eWeek: Cryptography Guru Paul Kocher Speaks Out

Bill Frantz frantz at pwpconsult.com
Mon May 5 18:58:39 EDT 2003


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.

The system for watermarking audio I heard described at Stanford worked
approximately:

(1) Use the public key, and a hash of the 8.6 millisecond block to be
watermarked to produce a spreading code which determines which bits carry
the watermark.

(2) Use a psycho - acoustic model to determine where the watermark can be
placed without significantly degrading the S/N ration of the block.

(3) Use the secret key to "sign" the watermark.

(4) Add the watermark to the block and output it.

Note that to recover the watermark, you need an un-watermarked copy of the
block, as well as the public key.  The system described continued to
watermark every 8.6 ms block in the entire work.

Cheers - Bill


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