Cryptography Reference
In-Depth Information
avoid detection. Watermarks also try to hide— but usually to stay
out of the way, not to avoid being discovered. Most consumers and
pirates will know the watermark is there soon after they try to make
a copy. The real challenge is keeping the consumer or pirate from
making a copy and removing the watermark.
Digital Watermarking
by Ingemar J. Cox,
Matthew L. Miller, and
Jeffrey A. Bloom is a
good survey of a quickly
growing field.[CMB01]
This too is not easy. The ideal watermark will stick with a docu-
ment even after editing, cropping, compression, rotation, or any of
the basic forms of distortion. Alas, there are no ideal watermarks out
there to date, although many offer some form of resistance to basic
distortions.
Defending against basic copying is easy. A digital copy of a doc-
ument will be exact and carry any watermark along with it. But not
all copies are exact. Artists often crop or rotate an image. Compres-
sion algorithms for sound or image files add subtle distortions by re-
producing only the most significant parts of the information stream.
Pirates seek to reproduce all of the salient information while leaving
the hidden information behind. Defending against all of the possible
threatsispracticallyimpossible.
This shouldn't come as a surprise. Making a copy of a document
means duplicating all of the sensations detectable by a human. If the
sky is a clear, bright blue in the document, then it should be a clear,
bright blue in the copy as well. If a bell rings in the document, then
it should ring with close to the same timbre in the copy. But if some
part of a document can't be perceived, then there's no reason tomake
acopyofthatpart.
The watermark creator faces a tough conundrum. Very-well-
hidden information is imperceptable to humans and thus easy to
leave behind during copying. The best techniques for general steg-
anography are often untenable for watermarks. Compression al-
gorithms and nonexact copying solutions will strip the watermarks
away.
But information that's readily apparent to human eyes and ears
isn't artistically desirable. Distortions to the music and the images
can ruin them or scar them, especially in an industry that often
pushes the quality of the reproduction in marketing.
If the ideal watermark can't be created, there's no reason why a
practical one can't solve some of the problems. Adobe Photoshop, for
instance, comes with a tool for embedding a watermark designed by
Digimarc. The software can insert a numerical tag into a photo that
can then be used to find the rightful owner in a database. The solu-
tion that uses some of the wavelet-encoding techniques from Chap-
ter 14 can resist many basic distortions and changes introduced, per-
haps ironically, by Photoshop. The technique is not perfect, however,
