[13214] in cryptography@c2.net mail archive
Re: eWeek: Cryptography Guru Paul Kocher Speaks Out
daemon@ATHENA.MIT.EDU (Peter Wayner)
Tue May 6 12:13:26 2003
X-Original-To: cryptography@metzdowd.com
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In-Reply-To: <v03110708badc9a90b278@[192.168.1.5]>
Date: Mon, 5 May 2003 22:00:10 -0400
To: Bill Frantz <frantz@pwpconsult.com>, cryptography@metzdowd.com
From: Peter Wayner <pcw2@flyzone.com>
At 3:58 PM -0700 5/5/03, Bill Frantz wrote:
>It occurred to me that one way to attack a watermarking system that I
>haven't heard proposed before is instead of removing the watermark, bury it
>in other watermarks. While this approach would degrade the quality, it
>might not degrade it enough to bother the people who trade video camera
>pointed at the screen copies of movies.
Some watermarking systems deliberately avoid this problem. For
instance the early work (circa 95) of Ingemar Cox et al at NEC
described a system where multiple watermarks could be written without
overwriting each other. There were other limitations to the system,
but it's possible to build a system where the watermarks don't "write
over each other".
The easiest way to see this is to think about radio stations. As long
as they're broadcasting on different frequencies, they don't
interfere too much. If you put your watermark at one frequency, then
another frequency won't interfere.
The problem is that the detector generally needs to know the
"frequency". That means an attacker can often find it out and write a
new watermark at exactly that "frequency". So there's something to
your idea, it just requires a bit more work.
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