BOB GARFIELD:  This is On the Media. I'm Bob Garfield. For years, we've been reporting on the great firewall of China, the digital security apparatus that eavesdrops on and sometimes blocks online chatter. And we've long known a handful of the search terms that have been targeted by government censors, Falun Gong, for example, Tiananmen Square. Now we’re getting even more insight into what words get flagged, thanks to computer science graduate student Jeffrey Knockel who, using one word on the Chinese version of Skype, known as TOM Skype, has been able to divine thousands of words and phrases that get Skype chat users put under surveillance.

JEFFREY KNOCKEL:  Well, it was the F word.

  [BOB LAUGHS]

I don’t know if I’m allowed to say it on –

BOB GARFIELD:  [LAUGHS] It was the F word. So how did you do it?

JEFFREY KNOCKEL:  We knew that the F word was triggering surveillance based on some previous work, so what we did was we redirected TOM Skype. Instead of downloading the list of encrypted words from TOM servers, we had a download list of encrypted words that we gave it. So we initially started out with just the regular list of encrypted words that it would have gotten anyways, and then we used a technique called binary search where you divide the list in half repeatedly. So what we would do is we would delete say the first half of the list. If this word was still triggering surveillance after we deleted the first half of the list, then we knew it was on the second half. We can repeat this process until we know exactly which line on the list corresponds to the F word.

BOB GARFIELD:  And before long, you had come up with such subversive terminology as “Human Rights Watch,” “Reporters Without Borders,” “BBC News.” But then there were others that seemed to be less about political threats than kind of moral turpitude issues like “kinky cinema” and “hired killer,” “two girls, one cup” and a bunch of other swearwords, in addition to the big F.

JEFFREY KNOCKEL:  “Two girls, one cup” is one of the more surprising key words that we found on there. If our audience isn’t aware, this refers to an obscene sexual video that made its rounds on the Internet a couple of years ago.

BOB GARFIELD:  Now, you know that this flagging system is built in to TOM Skype. Do you know for certain that TOM Skype turns this information over to the state or to the party?

JEFFREY KNOCKEL:  We don't know what they do with this information after they get it. All we see is the information being sent to TOM servers. They could be a lot more transparent. One problem is that if say I’m using Skype and I’m talking to another person using TOM Skype, the text messages that I’m sending them are still being surveilled just by virtue that they’re all going in and out of the person's TOM Skype client that I’m talking to. There’s no indication that – no warning their messages may be under surveillance.

BOB GARFIELD:  Well Jeff, thank you very much.

JEFFREY KNOCKEL:  Thank you.

BOB GARFIELD:  Jeffrey Knockel is a graduate student in computer science at the University of New Mexico.