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DIGITAL WATERMARKING
Description
Definition of Digital Watermarking
Digital watermarking is a process of embedding unobtrusive marks or labels into digital content. These embedded marks are typically imperceptible (invisible) that can later be detected or extracted (Yeung, Yeo, & Holliman, 1998). The concept of digital watermarking is associated with steganography.
Steganography is defined as covered writing. It has a long history of being associated with methods of secret communication. Steganography does not immediately arise the suspicion of something secret or valuable. Instead, it hides an important message in an unimportant one. Therefore, digital watermarking is a way to hide a secret or personal message to protect a product’s copyright or to demonstrate data integrity (Voyatzis & Pitas, 1999).
Purpose of Digital Watermarking
Watermarks added to digital content serve a variety of purposes. The following list details six purposes of digital watermarking (Memon & Wong, 1998).
• • Ownership Assertion – to establish ownership of the content (i.e. image)
• • Fingerprinting – to avoid unauthorized duplication and distribution of publicly available multimedia content
• • Authentication and integrity verification – the authenticator is inseparably bound to the content whereby the author has a unique key associated with the content and can verify integrity of that content by extracting the watermark
• • Content labeling – bits embedded into the data that gives further information about the content such as a graphic image with time and place information
• • Usage control – added to limit the number of copies created whereas the watermarks are modified by the hardware and at some point would not create any more copies (i.e. DVD)
• • Content protection – content stamped with a visible watermark that is very difficult to remove so that it can be publicly and freely distributed
Unfortunately, there is not an universal watermarking technique to satisfy all of these purposes (Memon & Wong, 1998). The content in the environment that it will be used determines the digital watermarking technique. The following section describes some digital watermarking techniques.
Digital Watermarking Techniques
The most important properties of any digital watermarking techniques are robustness, security, imperceptibility, complexity, and verification. Robustness is defined as if the watermark can be detected after media (normal) operations such as filtering, lossy compression, color correction, or geometric modifications. Security means the embedded watermark cannot be removed beyond reliable detection by targeted attacks. Imperceptibility means the watermark is not seen by the human visual system. Complexity is described as the effort and time required for watermark embedding and retrieval. Lastly, verification is a procedure where by there is a private key or public key function (Dittmann, Mukherjee, & Steinebach, 2000).
Each of these properties must be taken into consideration when applying a certain digital watermarking technique. The following sections describe a few of the most common digital watermarking techniques.
Spatial and Frequency Domain
Spatial and frequency domain watermarking are applied to graphic images and text. Spatial domain watermarking slightly modifies the pixels of one or two randomly selected subsets of an image. Modifications might include flipping the low-order bit of each pixel. However, this technique is not reliable when subjected to normal media operations such as filtering or lossy compression (Berghel, 1998).
Frequency domain watermarking technique is also called transform domain. Values of certain frequencies are altered from their original. Typically, these frequency alterations are done in the lower frequency levels, since alternations at the higher frequencies are lost during compression. The watermark is applied to the whole image so as not to be removed during a cropping operation. However, there is a tradeoff with the frequency domain technique. Verification can be difficult since this watermark is applied indiscriminately across the whole image (Berghel, 1998).
The Zhao Koch Algorithm and The Fridrich Algorithm
The Zhao Koch Algorithm and The Fridrich Algorithm watermark techniques are applied to MPEG videos. The Zhao Koch Algorithm embeds a copyright label in the frequency domain of the video. The algorithm randomly selects three coefficients from the discrete cosine transform encoded block and manipulates them to store a single bit of information using a secret key. This single bit information can be the name or address of the owner. The watermark can be easily embedded into the video with minimal operation. Thus, complexity is not an issue. However, the Zhao Koch Algorithm watermark technique is not robust against normal media operations such as scaling or rotation (Dittmann, Stabenau, & Steinmetz, 1998).
The Fridrich Algorithm watermark technique is where a pattern is overlaid in the low frequency domain. The pattern is created using a pseudo random number generator and a cellular automation with voting rules. The pseudo random number generator creates a white and black initial pattern that is the same size as the image. A cellular automation with voting rules is then applied until there is a convergence to fixed points. The voting rule randomly patches the pattern into these connected points. Thus, the pattern is now overlaid into the image. This algorithm is resistant to normal media operations. However, verification using this algorithm is not reliable. This is because the watermark technique does not include detail information about the owner when the pattern is created and overlaid (Dittmann et al., 1998).
Attacks
Digital watermarking does not have the same capability or level of security as data encryption. It does not prevent the viewing or listening of content, nor does it prevent accessing that content. Therefore, digital watermarking is not immune to hacker attacks (Yeung et al., 1998). The following are some intentional attacks on watermarks (Cox, Miller, & Bloom, 2000).
• • Active Attacks – hacker tries to remove the watermark or make it undetectable. An example is to crop it out.
• • Passive Attacks – hacker tries to determine whether there is a watermark and identify it. However, no damage or removal is done.
• • Collusion Attacks – hacker uses several copies of one piece of media, each with a different watermark, to construct a copy with no watermark.
• • Forgery Attacks – hacker tries to embed a valid watermark of their own rather than remove one.
• • Distortive Attacks – hacker applies some distortive transformation uniformly over the object in order to degrade the watermark so that it becomes undetectable/unreadable (Collberg & Thomborson, 1999).
These intentional attacks are just one of the barriers of why authors do not put their works into digital format. However, the government has stepped in to help these authors by establishing new laws.
Current Laws
There are three main laws concerning digital copyright. They are the Digital Millennium Copyright Act (DMCA), the Collections of Information Antipricay Act, and changes to Article 2B of the Uniform Commercial Code. Each of these laws is briefly described below.