[quote=marcan,Mar 30 2005, 11:27 AM]
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Hi 2B,
You right, I never craked a watermark (and you?)
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Of course not - that would be against the DMCA and could get me thrown into jail when I travel to the USA!
Seriously, I was given some watermarked content to play with at university (though I never did find out where it came from - my Professor was very secretive! However, thinking about it, it was probably just someone else's PhD project) - quite easy to detect and crack, though psychoacoustics wouldn't help. It wasn't a particularly good watermark, but it was still interesting to play with.
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I did read the Felten report long time ago now. Now it doesn't mean that my statement is false.

The only statement I'm not sure about is the psychoacoustic model able to crack it.
Could you tell me why it won't work? I'd like to know.
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There are a lot of inaudible changes you could make to an audio file. Pick any "transparent" encoder from the HA list - they'll all do quite different things, all inaudible. Or just make a program to add noise right up to the masking threshold - you'd expect that to trash any inaudible watermark, wouldn't you - because it's destroyed
all the inaudible space in the signal?
But it's also inaudible (though easily detectable) to speed up a track by, say, 0.1%. Just by listening, I can't hear the difference, so the change is (by definition) inaudible. If you run the little program I've just suggested, and add noise right up to the masking threshold, the track is still 0.1% fast, which is still detectable!
Obviously no watermark consists of just increasing the speed of a track by 0.1% (in fact that's a great way to break many primitive watermarks). But it illustrates how you can think you're trashing all inaudible content in the track, yet there's another inaudible property - another inaudible dimension if you like - that you haven't touched by your psychoacoustic based crack.
How many inaudible or near-inaudible dimensions do you think there are in audio? Unless you can think of them all, and totally trash them all, without affecting the audio quality, your blind "psychoacoustic hack" approach won't work.
Further more, the watermark might be periodic (in some domain - not necessarily time) - and the watermark encoder and detector know this periodicity, but you don't - that means they can average across this period, and uncover information that you would think had been buried beneath your inaudible noise/changes.
Contrast this with an intelligent hack, where you know something about the watermark - even just the domain it works in, or the periodicity (in the above example) or whatever. Then you can carry out a surgical attack. If you're good, you can actually restore the audio to the state it was in
before being watermarked - in which case, it could sound
better after the hack. You're never going to achieve that with blind psychoacoustic "trashing of inaudible parts".
Hope this makes sense - it's all quite apparent from the hack SDMI challenge responses - from the information in that paper, it's possible to simulate how the watermark might work, and to realise just how robust it is until you make a surgical attack, when it falls apart.
It's supposed to be security by obscurity, but that's never a very secure approach when the algorithm is patented, and therefore not obscure!
If the detectors weren't crackable, and the algorithm was totally novel and wasn't published anywhere (patent or journal), then it would be exceptionally difficult to crack. I'm not sure how that works as a business model though. There appears to be a school of thought to patent enough of the algorithm to stop anyone copying it, but not enough to help anyone to crack it.
The problem is, with that kind of secrecy, there's probably a huge flaw in the algorithm which no one involved has spotted. If there isn't, then the weak point becomes the inventors themselves - can they keep a secret?
However, who would deploy a system based only on the assurance of the inventor that it was uncrackable?
Cheers,
David.