The Chinese government has restricted foreign reporters from entering Tibet, but amateur photos and videos of protesters have found their way onto YouTube and various media sites outside the Great Firewall. Is it possible to trace who took those pictures?
Probably not, unless the owner registered the camera with the manufacturer. A little detective work can easily pinpoint the make and model of the camera that took them, but it would be hard to extract identifying information from the digital images themselves.
Most JPEG files include pieces of information called metadata that cover everything from when the photo was taken to how long the exposure lasted. Manufacturers usually include the make and model of the camera as part of this information, but it's easy to delete or falsify these tags by using editing software like Photoshop. Listing the camera's serial number is less common, but, when available, this tidbit can be used to track down the country where the device was sold, or even, if there's a superb paper trail, the store. When the final Harry Potter novel was leaked online after a fan photographed every single page last summer, investigators gathered from the metadata that a Canon Rebel 350 was used to take the pictures. Some Canon models also automatically include the camera's serial number in the metadata, but it's not clear if the culprit was ever caught. Of course, if you've registered the device with the manufacturer, a photo's metadata can lead straight to you.
It's harder to generalize about tracing cell-phone pictures, since manufacturers may choose to include less metadata because of space considerations. U.S. cellular plans make it a bit easier to connect phones with their owners, but this is less true in Asia, where people buy minutes of air time rather than subscription plans. If, however, the metadata on a photo includes a piece of information known as the IMEI number, it's theoretically possible to track down the camera phone while it is turned on, triangulate the position of the person carrying it to within a mile, then chase him down.
Even if all the metadata has been erased, you can still uncover the camera's make and model. To do this, search the JPEG file for something called the quantization table. This series of numbers reflects the way the image has been compressed. Since manufacturers use different compression methods, a quantization table can narrow the field to a few camera models. (Something else that's also embedded into digital files from certain cameras: a thumbnail of the original photo. Even if you edit out faces from the photo, a low-resolution copy of the undoctored image will still be available.)
If you have a lot of digital images (say, 100 photos or five minutes of video) and a suspected camera on hand, a process similar to handgun ballistics is an option. To prove that a particular camera took those photos, you'd need to examine the "noise" patterns in the pictures. Sensors aren't perfect, so each pixel of color contains tiny variations—say, random colors when the whole pixel should be sky blue. If you tease out the noise from each photo and then average the noise to form a pattern, you may be able to match them to new photos from the camera.
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Explainer thanks Donald Allison of Stroz Friedberg, LLC; Hany Farid of Dartmouth College; and Nasir Memon of Polytechnic University.