stego for the censored II

Sampo Syreeni decoy at iki.fi
Sun Feb 11 07:28:07 PST 2001


On Sat, 10 Feb 2001, Steve Schear wrote:

>Not entirely true.  If watermarking become ubiquitous then their presence
>is no longer suspicious.  If the marks contain encrypted content then
>almost no one will know or care as they are passed and published and only
>the intended recipients will receive the messages.

If stego is meant to be used between individuals, it's unlikely that a
watermark would be present in the content naturally - after all, the marks
are meant for CRM applications and individuals rarely take such good care
of their private communications. And as for watermarks on commercial
content, anyone caring about the presence of hidden communications can
obtain a licence so that the watermarks can be completely detected. If the
eavestropper is capable of full detection (not just statistical guesses),
commercial watermarks fail to provide a meaningful hidden channel. (I.e.
one would expect databases of valid marks to be available to an adversary
of means.)

In fact, many of the meaningful adversaries can perform a removal attack on
commercial watermarks, since the licences are available to corporations and
governments and, in the absence of workable asymmetric watermarks,
detection equals capability to remove. This means that one cannot
necessarily even use an existing watermark to hide another carrying your
message.

I think covert communication has very different needs from CRM. E.g.
requiring non-detectability means that you have to model the channel
statistics very closely, leading to a low bandwidth covert channel. CRM
does not suffer from this, since a watermark only needs to be perceptually
insignificant.

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





More information about the cypherpunks-legacy mailing list