiation from one or more illumination positions relative to the at least one portion.