Chinese human rights activist Shi Tao was sentenced to 10 years in prison in 2005 after authorities tracked him down using data provided by Yahoo.
The Internet service supplied information that it garnered about his location when he accessed his Yahoo e-mail account. That was enough to find him and put him in jail.
Now, human rights activists are looking to a new generation of Internet privacy tools to keep companies from gathering such data, hoping that it will protect dissidents like Shi.
One, called Tor, scrambles information, then sends it over the Web. It hides the user's location and gets past firewalls. Those features make it popular with activists in countries like China and Iran.
Tor is a tunnel. What you send into it comes out the other end, untouched, said Andrew Lewman, executive director of the Tor Foundation, which is funded by the U.S. government.
It lets surfers get around Internet censorship software - whether installed by governments or companies seeking to keep workers from using social networking sites like Facebook.
Tor also can protect against identity theft and monitoring by parents, suspicious spouses and bosses. It may even be able to evade the warrantless wiretapping program started in the United States following the September 11 attacks.
When a user shuts down a browser running on Tor, all information exchanged during the Web session is deleted.
The U.S. government is one of Tor's biggest financial backers. It contributed $250,000 of the $343,000 in income the nonprofit reported in 2007, the most-recent year for which financial data is available.
We are trying to encourage a certain freedom of the Internet, said Ken Berman, director of information technology at the U.S. Broadcasting Board of Governors, which oversees Voice of America.
Tor use has risen in China as authorities block access to sites that the government has banned for political reasons. They include Google's e-mail service, Lewman said.
People who were never were never concerned about censorship suddenly had it thrown in their face when they couldn't get to Gmail anymore, Lewman said. Average people said 'How do I get around this?'
In May, ahead of the 20th anniversary of the Tiananmen Square crackdown, the Chinese government also blocked access to Twitter and Microsoft Corp's Hotmail.
Connections to Tor from Iran surged five-fold in June as protest organizers used social network services Facebook and Twitter to coordinate demonstrations in the wake of the country's disputed presidential election.
These are great features. These are exactly the kinds of things Iranians need, said Sam Sedaei, an independent economic researcher who studies human rights in Iran.
The nonprofit group Human Rights in China plans to test a newer version of Tor to secure its communications. It is also developing tools to fight surveillance.
As activists, we want anonymity and security. The challenge is to keep up with the new technology, said Human Rights in China Executive Director Sharon Hom.
Tor runs on a free software package available on the group's website, http://www.torproject.org . It includes a customized version of the Firefox browser and other programs.
The service connects a user to a second PC that links to a third computer, which does not know the location of the first machine. When the data stream hits the Internet, it is impossible to trace the identity of person accessing the Web.
One drawback that has hurt adoption is speed. Not all users volunteer to let traffic flow through their computers, which makes the service far slower than regular Web browsing.
It has another. Tor's features can help criminals evade detection as they use the Web for activities ranging such as spam, identity theft or pedophilia.
At the same time, police can use it to cloak their identities when they go undercover to conduct online stings.
Tor competes with several other technologies, including one known as Freegate, which China's banned Falun Gong movement developed to allow its members to communicate in secrecy.
Freegate runs on a dedicated network paid for by a U.S.-based company that owns the product, Dynamic Internet Technology, which is run by members of Falun Gong.
DIT also sells an e-mail service that evades spam-filters installed to weed out correspondence related to human rights and other sensitive topics. Customers include the Voice of America and Human Rights in China.
It distributes about 250,000 e-mails with Human Rights in China's electronic newsletter, about 80 percent of which make it past the censorship filters, according to Hom.
(Reporting by Jim Finkle. Editing by Robert MacMillan)