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A dissident in China uses Web-based e-mail to contact a journalist in Canada. An intelligence agency wants to surveil a foreign website. Like every operation on the Internet, these activities leave tracks. Online anonymity measures provide a way around this problem; one of the most advanced is Tor, or the Onion Router.

Computer scientist Roger Dingledine developed Tor under a contract with the U.S. Naval Research Lab; today, the software is distributed by the Tor Project, under the fiscal sponsorship of the Electronic Frontier Foundation.

To disguise Internet traffic's origins, Tor plots a route through any three of more than 700 volunteer-run Onion routers around the world. It sets up a two-way link between the sender's computer and the final router in the chain; data passed between them is encrypted in three layers, and each router in the chain peels off one layer along the way. Each data packet "remembers" only the address of the last router it visited. That way, even if the data is intercepted before the final router hands it off to the recipient, it's difficult to trace back to the sender.

Editors’ note: This text is corrected version of the story that ran in the September/October 2006 print issue of Technology Review.