One of the most unnerving things about modern communications technology is the way devices constantly leak information about their physical whereabouts—to mobile carriers, network operators, e-mail providers, web sites, governments, even shopping mall owners. Many of these information leakages are simple historical accidents. The designers of technologies never considered that technical standards would let everyone around you notice your device‘s presence. They never considered that technical choices would let web sites infer when two people are (or aren‘t) spending the night in the same residence, or let your phone company follow you around virtually from moment to moment.