1. A handheld device comprising: a motion detection module operable to detect motion of the device within three dimensions; a memory maintaining a spatial signature comprising a particular movement of the device with respect to a reference position; and a signature detection module operable to determine an input signature by identifying an initial position of the device and tracking movement of the device in reference to the initial position using the motion detection module, to determine whether the input signature substantially matches the spatial signature, and to generate an authorization in response to determining that the input signature substantially matches the spatial signature.