A mobile device implements a state machine with full authentication, continuous authentication, and invalidation states. To access the device, the full authentication state requires the user to confirm his or her identity using some robust authentication technique. Upon success, the state machine transitions to the continuous authentication state wherein data samples are captured as the user interacts with the device and compared with stored exemplary fingerprints. Preferably, the state machine enforces a negative identification technique to determine whether the individual currently interacting with the touchscreen is not the user from which the exemplary fingerprints were generated. Upon such negative authentication, the state machine transitions to the invalidation state. In this state, operations (e.g., screen lock) are performed to secure the device against an unauthenticated use. On a next interaction, the state machine transitions to full authentication requiring robust authentication to continue operating in a normal mode.