Digital watermarks are catching on quickly as
digital artists and publishers begin to post and
distribute their copyright works online. Watermarks
provide a cheap means of copyrighting information
and a way to notify would-be users how to contact
the copyright owner to gain permission for use and
pay copyright fees. It's the way to go if a
copyright holder who needs to distribute works
without fear of losing major revenue to unauthorized
use.
Digital watermark technology still has a long way to go, however.I say this
because while watermarks digitally copyright online materials, they are by no
means foolproof, nor do they prevent an unauthorized user intent on reproducing
the work.
A digital watermark is similar in function to a traditional watermark. It
identifies a person's stationery so a digital watermark underlying a graphic,
photo, or document identifies the creator or owner of a certain work. Some forms
of watermarking are referred to as signatures, or fingerprints. I was taught
there is a functional distinction between watermarks and fingerprints. But be
aware that digital watermarking and fingerprinting are often used
interchangeably.
A watermark identifies the copyright owner and even relays permission
information, a fingerprint is essentially a recorded digital imprint left by the
user. An intelligent fingerprint is a telltale script of the violator's location
and use(s) of the copyrighted work. To enforce copyright protection, you just
follow the fingerprint trail. A publisher "reads" the fingerprints to determine
who used the work, how it was used, and how many times it was used. Many
watermarking software makers are now offering integrated intelligent
fingerprinting features.
The more rudimentary watermarking techniques require the original work with
which to compare the digital watermark or the coded value (checksum) used in
algorithm-based encoding. The more advanced techniques highlighted below have
been developed to eliminate the use of an original to extract the watermarked
message.
Spread-spectrum watermarking and steganography are the most common forms of
digital watermarking today. Steganography encodes, or hides, coded sequences
within the binary file of the image, video, or audio. A classic example of
steganography is writing a secret message with invisible ink between the lines
of a letter. Likewise, digital steganography hides a message in a computer image
or digital audio clip. Digital watermarks can be uniquely created so that each
watermarked image persists with each digital copy made, and persistent watermark
reproduction is key to deterring digital reproduction. The encoded watermark is
often referred to a work's digital signature.
The most available form of watermarking is based on spread spectrum
communications techniques. The watermark information is hidden using any
modulation scheme (any method of communicating over a continuous channel that
works at low signal-to-noise ratios). MIT Media Lab and Dice Company watermarks
are prime examples of how spread spectrum-based–watermarks are created via the
direct-sequencing technique. Think of the original image as the noise and the
hidden message as the signal. In the direct-sequence method, the hidden message
chooses a key and uses it to generate a pseudorandom carrier function that is
then modulated by the information to be encoded, and added to the original
image. To extract the information, an image is demodulated with the carrier
generated by the original key.
MIT Media Lab's PixelTag media watermark encodes bits of copyright information
in the pixel brightness values, rather than the image itself. The benefit of the
PixelTag technique is that the watermark is retained despite changes in format
or digital-to-analog conversion such as what occurs in printing. Another key
feature is the original image is not required to extract the watermark
information, enabling the use of a Web crawler to systematically search for
illegal copies of images. The decoding process reveals an actual message (e.g.
author's email address) instead of a checksum.
On the other hand, Dice Company's Argent refers to its watermarks as digital
signatures, but the process of embedding one into a digital sample stream is not
accomplished via a digital signature calculation. These signatures are single,
continuously integrated numbers, or messages, over a large area of the carrier
signal. Dice's definition of watermark is "a continuous integration of...many
repetitions of an informational message over arbitrary carrier signal areas."
The information encoded by Argent can be digitally signed to certify its
validity when it is extracted. Argent watermarks are removed with an authorized
key. Without a key, removal will damage the content.
Two well-known developers in the field of digital rights management, Dr. Jian
Zhoa and R. Eckhard Koch, employ a pseudorandom position sequence embedded
watermark using their new SysCop system. SysCop has a two-step process to embed
and retrieve copyright labels. The first step is to generate a pseudorandom
position sequence for selecting blocks where the code is embedded, using
extracted image data together with a user-supplied key. This step produces the
actual copyright code and a random sequence of locations for code embedding. The
second step embeds the code at specified locations, using a set of embedding
methods dependent on the type or even the content of the multimedia materials.
It also retrieves the code from the blocks specified in the position sequence
using different watermarking methods.
The embedded copyright label is reportedly tamperproof and does not reduce the
quality of the multimedia data. The method is said to resist damage from data
compression, low pass filtering and file format conversion. A Web interface to
the SysCop has been developed to provide Web users with services to mark their
materials.
(Digimarc is one of the better known watermarks as it is available as an Adobe
Photoshop plug-in.) A Digimarc watermark placed throughout the image is created
by imitating naturally occurring image variations. To further hide the
watermark, the Digimarc automatically varies the intensity of the watermark in
order to remain invisible in both flat and detailed areas of an image. The
Digimarc watermark contains both ownership and usage permission information. To
read the information, the user must have a Digimarc reader, also bundled with
tools like Adobe Photoshop.
Although the watermark is embedded digitally within the image, it remains part
of the image even when printed and can be read later by scanning the image into
a computer. A Digimarc watermark is reportedly retained through copying,
editing, and most file format conversions.
Highwater's FBI fingerprints which uses FBI Write and FBI Detector is another
Adobe Photoshop watermark plug-in. The tools apply and detect FBI fingerprints.
The fingerprint does not increase the image file size, nor degrade the
reproducible quality. FBI offers two key advantages. It can detect when a part
of a protected image has been used, such as in a photo montage. Most
importantly, FBI fingerprints can be detected in printed output, giving owners
an extra level of copyright control.
Watermarking alone does not deter reproduction. The watermarks need to be
combined with other technical mechanisms (e.g. cryptographic containers) to
physically prohibit the reproduction of works. Optical character recognition
(OCR) processing software used in scanners raises some technical challenges to
preserving watermarks. Intelligent page analysis software is needed to correctly
identify and reproduce the watermark's unique signature or line spacing when OCR
processing. Otherwise, the watermark becomes scrambled or destroyed.
Background watermarks also can be destroyed by the same tools used to clean up
water stains and other discolorations on scanned documents. Watermarking does
not restrict users from copying or printing. If the rights holder can restrict
printing a document or image, then he or she is assured that the secured
materials cannot be reproduced beyond the desktop.
The most obvious solution is to integrate watermarks into cryptographic
containers, as does IBM Cryptolope. While this is the best and most effective
means of displaying and enforcing copyright information, it is not a
cost-effective solution for the free agent. It depends on what a copyright owner
can afford to lose through unenforced copyright reproduction. Hey, it's a risky
business.