New Technique For Making JPEG Images Copy-Evident 139
Gunkerty Jeb writes "The days of wondering whether those drunken sex party photos are indeed the Olsen Twins, or if they are just the Mary-Kate and Ashley's faces photo-shopped on the bodies of Lindsay Lohan and Amy Winehouse are OVER! 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."
Not ready for commercialization (Score:5, Informative)
Re:PrtSc (Score:5, Informative)
Nice try at getting fast karma from a first post, but that's not going to work. The screen shot will already be of the compressed image, and will still show signs of re-compressing it.
Doesn't Work (Score:5, Informative)
So I downloaded their test image here: http://www.cl.cam.ac.uk/~abl26/copy-google.jpg [cam.ac.uk] that they claim gets a message on it when compressed by google proxy http://www.google.com/gwt/x/i?u=http%3A%2F%2Fwww.cl.cam.ac.uk/~abl26/copy-google.jpg&wsi=223e8e5df695e99c&ei=6ixQTebOCoPoxQW8rYlv&wsc=yq&whp=012e012f72be [google.com]
But when I take the original and re-save it in Photoshop CS5 I don't see the void lettering. I reduced the JPEG quality and kept trying and at quality 1, the lowest setting I was starting to see a pattern, but no words appeared.
I'd say their idea is nice, but doomed to failure, not least they mention "If you can’t see the message in the recompressed image, make sure your browser is rendering the images without scaling or filtering." which would be the obvious source of attack on such a method should it actually work in practise.
Re:Doesn't Work (Score:4, Informative)
Apparently it only works at very specific quality settings. Re-saving with GIMP, I can see the message at quality 24, 38, 41 (barely), 43, 60, 65 (barely), 69 (barely), 72 (barely), and a few others even less.
As far as I can tell (I haven't read the paper), it works by setting up a static hard to compress pattern, and then slightly altering that pattern in certain macroblocks so that they just push the boundary into a different kind of compression artifact at certain quality/quantizer levels. So the entire image is compressed one way at one quality, a different way at another quality, and at the threshold between them there's a quality level where the message blocks compress differently and you can see them.
Also, recompressing has a high chance of destroying the effect permanently. For example, saving at quality 51 destroys the message, and re-compressing at any quality level no longer makes it visible.
Re:Scaling should not be affected (Score:5, Informative)
If you read the article, they are depending on the JPEG compression artifacts for the watermark display. Resizing should not cause this because you are deling with the uncompressed image data.
Resizing and then saving as a JPEG will result in re-compression and the watermark appearing. Saving as anything else bypasses this completely.
This is only useful when you know what conditions will be applied. The example they give near the end, uploading to youtube, will apply only as long as youtube does not change their settings. Then you have to change your thresholds and all of your protected videos in the wold are unprotected.
Re:social problem, technical solution (Score:4, Informative)
I don't think you understand what they mean by clipping in this sense. In JPEG compression, the DCT is performed, the matrix is formed, the coefficients are scaled, quantized(clipped), and then Losslessly compressed. The scaling/clipping(from quantization) IS the core of the low/med quality JPEG compression compression. It is not like "clipping" in the audio sense where it is horribly distorted due to over amplification, the quantization is intentional and in DCT space.
Also, as stated elsewhere on this forum, when a browser resizes a picture, the cache is not recompressed, which is when this intentional dithering becomes apparent.