Digital detectives discern Photoshop fakery
New software combs for clues in al Qaeda tapes, Harry Potter pages, and celebrity waistlines.
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With the pervasiveness of computer editing software, investigators and courts are learning to deal with digital fraud. Since pictures and documents stand as the bedrock of evidence, Farid has applied his studies to help judges and juries determine what's real and what's been altered. Recently he testified in an intellectual property lawsuit. The plaintiff accused the defendant of stealing software and offered a computer screen shot of their programming as proof. After running tests on the evidence, Farid determined the screen shot was faked.Skip to next paragraph
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"They tried to fool the court," he says. "I think the case has now turned from a civil suit to a criminal case going the other way."
But after working on two dozen cases, Farid has found there are far more accusations of fraud than there are actual instances of foul play. In another case, a man insisted that someone had digitally added his signature to a scanned document. If the charges were true, then a computer could probably detect tiny discrepancies between the signature and the rest of the image. Farid could not detect any and concluded that the man had in fact signed the document.
How Al Qaeda alters its videos
Spotting inconsistencies in pictures is a major aspect for computer forensics.
"One helpful aspect of digital files is that they leave records, whether you know it or not," says Nasir Memon, a computer science professor at Polytechnic University in New York City. "Whatever you do to an image, it will leave tell-tale signs – artifacts hidden in pictures."
Scouring for these digital imperfections led computer security consultant Neal Krawetz to develop techniques of determining what specific areas of an image have been altered. He can even trace the history of those changes. Dr. Krawetz digitally dissected an image from a 2006 video of Al Qaeda No. 2 Ayman al-Zawahiri sitting in an office that's decorated by a banner with writing on it. (.) Through a computer analysis, Krawetz found that Dr. Zawahiri probably posed in front of a sheet and then was superimposed – much like the green-screen technology used in movies.
In fact, he concluded that the image is a composite of up to four different layers: Zawahiri, the subtitles and As-Sahab logo, the background, and finally the writing behind his head. "Apparently someone added those letters to the image afterward," he says.
How can he tell? If the picture was unedited, the quality across the frame would be uniform. But many digital images lose precision every time they are saved. With each modification, older additions to the collage deteriorate. This disparity is often undetectable to the eye, but Krawetz's software can sniff out the variations.
The Harry Potter photo case
A few days before the release of the seventh Harry Potter novel in July, someone posted digital photos of every page in the final book on the Internet. The culprit has yet to be caught, but digital forensics experts know a lot about this leaker.