A Tor transmission these days might start in Addis Ababa, hop to Dallas, then to Stockholm and finally Johannesburg. (There are some 2,000 Tor relay nodes at any one time across the globe.) The only thing the Johannesburg recipient can discover is that the data came from Tor, and Tor has successfully identified itself with no person or group, only with ideological incoherence. For the person trying to get a message out through Tor, this means he communicates exactly as much as he chooses and no more. With Tor, you “only reveal the information that you type,” Appelbaum says. “As opposed to all the other information that comes along when you use your computer.”

So why can’t a Tor transmission merely be intercepted sometime in its swerving and illogical flight path? This is the innovation of Tor’s code. A complete chain of nodes is never fully readable at a single relay point. Not only is the communication fully encrypted during transit, but its traffic pattern also is more than unknown; practically speaking, it’s unknowable. This central insight of Tor is brilliantly tautological: What can’t be known can’t be found out — even by interested parties with state-of-the-art or brutal interrogation and intelligence-gathering techniques. As Appelbaum put it: “Even though the government has a monopoly on violence, violence cannot solve math problems.”

Appelbaum, who is a developer for Tor and a research scientist at the University of Washington, had some cause for these sinister ruminations about governments while we spoke. Hours earlier, Julian Assange, the brazen founder of WikiLeaks, was arrested in London on Swedish sex-crime charges. Appelbaum himself, who is 27, was detained for several hours last summer by federal agents at Newark Liberty Airport, where he was questioned about his connection to WikiLeaks and Assange as well as his convictions about the wars in Afghanistan and Iraq.

Appelbaum said he didn’t discuss those topics with the agents in Newark, and we agreed to avoid them also. Instead he spoke volubly, on a temporary Web-based phone line, about Tor and privacy. Appelbaum, whose work as a Tor evangelist requires him to persuade large numbers of people to trust Tor, calls traditional journalism a system of “privacy by policy” — where the policy is set ad hoc by the journalist and the source. (“This is off the record; the story should run on Monday; you must let me use this if you expect the Monday story to run,” etc.) Tor, by contrast, offers what Appelbaum calls “privacy by design” — a kind of privacy that is built into the code, which is available, like Tor software, free, so users can inspect it. You couldn’t violate Tor’s privacy if you wanted to.