1. A method for unobtrusively recognizing a user of a mobile device, the method comprising the steps of: (a) unobtrusively and continuously collecting a stream of motion data from the mobile device during normal device usage by monitoring standard authorized-user interaction with the device, without any form of challenge or device-specified action; (b) determining a plurality of motion-states from said stream of motion data, wherein a motion-state refers to a placement and a speed of the mobile device at a point in time; (c) demarcating said stream of said motion data into user motion-sequences based on changes in said plurality of motion-states; (d) calculating a plurality of user motion-characteristics from said user motion-sequences locally within the mobile device; (e) generating a motion-repertoire from a subset of said plurality of said user motion-characteristics, whereby said motion-repertoire enables unobtrusive recognition of the user; and (f) detecting unidentified motion-sequences having motion-characteristics that are not associated with said motion-repertoire, thereby enabling unobtrusive recognition of unidentified usage.