Abstract: A lock (19) which by physical or logical means controls human access to an objective (29) such as a restricted area or a computer system is controlled by a data processing system (10) which has access to a store of authorised users (14) and a store of complex images (15), specifically images of human faces. Each authorised user knows certain ones of the complex images, and these key images are linked to an identity statement unique to that user. On receipt of an identity statement signifying an authorised user, a matrix of images including the key images is presented to the user on a display (12), and by means of a suitable input device (20) the user must identify which images are his key images in order to confirm authorisation and be permitted access. The user and image stores (18) may be carried in credit card form as a section of an optical disk.