Techniques for Multiple Watermarking

2. Introduction

Digital media has become an integral part of modern lives. Whether surfing the web, watching satellite television, playing a digital video disc, or listening to digital music, a large part of our professional and leisure time is filled with all things digital. This is the basis for modern scientific and economic developments which is centered around the distribution of digital data to a worldwide audience. However, the replacement of analog media by their digital counterparts together with the explosion of the high speed internet and wireless networks has had also a perhaps unintended consequence. It has also become relatively easy to illegally use and redistribute media at will. Securing the data exchanged in a networked media environment against copyright violation, unauthorized usage and illegal redistribution is thus a central and at time critical issue. This is because traditional means of network security consisting, e.g., in firewalls, virtual private networks and intrusion detection systems can hardly cope with security issues when used as stand-alone technologies. Additional security-assisting functionalities relying on watermarking and data hiding and techniques are highly instrumental in this trend (Voloshynovskiy, Deguillaume, Koval, and Pun, 2003).

Watermarking consists in inserting unperceived data in a signal (referred to as cover- or carrier signal) in an attempt to establish ownership, usage rules or track media redistribution. It gained importance in the mid 1990s mainly as a potential solution for digital rights management. The key aspects of watermarking that make it attractive are the imperceptibility of the embedded data, its statistical covertness and its ability to withstand channel degradations, including several cycles of digital-to-analog and analog-to-digital conversions and various signal processing operations. These two conflicting requirements are often called robustness and transparency requirement, respectively.

Much of prior work on watermarking and data hiding concentrated on designing one single watermark with prescribed requirements. This is suitable for applications such as copyright protection where the embedding of just a few bits of information expected to be detectable with very low probability of false alarm is sufficient to serve as an evidence of copyright (Swanson, Kobayashi, and Tewfik, 1998). However, in many other practical situations this might be not sufficient and one might need to embed more than one watermark into the same host. Such situations are referred to as multiple watermarking. The problem of inserting multiple watermarks into a digital media has been introduced in the literature in the pioneering work by Mintzer and Braudaway (1999). Then, there has been a growing body of works focusing on multiple watermarking techniques, algorithms and limitations (see, e.g., Cox. et al., 1997; Liu et al., 2004; Khisti et al., 2007; Zaidi et al., 2007, 2009).

The problem of watermarking is relatively well understood in the single watermark case (Chen and Wornell, 2001; Moulin and O'Sullivan, 20003), but it lacks theoretical foundation in the multiple watermarks case. The goal of this chapter is to provide technical ground and insights onto how multiple watermarks can be embedded efficiently into the same host signal. We adopt communication and information theoretic inclinations, and we argue that this problem has tight relationship to conventional multiuser information theory. Then we show that by virtue of this tight relationship design and optimization of algorithms for multiple watermarking applications can greatly benefit from recent advances and new findings in multiuser information theory. Harnessing information-theoretic tools to the investigation of the problem of multiple watermarking not only provides a yardstick by which the efficiency of this technique can be measured, but in fact provides the right guidance to the appropriate design of efficient embedding schemes in practice.