limits of watermarking (Re: First Steganographic Image in the Wild)

[Adam Back]

On Tue, Oct 16, 2001 at 11:30:05AM -0700, Greg Broiles wrote:
> Adam Back wrote:
> >Stego isn't a horseman, and the press drumming up scare stories around
> >stego is ludicrous.  We don't need any more stupid cryptography or
> >internet related laws.  More stupid laws will not make anyone safer.
> 
> I agree, but if Congress isn't careful (and they don't seem to be in a
> careful mood these days), they'll end up outlawing watermarking in
> digital "content", which would do to the DRM (digital rights management)
> industry what they tried to do to security researchers with the DMCA.
> 
> Perhaps the RIAA and SDMI folks will now come out in favor of
> steganography in order to save their businesses.
> 
> Or maybe they be forced to rewrite their complicated protection schemes
> to enable "stego escrow", so that federal agents can monitor the secrets
> hidden inside published content, to make sure there aren't any hidden
> messages in Anthrax albums.

So I presume your discussion on the applicability of stego techniques
to the detection of unauthorised copying refers to the framework where
content is personalised by having something identifying the purchaser
encoded in it at time of delivery to the purchaser.

Steganography means hiding the existance of a message -- making it
hard to distinguish content without a stegotext from content with a
stegotext embedded in it.

Copymarks are about making it hard for the user to remove the message
without massively degrading the quality (*).  This means you want some
or all of the purchaser identifying information to be hard to locate
-- because once it is located it can be removed.

But watermarks don't have to be invisible -- just hard to remove
without degrading the image quality.  This tends to mean spread
spectrum techniques, and unpublished parameters of where the signal
will be stored so that there is no publicly constructable
discriminator, and no black-box discriminators queryable either.

However this framework inherently violates Kerchoff's principle.

Another framework is to have players which will only play content with
certified copy marks (no need for them to be visible -- they could be
encoded in a logo in the corner of the screen).  The copymark is a
signed hash of the content and the identity of the purchaser.

This could be relatively robust, except that usually there is also a
provision for non-certified content -- home movies etc -- and then the
copy mark can be removed while still playing by converting the content
into the home movie format, which won't and can't be certified.

Just to say that copymarks and steganography are related but different.

In my opinion copymarks are evil and doomed to fail technically.
There always need to be playble non-certified content, and current
generation watermarks seem easy to remove; and even if some really
good job of spread spectrum encoding were done, someone would reverse
engineer the players to extract the location parameters and then they
too would be removable -- and in the end even if someone did manage to
design a robust watermarking scheme respecting Kerchoff's principle,
the identity information is weakly authenticated, and subject to
identity theft or the content itself could be stolen or plausibly
deniably claimed to have been stolen and this only has to happen once
for each work.

All with no comments on the US Congress being careful of course --
they are ham-fisted at the best of times, and they have degraded far
beyond their normal state.

Adam

(*) This in itself is pretty hard -- reportedly stirmark [1] (a small
random shearing image transform) gets rid of all evaluated watermarks.

[1] Fabien A.P. Petitcolas, Ross J. Anderson, Markus G. Kuhn: "Attacks
on copyright marking systems Information Hiding", Second International
Workshop, IH'98

http://www.cl.cam.ac.uk/~mgk25/stirmark.html



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