Watermarks - Disappearing Cryptography - page 317

choose where to hide the data. Many can carry multiple mes-
sages without any problem, especially if error correction han-
dles occasional collisions.
Unfortunately, these ideal solutions are often fragile and thus
undesirable for other reasons. This is another trade off. Lo-
calizing the information in the watermark reduces the chance
that another random watermark will alter or destroy it, but it
also increases the chance that a small change will ruin it.

Accuracy Many watermarking schemes achieve some robustness to
distortion by sacrificing accuracy. Many rely on finding the best
possible match and thus risk finding the wrong match if the
distortion is large enough. These algorithms sacrifice accuracy
in a strange way. Small changes still produce the right answer,
but large enough changes can produce a dramatically wrong
answer.

Fidelity One of the hardest effects of watermarks to measure is the
amount of distortion introduced by the watermarking process
itself. Invisible or inaudible distortions may be desirable, but
they're usually easy to defeat by compression algorithms that
strip away all of the unnecessary data.
The best schemes introduce distortions that are small enough
to be missed by most casual observers. These often succeed by
changing the relative strength or position of important details.
One classic solution is to alter the acoustics of the recording
room by subtly changing the echos. The ear usually doesn't
care if the echoes indicate a 8

20 room. At
some point, this approach fails and the art is finding the right
way to modulate the accoustics without disturbing the greatest
number of listeners.
Many of the wavelet-encoding techniques from Chapter 14
succeed by changing the relative strength of the largest coef-
ficients assembled to describe the image. The smallest coef-
ficients are easy to ignore or strip away, but the largest can't be
removedwithout distorting the image beyond recognition. The
solution is to change the relative strengths until they conform
to some pattern.

× 8 room or a 20 ×

Resistance to Framing One potential use for watermarks is to iden-
tify the rightful owner of each distinct copy. Someone who
mightwanttoleakorpiratethedocumentwilltrytoremove
that name. One of the easiest techniques is to buy multiple
copies and then average themtogether. If one pixel comes from