05-05-2012, 03:45 PM
Digital Watermarking
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1 Introduction
The advent of the Internet has resulted in many new opportunities for the creation and delivery of content in digital form. Applications include electronic advertising, realtime video and audio delivery, digital repositories and libraries, and Web publishing. An important issue that arises in these applications is the protection of the rights of all participants. It has been recognized for quite some time that current copyright laws are inadequate for dealing with digital data. This has led to an interest towards developing new copy deterrence and protection mechanisms. One such effort that has been attracting increasing interest is based on digital watermarking techniques. Digital watermarking is the process of embedding information into digital multimedia content such that the information (which we call the watermark) can later be extracted or detected for a variety of purposes including copy prevention and control. Digital watermarking has become an active and important area of research, and development and commercialization of watermarking techniques is being deemed essential to help address some of the challenges faced by the rapid proliferation of digital content.
In the rest if this chapter we assume that the content being watermarked is a still image, though most digital watermarking techniques are, in principle, equally applicable to audio and video data. A digital watermark can be visible or invisible. A visible watermark typically consists of a conspicuously visible message or a company logo indicating the ownership of the image as shown in Figure 1. On the other hand, an invisibly watermarked image appears very similar to the original. The existence of an invisible watermark can only be determined using an appropriate watermark extraction or detection algorithm. In this chapter we restrict our attention to invisible watermarks.
An invisible watermarking technique, in general, consists of an encoding process and a decoding process. A generic watermark encoding process is shown in Figure 2. Here, the watermark insertion step is represented as:
X` = EK(X,W) (1)
where X is the original image, W is the watermark information being embedded, K is the user_s insertion key, and E represents the watermark insertion function. We adopt the notation throughout this chapter that for an original image X, the watermarked variant is represented as X`. Depending on the way the watermark is inserted, and depending on the nature of the watermarking algorithm, the detection or extraction method can take on very distinct approaches. One major difference between watermarking techniques is whether or not the watermark detection or extraction step requires the original image. Watermarking techniques that do not require the original image during the extraction process are called oblivious (or public or blind) watermarking techniques. For oblivious watermarking techniques, watermark extraction works as follows:
ˆW= DK0(ˆX 0) (2)
where ˆX 0 is a possibly corrupted watermarked image, K0 is the extraction key, D represents the watermark extraction/detection function, and ˆW is the extracted watermark information (see, Figure 3). Oblivious schemes are attractive for many applications where it is not feasible to require the original image to decode a watermark. Invisible watermarking schemes can also be classified as either robust or fragile. Robust watermarks are often used to prove ownership claims and so are generally designed to withstand common image processing tasks such as compression, cropping, scaling, filtering, contrast enhancement, printing/scanning, etc., in addition to malicious attacks aimed at removing or forging the watermark.
1.1 Applications
Digital Watermarks are potentially useful in many applications, including:
Ownership assertion. Watermarks can be used for ownership assertion. To assert ownership
of an image, Alice can generate a watermarking signal using a secret private key, and then embed it into the original image. She can then make the watermarked image publicly available. Later, when Bob contends the ownership of an image derived from this public image, Alice can produce the unmarked original image and also demonstrate the presence of her watermark in Bob’s image. Since Alice’s original image is unavailable to Bob, he cannot do the same. For such a scheme to work, the watermark has to survive image processing operations aimed at malicious removal. In addition, the watermark should be inserted in such a manner that it cannot be forged as Alice would not want to be held accountable for an image that she does not own.
Fingerprinting. In applications where multimedia content is electronically distributed over a network, the content owner would like to discourage unauthorized duplication and distribution by embedding a distinct watermark (or a fingerprint) in each copy of the data. If, at a later point in time, unauthorized copies of the data are found, then the origin of the copy can be determined by retrieving the fingerprint. In this application the watermark needs to be invisible and must also be invulnerable to deliberate attempts to forge, remove or invalidate. Furthermore, and unlike the ownership assertion application, the watermark should be resistant to collusion. That is, a group of k users with the same image but containing different fingerprints, should not be able to collude and invalidate any fingerprint or create a copy without any fingerprint.
Copy prevention or control. Watermarks can also be used for copy prevention and control.
For example, in a closed system where the multimedia content needs special hardware for copying and/or viewing, a digital watermark can be inserted indicating the number of copies that are permitted. Every time a copy is made the watermark can be modified by the hardware and after a point the hardware would not create further copies of the data. An example of such a system is the Digital Versatile Disc (DVD). In fact, a copy protection mechanism that includes digital watermarking at its core is currently being considered for standardization and second generation DVD players may well include the ability to read watermarks and act based on their presence or absence.
Another example is in digital cinema, where information can be embedded as a watermark in every frame or a sequence of frames to help investigators locate the scene of the piracy more quickly and point out weaknesses in security in the movie’s distribution. The information could include data such as the name of the theater and the date and time of the
screening. The technology would be most useful in fighting a form of piracy that’s surprisingly common, i.e., when someone uses a camcorder to record the movie as it’s shown in a theater, then duplicates it onto optical disks or VHS tapes for distribution.
Fraud and tamper detection. When multimedia content is used for legal purposes, medical applications, news reporting, and commercial transactions, it is important to ensure that the content was originated from a specific source and that it had not been changed, manipulated or falsified. This can be achieved by embedding a watermark in the data. Subsequently, when the photo is checked, the watermark is extracted using a unique key associated with the source, and the integrity of the data is verified through the integrity of the extracted watermark. The watermark can also include information from the original image that can aid in undoing any modification and recovering the original. Clearly a watermark used for authentication purposes should not affect the quality of an image and should be resistant to forgeries. Robustness is not critical as removal of the watermark renders the content inauthentic and hence of no value.
ID card security. Information in a passport or ID (e.g., passport number, person’s name, etc.) can also be included in the person’s photo that appears on the ID. By extracting the embedded information and comparing it to the written text, the ID card can be verified. The inclusion of the watermark provides an additional level of security in this application. For example, if the ID card is stolen and the picture is replaced by a forged copy, the failure in extracting the watermark will invalidate the ID card.
The above represent a few example applications where digital watermarks could potentially be of use. In addition there are many other applications in rights management and protection like tracking use of content, binding content to specific players, automatic billing for viewing content, broadcast monitoring etc. From the variety of potential applications exemplified above it is clear that a digital watermarking technique needs to satisfy a number of requirements. Since the specific requirements vary with the application, watermarking techniques need to be designed within the context of the entire system in which they are to be employed. Each application imposes different requirements and would require different types of invisible or visible watermarking schemes or a combination thereof. In the remaining sections of this chapter we describe some general principles and techniques for invisible watermarking. Our aim is to give the reader a better understanding of the basic principles, inherent trade-offs, strengths, and weakness, of digital watermarking. We will focus on image watermarking in our discussions and examples. However as we mentioned earlier, the concepts involved are general in nature and can be applied to other forms of content such as video and audio.
1.2 Relationship with Information Hiding and Steganography
In addition to digital watermarking, the general idea of hiding some information in digital content has a wider class of applications that go beyond mere copyright protection and authentication. The techniques involved in such applications are collectively referred to as information hiding. For example, an image printed on a document could be annotated by information that could lead an user to its high resolution version as shown in Figure 4. Metadata provides additional information about an image. Although metadata can also be stored in the file header of a digital image, this approach has many limitations. Usually, when a file is transformed to another format (e.g., from TIFF to JPEG or to bmp), the metadata is lost. Similarly, cropping
or any other form of image manipulation destroys the metadata. Finally, the metadata can only be attached to an image as long as the image exists in the digital form and is lost once the image is printed. Information hiding allows the metadata to travel with the image regardless of the file format and image state (digital or analog). Metadata information embedded in an image can serve many purposes. For example, a business can embed the website URL for a specific product in a picture that shows an advertisement for that product. The user holds the magazine photo in front of a low-cost CMOS camera that is integrated into a personal computer, cell phone, or a palm pilot. The data is extracted from the low-quality picture and is used to take the browser to the designated website. Another example is embedding GPS data (about 56 bits) about the capture location of a picture. The key difference between this application and watermarking is the absence of an active adversary. In watermarking applications like copyright protection and authentication, there is an active adversary that would attempt to remove, invalidate or forge watermarks. In information hiding there is no such active adversary as there is no value associated with the act of removing the information hidden in the content. Nevertheless, information hiding techniques need to be robust against accidental distortions. For example, in the application shown in Figure 4, the information embedded in the document image needs to be extracted despite distortions incurred in the print and scan process. But these distortions are just a part of a process and not caused by an active adversary.
Another topic that is related to watermarking is steganography (meaning covered writing in Greek), which is the science and art of secret communication. Although steganography has been studied as part of cryptography for many decades, the focus of steganography is secret communication. In fact, the modern formulation of the problem goes by the name of the prisoner’s problem. Here Alice and Bob are trying to hatch an escape plan while in prison. The problem is that all communication between them is examined by a warden, Wendy, who will place both of them in solitary confinement at the first hint of any suspicious communication. Hence, Alice and Bob must trade seemingly inconspicuous messages that actually contain hidden messages involving the escape plan. There are two versions of the problem that are usually discussed – one where the warden is passive, and only observes messages and the other where the warden is active and modifies messages in a limited manner to guard against hidden messages. Clearly the most important issue here is that the very presence of a hidden message must be concealed. Whereas in digital watermarking it is not clear that a good watermarking technique should also be steganographic.
1.3 Watermarking Issues
The important issues that arise in the study of digital watermarking techniques are:
Capacity: what is the optimum amount of data that can be embedded in a given signal? What is the optimum way to embed and then later extract this information?
Robustness: How do we embed and retrieve data such that it would survive malicious or accidental attempts at removal?
Transparency: How do we embed data such that it does not perceptually degrade the underlying content?
Security: How do we determine that the information embedded has not been tampered, forged or even removed?
Indeed, these questions have been the focus of intense study in the past few years and some remarkable progress has already been made. However, there are still more questions than answers in this rapidly evolving research area. Perhaps a key reason for this is the fact that digital watermarking is inherently a multi-disciplinary topic that builds on developments in diverse subjects. The areas that contribute to the development of digital watermarking include at the very least the following:
Information and Communication Theory
Decision and Detection Theory
Signal Processing
Cryptography and Cryptographic Protocols
Each of these areas deals with a particular aspect of the digital watermarking problem. Generally speaking, information and communication theoretic methods deal with the data embedding (encoder) side of the problem. For example, information theoretic methods are useful in the computation of the amount of data that can be embedded in a given signal subject to various constraints such as peak power (square of the amplitude) of the embedded data or the embedding induced distortion. The host signal can be treated as a communication channel and various operations such as compression/decompression, filtering etc. can be treated as noise. Using this framework, many results from classical information theory can be and indeed have been successfully applied to compute the data embedding capacity of a signal.