A method of user authentication on a computing device is proposed, together with computing device on which the method is implemented. In the method a modified base image is overlaid with a modified overlay image on a display and movement of either the modified base image or modified overlay image is responsive to receiving an input starting outside a display area of a touchscreen. Positive authentication is indicated in response to the base image reference point on the modified base image being aligned with the overlay image reference point on the modified overlay image.