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Digital detectives discern Photoshop fakery

New software combs for clues in al Qaeda tapes, Harry Potter pages, and celebrity waistlines.

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Embedded in each photo file is information called metadata, which reveals that the shots were taken by a Canon EOS Digital Rebel 300D. The tags disclose the year the camera was made, when the pictures were taken, even the serial number for the specific camera. "Now, if that person ever brings in his digital camera for repairs, they got him," says Bruce Schneier, CTO for the network security firm BT Counterpane. "This information is secretly hidden in all kind of electronics. Even [some Xerox] color printers hide information in printed pages that can be tracked back to your specific printer."

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Mr. Schneier worries about the consequence such technology has on privacy. These buried codes are designed to track down crooks, but he warns that smart criminals will know how to scrub the metadata out of their files. The people they're hurting, he argues, are law-abiding citizens.

"Data is more and more traceable and now is being used to connect you to things that should be private," he says. "Do we really want ... cameras that link your pictures back to you?"

How to detect a fake

Spotting fake photos and forgeries doesn't always require high-powered algorithms. While image manipulation software is now easier to use, it still takes considerable skill to wield the tools well, says Cynthia Baron, author of "Photoshop Forensics."

"We're an intensively visual society, and yet we're not very good at visually scrutinizing," Ms. Baron says. But if you know what to look for, there are many ways to tell. Here are a few:

•Frankenstein images. A favorite trick is digitally sewing a celebrity's head onto another's body. Earlier this month, the Republican Party of Kentucky printed a campaign brochure with a fake photo of the Democratic candidate for governor, Steve Beshear, looking sleazy in a casino. The image is labeled "not an actual photo" and was designed to mock the candidate's stance on gambling. But the picture also demonstrates two clear signs of a stitched photo: an unnatural tilt of the head and an awkward seam where the head meets the collarbone.

•Too-straight lines. Few unmanipulated photos contain 90-degree angles. But computers love square corners. If a protester's sign has a perfectly straight edge all the way around it, it might be a fake.

•Recurring objects and patterns. After an Israeli airstrike in Lebanon last August, the Reuters news agency website ran a photo of smoke billowing from a building. But one of the thick puffs had a repeating pattern – an obvious signal, Baron says, that the photographer had copied and pasted parts of the image to make the damage look more severe.

•Truth in shadows. Manipulators often create collages without making sure that the lighting on all the objects matches up. An astute viewer can pick out such inconsistencies.

•If it looks too good to be true, it probably is. Andy Roddick surprised even himself when he saw the June-July cover of Men's Fitness magazine. In the image, the tennis star's biceps appear thicker than the Wimbledon trophy. "Pretty sure I'm not as fit as the Men's Fitness cover suggests," he jokes on his blog. "Little did I know I have 22-inch guns and a disappearing birth mark on my right arm." Oops.