A document used to assert a personal right has a randomized unique pattern of normally invisible but mechanically detectable material (e.g., finely divided ferromagnetic material) in a zone overlapping any visible indicia identifying the owner. The document is presented to a mechanical reader which scans the pattern along predetermined lines and records resultant output signals, along with an identifying record. Upon subsequent presentation, it is similarly scanned, for comparison with the recording to confirm its authenticity. Methods of producing appropriate patterns are disclosed.