Digital Cameras Have Unique Fingerprints
Fridrich’s technique is rooted in the discovery by her research group of this simple fact: Every original digital picture is overlaid by a weak noise-like pattern of pixel-to-pixel non-uniformity.
Although these patterns are invisible to the human eye, the unique reference pattern or “fingerprint” of any camera can be electronically extracted by analyzing a number of images taken by a single camera.
That means that as long as examiners have either the camera that took the image or multiple images they know were taken by the same camera, an algorithm developed by Fridrich and her co-inventors to extract and define the camera’s unique pattern of pixel-to-pixel non-uniformity can be used to provide important information about the origins and authenticity of a single image.
The limitation of the technique is that it requires either the camera or multiple images taken by the same camera, and isn’t informative if only a single image is available for analysis.
Like actual fingerprints, the digital “noise” in original images is stochastic in nature that is, it contains random variables which are inevitably created during the manufacturing process of the camera and its sensors. This virtually ensures that the noise imposed on the digital images from any particular camera will be consistent from one image to the next, even while it is distinctly different.
In preliminary tests, Fridrich’s lab analyzed 2,700 pictures taken by nine digital cameras and with 100 percent accuracy linked individual images with the camera that took them.
There’s one important aspect of this fingerprint that the article did not talk about: how easy is it to forge? Can someone analyze 100 images from a given camera, and then doctor a pre-existing picture so that it appeared to come from that camera?
My guess is that it can be done relatively easily.