In China, Tor is one weapon in a large arsenal. But in Mauritania, Tor appears to have single-handedly overwhelmed state censorship. Nasser Weddady is a Mauritanian-born son of a diplomat, now living in the Boston area. He is a civil-rights activist who seeks to call attention to the slavery still practiced in his native country, where black Muslims work in servitude for Arab and Moorish farms and households, far from the international spotlight. In 2005, in response to Internet filtering in Mauritania, he translated a guide to using Tor into Arabic and arranged for its distribution to owners of cybercafés. The effect was stunning: the government stopped filtering. Officials "didn't know we were using Tor," says Weddady. "I'm not sure they know what Tor is. But they noticed that our communications were not disrupted, so the filtering was useless."

Such successes can be short-lived, of course, and Weddady predicts that the regime will regroup and resume filtering. "The ­Middle East in general is a civil-rights desert; it has some of the most sophisticated filtering operations in the world," he says. "Plenty of people I personally know are using Tor in that region." Users know that to any snooper, the messages they post appear to originate from a Tor relay somewhere else in the world, so cybercafé owners can't rat them out even if they want to. "Tor doesn't say, 'Just trust us not to give out your information'--it says, 'We have a design where nobody is in a position to give up your information, because no one person has it,'" says Seltzer, who volunteers on Tor's board. "I do believe Tor is the best solution for people who are trying to get access to blocked matter, or are trying to speak anonymously."

Bridging Tor's Gaps
Neither Tor nor any other tool is a perfect solution to Internet spying and censorship. As an open-source project, Tor publishes everything about its workings, including the addresses of its relays. That doesn't betray the actual source and destination of users' information, but it does mean that a government could obtain this list of addresses and block them. (So far, nobody has taken this step, though Iran, Saudi Arabia, and the United Arab Emirates did find a way to block Tor for a few months in 2008.) Second, using Tor can make Internet access painfully slow; online activities can take more than 10 times longer when using Tor, according to a study by Harvard's Berkman Center for Internet and Society. "It turns out the speed of light isn't so fast after all," Dingledine deadpans. And this problem is getting worse; in the past year, the number of users has increased faster than Tor's developers can add relays.

But the biggest limitation is simply that all these tools still reach only a narrow slice of the world's Internet users. Yes, if you're a business traveler in China and have technical savvy and bandwidth--or you hire someone to set you up--you can circumvent government filters. (It's generally understood that state security forces will rarely move to shut down circumvention tools unless they're publicly embarrassed by being outsmarted online.) But a recently released Berkman report by Zuckerman, faculty codirector John Palfrey, and researcher Hal Roberts has concluded--on the basis of data supplied in 2007 by makers of circumvention software--that only a few million people use the major circumvention tools worldwide. It's true that usage has grown since then--and this estimate doesn't count everybody who has figured out a way to use proxies. Still, China alone has 300 million Internet users, and the researchers believe that most of them aren't equipped to fight censorship. Meanwhile, the list of nations that censor is only growing. Two years ago, Turkey piled on, with particular zeal for stamping out criticism of the nation's founding father, Kemal Atatürk.