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Watermarking applications and their properties
Ingemar J. Cox, Matt L. Miller and Jeffrey A. Bloom
Abstract Broadcast monitoring We describe a number of applications of digital water-
In 1997, a scandal broke out in Japan regarding televi-
marking and the examine the common properties of robust-
sion advertising. At least two stations had been routinely
ness, tamper resistance, fidelity, computational cost and
overbooking air time. Advertisers were paying for thou-
false positive rate. We observe that these properties vary
sands of commercials that were never aired [16]. The prac-
greatly depending on the application. Consequently, we
tice had remained largely undetected for over twenty years,
conclude that evaluation of a watermarking algorithm is
in part because there were no systems in place to monitor
difficult without first indicating the context in which it is to
the actual broadcast of advertisements.
There are several types of organizations and individuals
interested in broadcast monitoring. Advertisers, of course,want to ensure that they receive the air time purchased frombroadcasting firms. Musicians and actors want to ensure
1. Introduction
that they receive accurate royalty payments for broadcastsof their performances.1 And copyright owners want to en-sure that their property is not illegally rebroadcast by pirate
Watermarking methods are often evaluated based on the
common properties of robustness, tamper resistance, and fi-
We can use watermarks for broadcast monitoring by
delity. However, examination of these properties without
putting a unique watermark in each video or sound clip prior
careful consideration of the application can often be mis-
to broadcast. Automated monitoring stations can then re-
leading. A watermark designed to serve security needs of
ceive broadcasts and look for these watermarks, identifying
the CIA must meet different requirements than one intended
when and where each clip appears. Commercial systems
for annotating home video. Thus, it is inappropriate to eval-
have been deployed for a number of years and the basic
uate these two watermarks according to the same standards.
concepts have a long history [15, 3, 19, 11, 4]. 2
In this paper, we examine how the requirements for wa-
Owner identification
termarking can vary with application. Section 2 briefly de-scribes eight existing and proposed applications of water-
Although a copyright notice is no longer necessary to
marks. This is followed, in Section 3, with a discussion of
guarantee copy rights, it is still recommended. The form of
several properties of watermarking systems, and how their
the copyright notice is usually “ c date, owner”. On books
definition and importance depend on the application.
and photographs, the copyright is placed in plane sight. Inmovies, it is appended to the end of the credits. And onprerecorded music, it is placed on the packaging. Applications
One disadvantage of such text copyright notices is that
they can often be removed from the protected material.
This section describes seven applications of watermark-
1A recent spot-check by the Screen Actor’s Guild found an average of
ing: broadcast monitoring, owner identification, proof of
$1000 in underpaid royalties per hour of US television programming [2].
2The earliest reference we have found [15] is assigned to the Muzak
ownership, authentication, transactional watermarks, copy
Corporation, famous for providing “elevator music”, and may be the source
of the many rumors that Muzak contained subliminal messages.
Packaging can be lost, movies can have the credits cut off,
one pixel of the image is modified, it will no longer match
and images can be spatially cropped. A digital watermark
the signature, so any tampering can be detected. However,
can be used to provide complementary copyright marking
this signature is metadata that must be transmitted along
functionality because it becomes an integral part of the con-
with the photograph, perhaps in a header field of a partic-
tent, i.e. the copyright information is embedded in the mu-
ular image format. If the image is subsequently copied to
sic to supplement the text notice printed on the packaging.
another file format that does not contain this header field,
The Digimarc corporation has marketed a watermark-
the signature will be lost, and the image can no longer be
ing system designed for this application. Their watermark
embedder and detector are bundled with Adobe’s popular
A preferable solution is to embed the signature directly
image processing program, Photoshop. When the detector
into the image using watermarking. This eliminates the
finds a watermark, it contacts a central database to identify
problem of ensuring that the signature stays with the im-
the watermark’s owner (who must pay a fee to keep the in-
age. It also opens up the possibility that we can learn more
about what tampering has occurred, since any changes madeto the image will also be made to the watermark. Thus,
Proof of ownership
there are several systems that can indicate the rough loca-tion of changes that have been made to the image. There
Multimedia owners may want to use watermarks not just
are also systems designed to allow certain changes, such as
to identify copyright ownership, but to actually prove own-
JPEG compression [17, 18], and only disallow more sub-
ership. To illustrate the problem, let’s quickly introduce
stantial changes, such as removing an individual from a
some characters who are well known in the watermarking
literature. Suppose Alice creates an image and puts it onher website, with a copyright notice “ c Alice 2000”. Bob
then steals the image, uses an image processing program to
Transactional watermarks (Fingerprinting)
replace the copyright notice with “ c Bob 2000”, and then
claims to own the copyright himself. How can the disputeresolved?
Monitoring and owner identification applications place
Traditionally, Alice could register the image with the Of-
the same watermark in all copies of the same content. How-
fice of Copyrights and Patents by sending a copy to them.
ever, electronic distribution of content allows each copy dis-
The OCP archives the image, together with information
tributed to be customized for each recipient. This capability
about the rightful owner. When the dispute between Al-
allows a unique watermark to be embedded in each indi-
ice and Bob comes up, Alice contacts the OCP to obtain
vidual copy. Transactional watermarks, also called finger-
proof that she is the rightful owner. If Alice did not regis-
prints, allow a content owner or content distributor to iden-
ter the image, then she should at least be able to show the
tify the source of an illegal copy. This is potentially valuable
film negative. However, with the rapid acceptance of digital
both as a deterrent to illegal use and as a technological aid
photography, there might never have been a negative.
In theory, it is possible for Alice to use a watermark em-
One possible application of transactional watermarks is
bedded in the image to prove that she owns it. However,
in the distribution of movie dailies. During the course of
this is not a trivial problem, as Craver et al [10] have noted.
making a movie, the result of each day’s photography is of-ten distributed to a number of people involved in its produc-
Authentication
tion. These dailies are highly confidential, yet occasionally,a daily is leaked to the press. When this happens, studios
As both still and video cameras increasingly embrace
quickly try to identify the source of the leak. Clearly, if
digital technology, the ability for undetectable tampering
each copy of the daily contains a unique transactional wa-
also increases. The content of digital photographs can eas-
termark that identifies the recipient, then identification of
ily be altered in such a way that it is very difficult to detect
the source of the leak is much easier.
what has been changed. In this case there is not even an
Another application of transactional watermarks was de-
original negative to examine. There are many applications
ployed by the DiVX corporation. DiVX marketed a modi-
where the veracity of an image is crucial, especially in legal
fied version of DVD. One of the security measures imple-
mented in DiVX hardware was a transactional watermark
Authentication is a well studied problem in cryptography
that could be used to identify a player used for piracy. If
[22]. Friedman [12, 13] first discussed its application to cre-
illegal copies of a DiVX movie turned up on the black mar-
ate a “trustworthy camera” by computing a cryptographic
ket, DiVX could use the watermark to track them to the
signature that is associated with an image. If even one bit of
Copy Control
digital conversions and lossy compression. More recently,there has been an increased concern that video and still im-
Transactional watermarks as well as watermarks for
age watermarks also be robust to geometric transformations.
monitoring, identification, and proof of ownership do not
Robustness is often thought of as a single-dimensional
prevent illegal copying. Rather, they serve as powerful de-
value, but this is incorrect. A watermark that is robust
terrents and investigative tools. However, it is also possi-
against one process may be very fragile against another. In
ble for recording and playback devices to react to embed-
many applications, robustness to all possible processing is
ded signals. In this way, a recording device might inhibit
recording of a signal if it detects a watermark that indicates
Usually, a watermark must survive common signal pro-
recording is prohibited. Of course, for such a system to
cessing only between the time of embedding and the time
work, all manufactured recorders must include watermark
of detection. For example, in television and radio broadcast
detection circuitry. Such systems are currently being devel-
monitoring, the watermark need only survive the transmis-
oped for DVD video [6] and for digital music distribution
sion process. For television, this means lossy compression,
[5]. Interestingly, the use of watermarks in video to control
analog transmission, and some small amount of horizontal
equipment dates back to at least 1989 [7] and in audio to
and vertical translation. It need not survive rotation, scaling,
high-pass filtering, or any of a wide variety of distortionsthat do not occur during broadcast. Covert communication
In some cases, robustness may be completely irrelevant,
or even undesirable. Watermarks used for covert commu-
One of the earliest applications of watermarking, or more
nication need not be robust at all, if the cover media will
precisely, data hiding, is as a method of sending secret mes-
be transmitted digitally without compression. A watermark
sages. The application has been formulated by Simmons
for simple authentication, which just indicates whether the
[21] as “the prisoner’s problem”, in which we imagine two
media has been altered, should be fragile.
prisoners in separate cells trying to pass messages back and
On the other hand, when the signal processing between
forth. Their problem is that they cannot pass these messages
embedding and detection is unpredictable, the watermark
directly, but rather, must rely on the prison warden to act as
may need to be robust to every conceivable distortion. This
a messenger. The warden is willing to carry innocuous mes-
is the case for owner identification, proof of ownership, fin-
sages between them, but will punish them if he finds that,
gerprinting, and copy control. It is also true for any applica-
for example, their messages relate to a plan for escape. The
tion in which hackers might want to remove the watermark.
solution is to disguise the escape-plan messages by hidingthem in innocuous messages. There are several commer-
Tamper resistance
cially available programs designed for this application, in-cluding StegoTools [1].
Tamper resistance refers to a watermarking system’s re-
sistance to hostile attacks. There are several types of tam-
Properties
per resistance. Depending on the application, certain typesof attacks are more important than others. In fact, there are
There are a number of papers that have discussed the
several applications in which the watermark has no hostile
characteristics of watermarks [8, 20, 14, 24]. Some of the
enemies, and tamper resistance is irrelevant. Some basic
properties discussed are robustness, tamper resistance, fi-
delity, computational cost, and false positive rate. In prac-
Active attacks. Here the hacker tries to remove the wa-
tice, it is probably impossible to design a watermarking sys-
termark or make it undetectable. This type of attack is
tem that excels at all of these. Thus, it is necessary to make
critical for many applications, including owner identi-
tradeoffs between them, and those tradeoffs must be cho-
fication, proof of ownership, fingerprinting, and copy
sen with careful analysis of the application. In addition, the
control, in which the purpose of the mark is defeated
application can affect the very definition of a property.
when it cannot be detected [9]. However, it is not a
In the following subsections, we look at each of the five
serious problem for authentication or covert commu-
properties listed above, and discuss how its importance and
Passive attacks. In this case, the hacker is not trying
Robustness
to remove the watermark, but is simply trying to deter-mine whether a mark is present. In most applications,
A watermark is said to be robust if it survives common
we are not concerned with this type of attack. In fact,
signal processing operations such as digital-to-analog-to-
we might even advertise the presence of the mark so
that it can serve as a deterrent. But in the case of covert
A small visible distortion caused by a watermark will not
communication, our primary interest is in preventing
Computational cost Collusion attacks. These are a special case of active
attacks, in which the hacker uses several copies of one
Different applications require the embedders and detec-
piece of media, each with a different watermark, to
tors to work at different speeds. In broadcast monitoring,
construct a copy with no watermark. Resistance to col-
both embedders and detectors must work in (at least) real
lusion attacks can be critical in a fingerprinting appli-
time. The embedders must not slow down the media pro-
cation, which entails putting a different mark in each
duction schedule, and the detectors must keep up with real-
copy of a piece of media. However, the number of
time broadcasts. On the other hand, a detector for proof of
copies that we can expect the hacker to obtain varies
ownership will be valuable even if it takes days to find a wa-
greatly from application to application. For example,
termark. Such a detector will only be used during ownership
in the DiVX application, a hacker can buy any number
disputes, which are rare, and its conclusion about whether
of DiVX players, and play one movie on all of them to
the watermark is present is important enough that the user
obtain any number of differently-watermarked copies.
On the other hand, in the film-studio dailies applica-
Furthermore, different applications require different
tion, each employee can only obtain one copy of the
numbers of embedders and detectors. Broadcast monitor-
watermarked material. A collusion attack would re-
ing typically requires a few embedders and perhaps several
quire that several employees conspire to steal the ma-
hundred detectors at different geographic locations. Copy
terial, which is an unlikely prospect.
control applications may need only a handful of embeddersbut millions of detectors. Conversely, in the fingerprinting
Forgery attacks. Here, the hacker tries to embed a
application implemented by DiVX, in which each player
valid watermark, rather than remove one. These are
embeds a distinct watermark, there would be millions of
our main security concern in authentication applica-
embedders and only a handful of detectors. In general, the
tions, since, if hackers can embed valid authentication
more numerous a device needs to be for a given application,
marks, they can cause the watermark detector to ac-
cept bogus or modified media. In addition, as pointed
The wide variation in dollar cost and in speed require-
out by Craver et al [10], this type of attack is a serious
ments means that there is a wide variation in the required
computational efficiency of watermark embedders and de-tectors. Fidelity False positive rate
A watermark is said to have high fidelity if the degreda-
tion it causes is very difficult for a viewer to perceive. How-
A false positive is a detection of a watermark in a piece of
ever, it only needs to be imperceptible at the time that the
media that does not actually contain that watermark. When
media is viewed. If we can be certain that the media will be
we talk of the false positive rate, we refer to the number of
seriously degraded before it is viewed, we can rely on that
false positives we expect to occur in a given number of runs
degradation to help mask the watermark. Such a case oc-
of the detector. Equivalently, we can discuss the probabil-
curs when we watermark video that will be transmitted over
ity that a false positive will occur in any given detector run.
NTSC, or audio that will be transmitted over AM radio. The
There are two subtly different ways to define this probabil-
quality of these broadcast technologies is so low that our
ity, that are often confused in the literature. They differ in
initial fidelity need not be very good. Conversely, in HDTV
whether the watermark or the media is considered to be the
and DVD video and audio, the signals are very high qual-
ity, and require much higher fidelity watermarks (though, of
In the first definition, the probability of a false positive
course, the quality of the content remains the same - a bad
is the probability that, given a fixed piece of media and a
movie is a bad movie whether on VHS or DVD).
randomly-selected watermark, the detector will report that
In some applications, we can accept mildly perceptible
the watermark is in the media. The watermarks are drawn
watermarks in exchange for higher robustness or lower cost.
from a distribution that is defined by the design of a water-
For example, Hollywood dailies are not finished products.
mark generation system. Typically, watermarks are gener-
They are usually the results of poor transfers from film to
ated by either a bit-encoding algorithm or by a Gaussian, in-
video. Their only purpose is to show those involved in a
dependent random number generator. In many cases, prob-
film production the raw material that has been shot so far.
ability of false positives, according to this first definition,
is actually independent of the piece of media, and depends
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