Princeton's Ed Felten has written a terrific article explaining the ins and outs of watermarking for audiovisual material, providing an excellent guide for anyone who wants to understand how the new proposals to mandate watermark detectors are doomed.
Congress is considering a bill to "plug the analog hole," that is, to prevent the use of recording equipment for capturing digital programs while they're been played back (one outcome of this is that you couldn't video your child's first step if he was taking it in the living-room with the TV playing in the shot).
The proposal is to use a "watermark" called VEIL, based on secret technology, and to require all people who build recorders to include VEIL detectors that can shut off the recorder if it appears that it's recording a watermarked program.
Felten is one of the world's leading experts on why watermarking fails, having led the effort to defeat the most ambitious, expensive watermarking system to date, the Secure Digital Music Initiative. In this article, "How Watermarks Fail," he talks about the ways that attackers can circumvent watermarks, in plain language that even I can understand:
[W]atermarks tend to be defeated if an adversary can get his hands on a watermarked file, and the same file without the watermark. By comparing the two, the adversary can determine where the watermark lives, which is usually sufficient to remove the watermark from other files. Alex used this method in deciphering the MediaMax watermark (as described in our Sony CD DRM paper), and my colleagues and I used it also in analyzing the SDMI watermarks back in 2000.
Almost as powerful as a Rosetta Stone attack is a comparison attack, where the adversary does not have an unwatermarked file, but does have the same file with several different watermarks in it. Any place where two of the files differ is a place where watermark information lives. Given several marked files, an attacker can locate all or most of the places the watermark is hidden, which is again the first step in removing the watermark.
(via Hack the Planet)
