Making JPEG Images Copy-Evident

A group of academic researchers at the University of Cambridge has developed a new technique for making JPEG images copy-evident, so that users can tell whether an image has been recompressed and copied.

A group of academic researchers at the University of Cambridge has developed a new technique for making JPEG images copy-evident, so that users can tell whether an image has been recompressed and copied.

The technique, presented in a paper by Andrew B. Lewis and Markus G. Kuhn, relies on a complex method for inserting a large message into an image, which will only become visible once the image is copied and recompressed at a different level of quality. The idea is to help make it obvious to human eyes that the JPEG they’re seeing isn’t an original.

In a blog post explaining the paper and the technique, Lewis said that the idea came from observing the techniques used to protect paper money and other valuable documents.

“Our algorithm works by adding a high-frequency pattern to the image
with an amplitude carefully selected to cause maximum quantization error
on recompression at a chosen target JPEG quality factor. The amplitude
is modulated with a covert warning message, so that foreground message
blocks experience maximum quantization error in the opposite direction
to background message blocks. While the message is invisible in the
marked original image, it becomes visible due to clipping in a
recompressed copy,” Lewis wrote.

The technique could be used to help copyright owners enforce their rights over specific images if they’re clearly being copied and used for prohibited activities.

“Ideally, we would even like to have control over the conditions under which the embedded
mark becomes visible. In some applications we prefer a targeted mark, which only
becomes visible when one particular a priori known processing step is applied. (Imagine
a video that shows a warning if it has been uploaded to a particular website where all
material is recompressed with fixed settings.) In other applications, we might prefer an
untargeted mark, which becomes visible with high probability as soon as any of a number
of possible processing parameters are applied,” Lewis and Kuhn write in their paper.

The unauthorized use of material such as pictures, video and text has been a major problem online basically since the inception of the Internet. Copyright owners, media companies and others have tried various techniques–mainly variations of one kind or another on digital rights management–most of which have failed badly and quickly. The method developed by Lewis and Kuhn isn’t designed specifically for the purpose of protecting copyrighted material, and it’s not a protection method in the way that DRM techniques are. But it could have applications in that domain.

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