RE: watermarking sucks (Re: stego for the censored II)
I realize that this is *slightly* simplistic, but comparing 2 (preferably 3 or more) copies of the data with different watermark contents should quickly reveal where and what constitutes the watermarking. Of course, there are methods to make this more onerous - MACing the watermarked data for one - but nothing a determined counterfeiter couldn't work around. It only takes one, after all. I'm currently working my way through Schneier's 'Secrets and Lies'. He has one comment I like to the effect that trying to make data not be copyable is a task on a par with trying to make water not be wet. Peter Trei
---------- From: Adam Back[SMTP:adam@cypherspace.org] [...]
Of course the whole concept of watermarking is broken at all levels, copying can not be prevented as the content can typically be reencoded and lose the watermark, quality is in heavy contention with the ease with which the watermark can be removed. Even if it is keyed. And ultimately content can plausibly deniably be stolen and all it takes is one copy.
Adam
On Mon, 12 Feb 2001, Trei, Peter wrote:
I realize that this is *slightly* simplistic, but comparing 2 (preferably 3 or more) copies of the data with different watermark contents should quickly reveal where and what constitutes the watermarking.
Not really. If the original version is not available, a properly constructed watermark will basically amount to a noise component. Comparing two noisy versions of the same data will not give you enough statistics to recover the noise completely. Averaging attacks (averaging over multiple independently marked copies to try and fade out the mark; an optimal attack if the marks are independent and flatly distributed) can be seen as a channel distortion, which isn't too difficult to compensate for, with sufficient redundancy in the mark. A good watermarking scheme resists averaging over ten or twenty independently marked copies. Beyond this, it becomes quite difficult to find paying customers ready to participate in collusion on a regular basis. Anyway, this is what CRM people are counting on. Sampo Syreeni <decoy@iki.fi>, aka decoy, student/math/Helsinki university
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Sampo Syreeni -
Trei, Peter