"There's no direct connection between the user in the censored country and the Web sites he's going to," said Nart Ville-neuve, director of technical research for the Psiphon project.
The secret surfing programs also add a layer of encryption, ensuring that messages are scrambled in transit and are thus unreadable. Even if the government set up a phony Falun Gong site to trap its citizens, it wouldn't be able to identify visitors by the Internet addresses of incoming data requests. The addresses belong to the proxy computers, not the Chinese citizen.
Anonymizer has set up its own commercial network of anonymous proxies. Tor relies on volunteers to run proxy software on machines attached to the Internet.
The addresses of these machines are added to the Tor network, and data are automatically routed through them. There are about 400 Tor servers serving about 200,000 users, according to Dingledine.
But all these systems have a key weakness. Eventually, government censors will figure out the Internet addresses of the proxies and block them, thus preventing anonymous surfing.
Anonymizer addresses this by letting users subscribe to an e-mail service that sends out a daily list of the latest unblocked proxies. Cottrell admitted that government censors can get the same e-mails and move to block the proxies. But "it's a major effort," he said. "Our experience is, it takes several days."
Dingledine said that he hopes to develop software that would let users automatically find the newest open proxies. "Then we can start looking at ways to do network discovery in a way that's harder to censor," he said.
Dingledine is presently the only software engineer working full time on Tor. But Ken Berman -- manager of the Internet anticensorship office at the Broadcasting Board of Governors, parent of the Voice of America -- said that his agency may provide funding for an enhanced version of Tor, aimed at outwitting the world's Internet censors.