Mobile devices are easily lost or stolen, compromising data and privacy.  We have designed a new file system for mobile devices, called Keypad, which provides an audit trail that indicates which files were (or were not) accessed following device loss and lets users disable file reading post-loss, even in the absence of network connectivity. A Keypad paper was presented at the EuroSys Conference, April 2011 and won the Best Student Paper Award. Faculty: Gribble, Kohno, Levy.