Cryptography Reference
In-Depth Information
number of distinct watermarks requires a small distance be-
tween different watermarks. A small distance means that only
a small distortion could convert one watermark into another.
Watermark Size Howmany “bits” of information are available? Some
watermarks simply hide bits of information. Counting the
number of bits stored in the document is easy.
Other watermarking schemes don't hide bits per se. They add
distortions in such a way that the shape and location of the
distortions indicate who owns the document. Hiding lots of
informationmeans having many different and distinct patterns
of distortions. In some cases, packing many different patterns
is not easy because the size, shape and interaction with the
cover document are not easy to model or describe.
Blind Detection Somewatermarks require providing some extra data
to the detector. This might be the original unwatermarked im-
age or sound file, or it could be a key. The best solutions offer
blind detection , which provides as little information as possible
to the algorithm that looks for a watermark. The ideal detector
will examine the document, check for a watermark and then
enforce the restrictions carried by the watermark.
Blind detection is a requirement for many schemes for con-
tent protection. Providing a clean, unwatermarked copy to the
computers of the users defeats the purpose. But this doesn't
mean that nonblind schemes are worthless. Some imagine sit-
uations where the watermark is only extracted after the fact,
perhaps as evidence. One solution is to embedded the Id num-
beroftherightfulownerofadocumentinawatermark.Ifthe
document later appears in open circulation, perhaps on the In-
ternet, the owners could use a nonblind scheme to extract the
watermark and track down the source of the file. They could
still hold the original clean copy without releasing it.
Resistance to Multiple Watermarks Storing more hidden informa-
tion is one of the easiest attacks to launch against a document
withhidden information. Using the same algorithmoften guar-
antees that the same hidden spots will be altered to carry the
newmessage.
An ideal watermark will carry multiple messages from multi-
ple parties, who can insert their data and retrieve it without
any coordination. Some of these least significant bit schemes
from Chapter 9 offer this kind of resistance by using a key to
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