Privacy advocates have fretted for some time about radio-frequency identification (RFID) tags and their potential for enabling detailed tracking of an individual’s activities. Some tags are obvious, such as those used for E-Z Pass toll collection on northeastern U.S. highways; others are more insidious, such as those tucked inside U.S. passports and on the London Underground’s travel cards. Rapidly expanding in use, such tags may soon even report the amount of material in recycle cans.

Melanie Rieback and her colleagues at Free University in Amsterdam are working on RFID Guardian, which they hope will enhance privacy: a portable, battery-powered personal firewall. The notion is that anyone ought to be able to see what RFID tags are nearby and who owns them—and then have the ability to selectively jam the readers. In this way, you could allow security—and no one else—to read your passport RFID chip.