A method of authenticating a user of a computing device is proposed, together with a computer readable medium and a 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 either the modified base image or modified overlay image is moved by the user. Positive authentication is indicated in response to alignment of a base image reference point and an overlay image reference point and an indication of a correct level of pressure applied to a pressure sensitive input device.