watermarking sucks (Re: stego for the censored II)

[Sampo Syreeni]

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 at iki.fi>, aka decoy, student/math/Helsinki university