EURASIP Journal on Applied Signal Processing 2002:2, 126–132
`© 2002 Hindawi Publishing Corporation
`
`The First 50 Years of Electronic Watermarking
`
`Ingemar J. Cox
`
`NEC Research Institute, 4 Independence Way, Princeton, NJ 08540, USA
`Email: ingemar@research.nj.nec.com
`
`Matt L. Miller
`
`NEC Research Institute, 4 Independence Way, Princeton, NJ 08540, USA
`Email: mlm@research.nj.nec.com
`
`Received 8 October 2001 and in revised form 28 October 2001
`
`Electronic watermarking can be traced back as far as 1954. The last 10 years has seen considerable interest in digital watermarking,
`due, in large part, to concerns about illegal piracy of copyrighted content. In this paper, we consider the following questions:
`is the interest warranted? What are the commercial applications of the technology? What scientific progress has been made in
`the last 10 years? What are the most exciting areas for research? And where might the next 10 years take us? In our opinion, the
`interest in watermarking is appropriate. However, we expect that copyright applications will be overshadowed by applications
`such as broadcast monitoring, authentication, and tracking content distributed within corporations. We further see a variety of
`applications emerging that add value to media, such as annotation and linking content to the Web. These latter applications
`may turn out to be the most compelling. Considerable progress has been made toward enabling these applications—perceptual
`modelling, security threats and countermeasures, and the development of a bag of tricks for efficient implementations. Further
`progress is needed in methods for handling geometric and temporal distortions. We expect other exciting developments to arise
`from research in informed watermarking.
`
`Keywords and phrases: digital watermarking, data hiding, steganography.
`
`1.
`
`INTRODUCTION
`
`In 1954, Emil Hembrooke of the Muzac Corporation filed a
`patent entitled “Identification of sound and like signals” [1]
`in which is described a method for imperceptibly embedding
`an identification code into music for the purpose of proving
`ownership. The patent states “The present invention makes
`possible the positive identification of the origin of a musical
`presentation and thereby constitutes an effective means of
`preventing such piracy, that is, it can be likened to a water-
`mark in paper.” Electronic watermarking had been invented!1
`Since that time, a number of watermarking technologies
`have been developed and deployed for a variety of applica-
`tions. Interest in embedded signaling continued throughout
`the next 35 years. For example, systems were developed for
`advertisement verification and device control both of which
`are discussed in the next section. However, electronic water-
`
`1To the best of our knowledge, this is the earliest reference to electronic
`watermarking. We do cite a patent dated 1953 [2] later as an example of
`device control. However, the patent description is ambiguous as to whether
`this is really watermarking or not. If readers are aware of earlier technology,
`please let us know.
`
`marking (particularly digital watermarking) did not receive
`substantial interest as a research topic until the 1990’s. In
`the first half of that decade, interest in the topic expanded
`rapidly and today entire conference proceedings are devoted
`to the subject.
`This increase in interest was motivated by copyright con-
`cerns that became acute with advances in computer tech-
`nology and the development of the Web. These technologies
`enable the perfect copying and distribution of copyrighted
`material to almost anywhere in the world at almost no cost.
`To address these concerns, a number of industry technology
`groups were established, perhaps the best known being the
`Copy Protection Technical Working Group (CPTWG) and
`the Strategic Digital Music Initiative (SDMI). The former is
`concerned with digital video content stored on DVD discs
`and the latter with digital music.
`These industry groups recognized that cryptography can
`only protect the distribution of content and that once a cus-
`tomer decrypts it, all protection is lost. Watermarking can
`complement cryptography, providing protection after de-
`cryption, even after the content has entered the analog world.
`Nevertheless, initial expectations of watermarking were prob-
`ably too high, particularly with respect to intentional efforts
`
`Sony Exhibit 1039
`Sony v. MZ Audio
`
`

`

`The First 50 Years of Electronic Watermarking
`
`127
`
`to remove a watermark from content. However, subsequent
`responses to requests for proposals met specifications and the
`slow adoption of the technology is, in our opinion, due pri-
`marily to the diverging business interests of the three indus-
`try groups—content owners and manufacturers of consumer
`electronic equipment and computers—that must reach a con-
`census.
`This leads us to the main question of the present paper: is
`the current business and academic interest in watermarking
`warranted?
`From a business perspective, the question is whether wa-
`termarking can provide economic solutions to real problems.
`Current business interest is focused on a number of appli-
`cations that broadly fall into the categories of security and
`device control. From a security perspective, there has been
`criticism that many proposed watermark security solutions
`are “weak,” that is, it is relatively straightforward to circum-
`vent the security system. While this is true, there are many
`business applications where “weak” security is preferable to
`no security. We therefore expect that businesses will deploy
`a number of security applications based on watermarking.
`In addition, many device control applications have no secu-
`rity requirement, since there is no motivation to remove the
`watermark. Device control, particularly as it pertains to the
`linking of traditional media to the Web, is receiving increased
`attention from businesses and we expect that this interest will
`increase. Business usages of watermarking are discussed in
`more detail in Section 2.
`From an academic perspective, the question is whether
`watermarking introduces new and interesting problems for
`basic and applied research. Watermarking is an interdis-
`ciplinary study that draws experts from communications,
`cryptography and audio and image processing. Interesting
`new problems have been posed in each of these disciplines
`based on the unique requirements of watermarking applica-
`tions. Commercial implementations of watermarking must
`meet difficult and often conflicting economic and engineer-
`ing constraints. These problems are addressed in more detail
`in Section 3.
`Our opinion is that current interest in watermarking is
`warranted, although expectations in the early 1990’s were of-
`ten too high. This raises the final question of this paper: what
`are the most exciting areas for research and where might the
`next 10 years take us? We address these questions in Section 4.
`
`2. COMMERCIAL APPLICATIONS
`
`Is watermarking important commercially? To answer this
`question, we begin by noting that a number of companies
`have employed watermarking for several years—decades in
`some cases. We regard this as empirical evidence that wa-
`termarking is, indeed, commercially viable. We then address
`the question more analytically, examining the practicality of
`some proposed watermarking applications, in light of current
`research in the field. We conclude that, although businesses
`may need to lower their expectations of performance, water-
`marks can serve most of these functions economically.
`
`2.1. Early uses of watermarking
`
`The applications of watermarking are well known and can be
`broadly classified as copyright control (owner identification,
`proof of ownership, transaction tracking, and copy control)
`broadcast monitoring and device control. What is less well-
`known is that watermarks have been deployed for some of
`these applications for several decades.
`Owner identification appears to have been pioneered by
`the Muzak Corporation [1]. Their system, which used a notch
`filter to block, with varying duration, the audio signal at
`1 kHz, encoded identification information using Morse code.
`The system remained in use until the early 1980’s, when a
`change in Muzak’s business model ended their interest in
`identifying music they owned [3]. More recently, each DiVX
`DVD player manufactured by the now defunct DiVX Corpo-
`ration, contained watermark embedding circuitry that sup-
`ported transaction tracking that was intended to deter piracy.
`Advertisement monitoring and audience measurement
`companies have also used embedded signaling for some time.
`Both Nielsen Media Research, now part of VNU, and Com-
`petitive Media Reporting (CMR), now part of Taylor Nelson
`Sofres, employ watermarking to provide advertisement veri-
`fication services and these systems have probably been in use
`for about 10 years. More recently, Verance Corporation has
`introduced a service to monitor television and radio broad-
`cast media using their audio watermarking technology.
`A number of companies have experimented with embed-
`ded signalling for device control purposes. In a 1962 patent
`assigned to Lynch Carrier Systems Inc., Noller [4] described
`a “inband signalling system” designed to control telephony
`equipment. In an even earlier patent assigned to Musicast
`Inc., Tomberlin et al. [2] proposed to distribute music to
`businesses by partnering with existing radio broadcasters.
`Their patent describes embedding a low frequency 30 Hz
`control signal at the point of transmission which will allow
`receivers to remove advertisements. Baer of the Sanders As-
`sociates Inc. was issued a patent in 1976 [5] for a video wa-
`termark intended for interactive television applications. In
`a 1981 patent assigned to Dolby Labs [6], Dolby describes
`“A sub-audible in-band tone system …for identifying an FM
`stereophonic radio broadcast which is specially encoded, as
`with dynamic range improvement encoding or quadraphonic
`encoding, …[and] which can control a visual display and
`switch in appropriate signal decoding circuitry when the tone
`is detected.” A few years later, in 1989, Interactive Systems
`Inc. was awarded a patent [7] for a “Method and appara-
`tus for in-band, video broadcasting of commands to interac-
`tive devices.” An early application of this technology was in
`the synchronization of children’s toys with live-broadcast or
`recorded video. Interactive Systems, has since become VEIL2
`Interactive Technologies. This company offers watermarking
`solutions for a number of different applications including in-
`teractive television, interactive toys, and advertisement mon-
`itoring.
`
`2VEIL stands for Video Encoded Invisible Light.
`
`

`

`128
`
`EURASIP Journal on Applied Signal Processing
`
`2.2. Potential applications
`
`In addition to the ongoing watermarking activities of Nielsen,
`CMR, and VEIL Interactive Systems, several new applications
`have sprung up in the 1990’s. Whether these applications
`prove economically viable remains to be seen, but we can
`offer some educated guesses based on how well the current
`state of technology satisfies the applications’ requirements.
`We discuss, in turn, the applications of transaction tracking
`(also known as fingerprinting), proof of ownership, copy con-
`trol, legacy system enhancement, and a range of applications
`we refer to broadly as database linking.
`
`2.2.1 Transaction tracking
`
`In transaction tracking, or fingerprinting, a unique water-
`mark is embedded into each copy of a Work. Typically, the
`watermark identifies the legal recipient of the copy, and can
`be used to trace the source of illegally redistributed content.
`Large-scale use of watermarks for transaction tracking, such
`as that implemented by DiVX,3 is known to be vulnerable to
`collusion attacks, which usually require fewer than 20 copies
`to be effective. In such an attack, an adversary obtains sev-
`eral copies of a single Work, each with a different watermark,
`and uses them to obtain an approximation of the original,
`unwatermarked Work. Most existing or envisioned water-
`marks can be removed using fewer than 20 copies [8, 9, 10].
`Thus, an adversary with 20 DiVX DVD players could produce
`watermark-free copies. Nevertheless, the system might still be
`worthwhile, since it would catch adversaries who lacked the
`dilligence or knowledge to perform these attacks, and this
`might prevent enough piracy to justify the system’s cost.
`On the other hand, smaller-scale transaction tracking ap-
`plications, in which collusion attacks are unlikely, can prob-
`ably be implemented with a very high degree of security. For
`example, if a Hollywood studio wishes to distribute movie
`dailies to a few key personnel, it is extremely unlikely that
`even two executives would collude in leaking these movie
`clips to the press. By using the original clip during the de-
`tection process (informed detection), a studio could design a
`watermark that is very difficult to remove.
`
`2.2.2 Proof of ownership
`Muzak’s original interest in watermarking was to distinguish
`between theirs and similar recordings. The most ambitious
`form of such an application, which has received much atten-
`tion in the watermarking literature, is the use of watermarks
`to actually prove ownership in a court of law.
`In 1996, Craver et al. pointed out that there is an inherent
`problem in using watermarks for proof of ownership [11].
`Specifically, with many watermarking methods, it is possi-
`ble for adversaries to make it appear as though all distributed
`
`copies of a Work contain their watermarks, even though those
`marks were never actually embedded. However, the original
`paper suggested a solution to this problem, involving a cryp-
`tographic link between the watermark and the original Work,
`and we have seen no weakness in this solution. We therefore
`believe that, with a properly designed system, it is technically
`possible to prove ownership with watermarks. Business and
`legal issues appear to be the only hurdles to adoption of such
`a technology.
`
`2.2.3 Copy control
`If every recording device contained a watermark detector,
`watermarks could be used to prevent copying of copyrighted
`material. Watermarking for copy-control has been the subject
`of much R&D effort through the latter half of the 90’s.4
`There are two main areas of difficulty in implementing a
`watermarking copy-control system—one technical, the other
`political. The technical problem is that everyone must be able
`to detect the watermarks and within this context, current
`technology can only provide weak security.5 Nevertheless
`weak protection against copying can still be economic. For
`example, it is very easy to circumvent the Macrovision sys-
`tem for preventing copying on VHS tapes (which is not based
`on watermarking). In fact, several legitimate pieces of video
`equipment remove Macrovision protection as a side effect.
`But Macrovision still prevents a great deal of casual copying,
`and studios have continued using it for several years. Thus,
`even if the copy protection provided by watermarking is weak,
`it may still be worthwhile.
`The more serious problem in implementing a watermark-
`ing copy-control system is the political problem of persuad-
`ing manufacturers to include watermark detectors in their
`recording devices. These detectors add cost yet do not neces-
`sarily add any value to the equipment. In fact, they reduce the
`value, since many consumers would like to be able to make
`illegal recordings. Thus, equipment manufacturers must be
`forced to include detectors, by a combination of laws and
`contractual obligations. The political wrangling that results,
`together with conflicts over patent rights, are probably greater
`impediments to the deployment of these systems than any
`technical problems.
`
`2.2.4 Authentication
`Authentication is well understood. A digital signature can be
`embedded as a watermark in a Work. And in fact, Epson offers
`a camera systems that does just this. An advantage of this
`arrangement is for legacy systems. There has been concern
`because embedding a signature alters the Work. However,
`the recent introduction of erasible watermarks [14] should
`dispell this concern.
`
`3Perhaps the most ambitious implementation of transaction tracking was
`deployed by the DiVX Corporation in the late 1990’s. Each DiVX-enabled
`DVD player embedded a unique watermark into video that it played. If the
`video was subsequently pirated and redistributed, the DiVX Corporation
`could use the watermark to identify the exact player used, and, thereby iden-
`tify the source of the pirated Work.
`
`4As noted earlier, two on-going, high-profile projects to deploy such copy-
`control systems have been undertaken. The Copy Protection Technical Work-
`ing Group (CPTWG) has worked on a system for protecting video on DVD
`since 1995, and the Secure Digital Music Initiative (SDMI) has worked on an
`audio system since 1999.
`5This is because the general availability of detectors permits adversaries
`to apply a sensitivity attack [12, 13].
`
`

`

`The First 50 Years of Electronic Watermarking
`
`129
`
`2.2.5 Legacy system enhancement
`and database linking
`
`Watermarking may also play a valuable role in enhancing the
`functionality of legacy systems while maintaining compati-
`bility with deployed devices. For example, Schreiber et al. [15]
`proposed “a compatible high-definition television system us-
`ing the noise-margin method of hiding enhancement infor-
`mation.” Although this high-definition television system was
`not adopted, similar proposals have more recently been made
`for digital radio [16, 17].
`Recently, Digimarc has pioneered a class of device control
`applications that link traditional print media to associated
`websites in a product called MediaBridge. Philips has also
`demonstrated an audio watermarking technology for music
`[18]. When the music is played, the audible signal can be
`digitized using the microphone present in many PDA’s and
`the PDA can decode the watermark and thereby identify-
`ing the song. If the PDA has a wireless Web connection, it
`can then link the song to an associated site that, for exam-
`ple, may provide additional information or offer the song
`for purchase. Similar technology has also been demonstrated
`by Microsoft [19].
`Work in these areas is still in its infancy. However, it is
`expected that security will not become an issue. Rather, ro-
`bustness, fidelity, and payload requirements are the key issues
`and we believe that these requirements can or will be met.
`The 1990’s also say a new business development, the cre-
`ation of companies that promoted watermarking as their core
`competence. This is as opposed to previous companies who
`exploited watermarking technology but promoted a prod-
`uct or service that watermarking was a part of. Whether
`these companies remain focused on developing the core wa-
`termarking technology or ultimately develop an application
`market, it is clear that watermarking has, is and will continue
`to be used.
`
`3. RESEARCH PROGRESS
`
`Is watermarking a worthwhile topic of research? To answer
`this, we need to ask whether watermarking is leading to in-
`teresting problems in basic research and whether engineering
`progress is leading to practical solutions.
`
`3.1. Basic research
`Very early work on watermarking was essentially heuristic, in
`part, because watermarking was not recognized as a distinct
`technology. This began to change in the late 1980’s and early
`1990’s when a number of published papers described a va-
`riety of different watermarking algorithms. A more rigorous
`understanding of watermarking then began to be developed,
`beginning in the mid-1990’s.
`Perhaps the most significant progress has been in the
`development of increasingly sophisticated models of water-
`marking. In the early 1990’s it became common to model wa-
`termarking as a communications channel in which the cover
`Work and any subsequent distortions between the time of em-
`bedding and detection were treated as noise. The constraint
`
`of imperceptibility was met by imposing a global power con-
`straint at the embedder.
`In these early systems the added watermark signal is in-
`dependent of the cover Work and we refer to this as blind
`embedding. Similarly, blind detection refers to the detection
`of a watermark signal in a cover Work, the detection being
`independent of the unwatermarked Work.
`In 1999, contemporaneous results from [20, 21, 22] rec-
`ognized that watermarking is more accurately modelled as
`communications with side information [23]. The resulting
`watermark algorithms are referred to as informed embedding
`and/or informed encoding [24]. This is because the added
`watermark pattern is a function of the cover Work.
`This model was further refined with the introduction in
`[25, 26, 27] of Costa’s paper, “Writing on Dirty Paper” [28] to
`the watermarking community. Costa examined the capacity
`of a channel with two additive white Gaussian noise sources,
`the first of which is known. In Costa’s analogy, the first noise
`source represent dirty paper. The watermark embedder writes
`a message on the dirty paper using only a limited quantity
`of ink. Then, during transmission, more unknown noise is
`added to the paper before its receipt at the detector, which
`has no knowledge of either the first or second noise source.
`Costa’s surpring result is that the channel capacity is inde-
`pendent of the first noise source. This result has profound
`implications for watermarking where the cover Work can be
`thought of as the first, known noise source. It implies that,
`with the right coding, the capacity of a watermarking system
`may be independent of the cover Work even when blind de-
`tection is utilized. This work has since been extended [27] to
`more closely approximate the case for watermarking.
`Watermarking must not only transmit a message, but it
`must also maintain the fidelity of the underlying cover Work
`while surviving common distortions that the cover Work may
`undergo. These fidelity and robustness constraints often con-
`flict. As noted previously, early watermarking systems applied
`a global power constraint to satisfy fidelity constraints. In
`1995, it was recognized that the fidelity constraint required
`a perceptual model that allowed the embedded watermark
`signal to be locally varied in response to the local properties
`of the corresponding cover Work [29]. Many watermarking
`systems have been developed that employ a variety of per-
`ceptual models and they are generally superior to algorithms
`with no such models [30, 31, 32]. These perceptually-based
`watermark embedders were early forms of informed embed-
`ding, since the added watermark signal is dependent on the
`cover Work.
`The watermark communications channel can often be
`considered to exist in a hostile environment. For example,
`when watermarking is employed for copyright purposes,
`there is often a strong incentive to remove the watermark.
`The last 10 years has seen significant progress in the de-
`velopment of attacks and counter attacks. Researchers have
`documented many different attacks that an adversary might
`apply, for example, collusion attacks [8, 33], ambiguity at-
`tacks [34], copy attacks [35], sensitivity and gradient de-
`scent attacks [12, 13]. In addition, solutions to some of these
`threats, such as for ambiguity and copy attacks, have also been
`
`

`

`130
`
`EURASIP Journal on Applied Signal Processing
`
`proposed [34, 36]. This effort has provided valuable insights
`into what the threats are, under what conditions these threats
`can be neutralized and the limitations of current systems.
`Spread spectrum communications was introduced at the
`same time as perceptual modelling in order to deal with
`the conflicting fidelity and robustness requirements [37, 38].
`Spread spectrum communications spreads a narrow band sig-
`nal over a much wider frequency band such that the signal-
`to-noise ratio in any single frequency is very low. However,
`with precise knowledge of the spreading function, the re-
`ceiver is able to extract the transmitted signal, summing up
`the signals in each of the frequencies such that the detec-
`tor signal-to-noise ratio is strong. These characteristics allow
`weak watermark signals to be embedded that, in many cases,
`can be reliably detected. Spread spectrum communications
`is also difficult for an adversary to detect or jam and this is a
`further advantage of the technology.
`Around 1998, more rigorous quantitative measures of
`performance were introduced based on traditional false alarm
`and bit error rate techniques [39]. Theoretical progress was
`coincident with the development of more sophisticated mod-
`els. Traditional false alarm and bit error rate techniques have
`been applied to watermarking [39]. In addition, more accu-
`rate noise models were developed, particularly for quantiza-
`tion noise. The effect of quantization noise on watermarking
`is important because cover Works are often heavily quantized
`as part of lossy compression. The effect of quantization was
`rigorously modeled in [40] in which it was recognized that
`dither modulation was analogous to watermarking.
`
`3.2. Applied research
`This conceptual and theoretical progress paralleled signifi-
`cant engineering progress by small and large companies as
`well as universities. Much of this effort has focused on meet-
`ing fidelity, robustness and economic constraints.
`Steady progress has been made, particularly with respect
`to the problem of geometric and temporal distortions. Several
`different strategies have been pursued that can be categorized
`as exhaustive search, explicit synchronization/registration,
`autocorrelation [41], invariants [42, 43], and implicit syn-
`chronization [44, 45, 46, 47]. While no breakthroughs are
`expected, a number of design choices are now available.
`There has also been significant experimentation with a va-
`riety of different marking spaces. For example, frequency de-
`compositions such as DCT, FFT, wavelet and Fourier-Mellin
`transforms. While there is no clear superiority of one space
`over another, considerable expertise has been developed for
`embedding watermarks in MPEG and JPEG encoded content.
`This work facilitates the design of very inexpensive watermark
`detectors that are suitable for large-scale deployment, for ex-
`ample, for DVD copy control applications.
`
`4. THE FUTURE
`
`Some of the advances discussed above are in their infancy, and
`much interesting work remains to be done. In some cases,
`we believe that significant results may be imminent, which
`makes an area exciting. In other cases, we do not see any
`
`breakthroughs on the horizon, but significant results would
`increase the suitability of watermarks for a wider variety of
`applications, and are therefore worth further study.
`We believe that informed watermarking offers signifi-
`cant near-term improvements. While proposed codes for in-
`formed embedding are computational efficient they are not
`robust to valumetric scaling. A solution was briefly proposed
`in [48], but further investigation is needed to realize compu-
`tationally efficient and robust codes.
`Handling geometric/temporal distortions in a blind de-
`tector remains a difficult problem. However, a number of
`different approaches have been investigated and incremental
`progress is being made. A breakthrough is probably not im-
`minent, but progress in this area would lead to significantly
`more robust systems.
`While many papers have illustrated the use of a variety of
`fidelity models, there has been very little work [49] on how
`to optimally embed a watermark with fidelity and robustness
`constraints. We expect this to become a fruitful new area
`of research.
`Not all watermarks need to be secure. This is especially
`true of applications for which there is no adversary, for ex-
`ample, linking media to the Web. And even weak security
`has value in many business environments. Nevertheless, it
`remains an open question whether a watermark system can
`be designed that permits public detection of the watermark
`while preventing an adversary from removing the watermark.
`The authors of the present paper are divided about whether
`it is even theoretically possible to do this. Sensitivity analysis
`and gradient descent attacks appear to threaten any water-
`marking system in which the detector is publicly available.
`A number of researchers have attempted to design secure,
`public watermarking systems, but all appear susceptible to
`attack. It would be interesting to know whether such a system
`is even possible.
`If the past is any prediction of the future, then it is clear
`that watermarking technology will continue to be used by
`businesses. It is also reasonable to expect that legacy systems
`will be enhanced through the use of embedded signalling in
`order to maintain backward compatibility. The linking of tra-
`ditional media to the Web is still in its infancy and it remains
`uncertain whether consumers will value services that facili-
`tate commerce and discovery. So we conclude this paper with
`an exercise for the reader. Imagine that all content is water-
`marked with a technology that is open, free and can be read
`by anyone. As such, any and all content is identifiable by con-
`sumer devices. What services might these devices provide?
`
`ACKNOWLEDGEMENT
`
`Portions reprinted, with permission, from 2001 IEEE Fourth
`Workshop on Multimedia Signal Processing, 225–230, © 2001
`IEEE [50].
`
`REFERENCES
`
`[1] E. F. Hembrooke, “Identification of sound and like signals,”
`United States Patent, 3,004,104, 1961.
`
`

`

`The First 50 Years of Electronic Watermarking
`
`131
`
`[2] W. M. Tomberlin, L. G. MacKenzie, and P. K. Bennett, “System
`for transmitting and receiving coded entertainment programs,”
`United States Patent, 2,630,525, 1953.
`[3] C. Walker, “Personal communications,” June 2001, Muzak
`Corporation.
`[4] W. E. Noller, “Inband signalling system,” United States Patent,
`3,061,783, 1962.
`[5] R. H. Baer, “Digital video modulation and demodulation sys-
`tem,” United States Patent, 3,993,861, 1976.
`[6] R. Dolby, “Apparatus and method for the identification of
`specially encoded FM stereophonic broadcasts,” United States
`Patent, 4,281,217, 1981.
`[7] R. S. Broughton and W. C. Laumeister, “Interactive video
`method and apparatus,” United States Patent, 4,807,031, 1989.
`[8] D. Boneh and J. Shaw, “Collusion-secure fingerprinting for
`digital data,” in Advances in Cryptology—Crypto ’95, vol. 963
`of Lecture Notes in Computer Science, pp. 452–465, Springer-
`Verlag, Berlin, 1995.
`[9] H. S. Stone, “Analysis of attacks on image watermarks with
`randomized coefficients,” Technical report TR 96-045, NEC
`Research Institute, Princeton, NJ, USA, May 1996.
`[10] J. Kilian, T. Leighton, L. R. Matheson, T. Shamoon, and R. E.
`Tarjan, “Resistance of watermarked documents to collusional
`attacks,” Technical report TR 97-167, NEC Research Institute,
`Princeton, NJ, USA, 1997.
`[11] S. Craver, N. Memon, B.-L. Yeo, and M. M. Yeung, “Can invis-
`ible watermarks resolve rightful ownerships?,” IBM Technical
`Report RC 20509, IBM Research, July 1996,
`in IBM Cyber
`Journal, http://www.research.ibm.com.
`[12] J. P. Linnartz and M. van Dijk, “Analysis of the sensitivity
`attack against electronic watermarks in images,” in Proc. 2nd
`Int. Workshop on Information Hiding, pp. 258–272, Portland,
`Ore, USA, April 1998.
`[13] I. J. Cox and J.-P. M. G. Linnartz, “Some general methods for
`tampering with watermarks,” IEEE Journal on Selected Areas in
`Communications, vol. 16, no. 4, pp. 587–593, 1998.
`[14] M. Goljan, J. Fridrich, and R. Du, “Distortion-free data em-
`bedding for images,” in Proc. 4th Int. Workshop on Information
`Hiding, pp. 27–41, Pittsburgh, Pa, USA, 2001.
`[15] W. F. Schreiber, E. H. Adelson, A. B. Lippman, et al., “A compat-
`ible high-definition television system using the noise-margin
`method of hiding enhancement information,” SMPTE Journal,
`vol. 98, no. 12, pp. 873–879, 1989.
`[16] H. C. Papadopoulos and C.-E. W. Sundberg, “Simultaneous
`broadcasting of analog FM and digital audio signals by means
`of adaptive precanceling techniques,” IEEE Trans. Communi-
`cations, vol. 46, no. 9, pp. 1233–1242, 1998.
`[17] B. Chen and C.-E. W. Sundberg, “Digital audio broadcasting
`in the FM band by means of contiguous band insertion and
`precancelling techniques,” IEEE Trans. Communications, vol.
`48, no. 10, pp. 1634–1637, 2000.
`[18] M. van der Veen, W. Oomen, F. Bruekers, J. Haitsma, T. Kalker,
`and A. N. Lemma, “Robust, multi-functional, and high quality
`audio watermarking technology,” in 110th Audio Engineering
`Society Convention, vol. 5345, Amsterdam, The Netherlands,
`May 2001.
`[19] D. Kirovski and H. S. Malvar, “Robust covert communication
`over a public audio channel using spread spectrum,” in Infor-
`mation Hiding Workshop, I. S. Moskowitz and J. McHugh, Eds.,
`pp. 354–368, Pittsburgh, Pa, USA, April 2001.
`[20] I. J. Cox, M. L. Miller, and A. L. McKellips, “Watermarking
`as communications with side information,” Proceedings of the
`IEEE, vol. 87, no. 7, pp. 1127–1141, 1999.
`[21] J. Chou, S. S. Pradhan, a