A method of comparing two nominally identical identities each of which can be represented by an assembly of arcs, comprising the operations of producing images of the entities, analysing each of the images into an assembly of arcs, producing respective data sets representative of the arcs in the assemblies, using the data sets to map one assembly of arcs onto the other, determining differences in the mapping related to differences between the images, comparing the determined differences in the mapping between the images with statistically determined differences in the mapping between a plurality of authentic images to establish a criterion of an acceptable level of probability of identity between the said two entities and providing an indication of the acceptance or rejection of the proposition that the two entities are identical.