Fabien
A. P. Petitcolas, Microsoft Research
Digital watermarking has been presented as a solution to copy
protection of multimedia objects and dozens of schemes and algorithms
have been proposed. Two main problems seriously darken the future of
this technology though.
Firstly, the large number of attacks and weaknesses which
appear as fast as new algorithms are proposed, emphasizes the limits of
this technology and in particu-lar the fact that it may not match users
expectations.
Secondly, the requirements, tools and methodologies to assess
the current technologies are almost non-existent. The lack of
benchmarking of current algorithms is bla-tant. This confuses rights
holders as well as software and hardware manufacturers and prevents them
from using the solution appropriate to their needs. Indeed basing
long-lived protection schemes on badly tested watermarking technology
does not make sense.
In this paper we will discuss how one could solve the second
problem by having a public benchmarking ser-vice. We will examine the
challenges behind such a service.
I. INTRODUCTION
Digital watermarking remains a largely `untested field and only very
few large industrial consortiums have published requirements against
which watermarking algorithms should be tested [1,2]. For instance the
International Federation for the Phonographic Industry led one of the
first large scale comparative testing of watermarking algorithm for
audio. In general, a number of broad claims have been made about the
�robustness� of various digital watermarking or
fingerprinting methods but very few researchers or companies have
published extensive tests on their systems.
The growing number of attacks against watermarking systems (e.g., [3,
4, 5]) has shown that far more research is required to improve the
quality of existing watermarking methods so that, for instance, the
coming JPEG 2000 (and new multimedia standards) can be more widely used
within electronic commerce applications.
We already pointed out in [6] that most papers have used their own
limited series of tests, their own pictures and their own methodology
and that consequently comparison was impossible without re-implementing
the method and trying to test them separately. But then, the
implementation might be very different and probably weaker than the one
of the original authors. This led to suggest that methodologies for
evaluating existing watermarking algorithms were urgently required and
we proposed a simple benchmark for still image marking algorithms.
With a common benchmark authors and watermarking software providers
would just need to provide a more or less detailed table of results,
which would give a good and reliable summary of the performances of the
proposed scheme. So end users can check whether their basic require-ments
are satisfied, researchers can compare different algorithms and see how
a method can be improved or whether a newly added feature actu-ally
improves the reliability of the whole method and the industry can
properly evaluate risks associated to the use of a particular solution
by knowing which level of reliability can be achieved by each contender.
Watermarking system designers can also use such evaluation to identify
possible weak points during the early development phase of the system.
Evaluation per see is not a new problem and significant work has been
done to evaluate, for instance, image compression algorithms or security
of information systems [7] and we believe that some of it may be re-used
for watermarking.
In section II will explain what is the scope of the evaluation we
envisage. Section III will review the type of watermarking schemes that
an automated evaluation service could deal with. In section IV we will
review what are the basic functionalities that need to be evaluated.
Section V will examine how each functionality can be tested. Finally,
section VI will argue the need for a third party evaluation service and
briefly sketch its architecture.
II. SCOPE OF THE EVALUATION
Watermarking algorithms are often used in larger system designed to
achieve certain goals (e.g., prevention of illegal copying). For
instance Herrigel et al. [ ] presented a system for trading images; this
system uses watermarking technologies but relies heavily on
cryptographic protocols.
Such systems may be flawed for other reasons than watermarking
itself; for instance the protocol, which uses the watermark , may be
wrong or the random number generator used by the watermark embedder may
not be good. In this paper we are only concerned with the evaluation of
watermarking (so the signal processing aspects) within the larger system
not the effectiveness of the full system to achieve its goals.
III. TARGET OF EVALUATION
The first step in the evaluation process is to clearly identify the
target of evaluation, that is the watermarking scheme (set of algorithms
required for embedding and extraction) subject to evaluation and its
purpose. The purpose of a scheme is defined by one or more objectives
and an operational environment. For instance, we may wish to evaluate a
watermarking scheme that allows automatic monitoring of audio tracks
broadcast over radio.
Typical objectives found across the watermarking and copy protection
literature include:
- Persistent identification of audio-visual signals: the mark
carries a unique identification number (similar to an I.S.B.N.),
which can be used as a pointer in a database. This gives the ability
to manage the association of digital content with its related
descriptive data, current rights holders, license conditions and
enforcement mechanisms. This objective is quite general as it may
wrap many other objectives described below. However one may wish to
have the data related to the work stored into the work itself rather
than into a central database in order to avoid connection to a
remote server.
- Proof of creatorship, proof of ownership: the embedded mark is be
used to prove to a court who is the creator or the right holder of
the work;
- Auditing: the mark carries information used to identify parties
present in a transaction involving the work (the distributors and
the end users). This audit trail shows the transfer of work between
parties. Marks for identifying users are usually referred to as
fingerprints;
- Copy-control marking: the mark carries information regarding the
number of copies allowed. Such marks are used in the digital
versatile disk copy protection mechanisms. In this system a work can
be copied, copied once only or never copied [ ].
- Monitoring of multimedia object usage: monitoring copyright
liability can be achieved by embedding a license number into the
work and having, for instance, an automated service constantly
crawling the web or listening to the radio, checking the licensing
and reporting infringement.
- Tamper evidence: special marks can be used in a way that allows
detection of modifications introduced after the mark has been added.
- Labeling for user awareness: this type of marks are typically
used by right holders to warn end users that the work they
�have in hands� is copyrighted. For instance, whenever
an end user tries to save a copyrighted image opened in a web
browser or an image editor, he may get a warning encouraging him to
purchase a license for the work.
- Data augmentation: this is not really in the scope of
�digital watermarking� but a similar evaluation
methodology can be applied to it.
- Labeling to speed up search in databases.
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