Farid's forgery test also examines another aspect of the MPEG compression: the error that's introduced when motion between frames is estimated. "The motion error turns out to be very valuable to us," he says. Between each frame of an MPEG file is a predictable type of motion error, but when frames are removed, this alters the error in a noticeable way. The combination of this error detection and the JPEG compression test is "very good at detecting when you delete a handful of frames," says Farid.

"I think it's a very interesting approach," says Edward Delp, professor of electrical and computer engineering at Purdue University, in Lafayette, Indiana. The technique is an extension of the earlier image-forensics work out of Farid's lab, he says.

"You're going to see more and more [video forgeries] in the future because it's so easy for people to acquire and process the videos," Delp says. The legal implications are important, especially in the case of surveillance video, from which a couple of frames featuring a person walking by could easily be removed. "The question going into court is, How do you prove that it's really the video that came out of the camera?" he says. "And we're going to need more tools to decide if [the video] has been tampered with and if it's authentic."

Farid's team is collaborating with Adobe, maker of the video-editing software Premier, to get a better understanding of how the company's editing tools might be reverse engineered.

The Dartmouth approach isn't foolproof for high-quality videos, however. Deleting frames in multiples of twelve can trick the system. "You can get around it--no doubt about it," Farid says. This is why he and his team are developing a suite of tools to detect tampering that use differing techniques, which aren't sensitive to the quirks of MPEG compression. "As with the image forensics, we expect each technique can be circumvented, but circumventing a larger set of tools will become increasingly more difficult," he adds. The current work, says Farid, is a good starting point, though, and the method will still be useful as an initial test to determine if videos have been doctored by unsavvy editors.