Their project is designed to let police identify whether suspects under surveillance have been communicating through voice over Internet Protocol (VoIP)--information that would be unavailable today if people choose to communicate surreptitiously. The eavesdropping technique already has been shown to work with Skype, the researchers say.
"From a privacy advocate's point of view, this is an attack on privacy," Xinyuan Wang, an assistant professor of software engineering and principal investigator, said Tuesday. "From a police point of view, this is a way to trace things."
To translate his research into a tool that could be used by police in a grant calls for the development of a prototype VoIP-tracing application to provide a "critical but currently missing capability in the fight on crime and terrorism."of the FBI's Carnivore system, Wang received a grant of $307,436 from the National Science Foundation this month. The
The NSF grant comes as federal police are fretting about criminals using VoIP to mask their communications. The Federal Communications Commission on Fridayfor some VoIP providers, and the FBI has been warning for that VoIP may become a " , terrorists and spies."
At the moment, two Skype users who wish to conceal the fact that they're chatting can direct their computers to bounce their conversation off a commercial anonymizing service, sometimes called a proxy service. Such services are offered by FindNot.com, Proxify.us and Anonymizer.com.
The FBI or any other government agency that's eavesdropping on both ends of the link would see that each person was connected to the anonymizing server--but couldn't know for sure who was talking to whom. The more customers who use the service at once, the more difficult it would be for investigators to connect the dots.
Wang discovered he could embed a unique, undetectable signature in Skype packets and then identify that signature when they reached their destination. The technique works in much the same way as a radioactive marker that a patient swallows, permitting doctors to monitor its progress through the digestive system.
"It's based on the flow itself," Wang said. "I embed a watermark into the flow itself, the timing of the packets. By adjusting the timing of select packets slightly, it's transparent. There's no overhead in the bandwidth, and it's very subtle. It's mingled with the background noise." (The anonymizing service tested was Findnot.com, which did not immediately respond to a request for comment on Tuesday.)
A paper co-authored by Wang and fellow George Mason researchers Shiping Chen and Sushil Jajodia describing their results is scheduled to be presented at a computer security conference in November. An early draft concludes that "tracking anonymous, peer-to-peer VoIP calls on the Internet is feasible" with only 3-millisecond timing alterations as long as the calls are at least 90 seconds long.
Peter Wayner, an author of books on cryptography who is teaching at Dartmouth College, predicts that an arms race could develop between VoIP programmers and eavesdroppers. The George Mason research "seems as likely to yield new techniques in anonymizing as it is to yield techniques for stripping people of their privacy," Wayner said.
"I think it's pretty academic right now," Wayner said. "It would take a lot of work to track people. They'd have to really be interested in someone to use it."
The George Mason researchers' technique does not try to decipher the contents of encrypted conversations (Skype, VoicePulse and PeerMe are VoIP providers that use encryption). In other words, it tries to glean only the identities of the participants, not what they say.