A classification of digital watermarking algorithms. The watermarking process consists of two main steps. The first step is watermark casting, in which the ...
Interoperability Issues in Watermark-Based Electronic Copyright Management M. Barni, F. Bartolini, V. Cappellini, A. Piva Dip. Ing. Elettronica, Univ. di Firenze, Via S. Marta 3, 50139, Firenze, Italy Abstract. Digital watermarking systems represent a solution to the ever increasing need for copyright protection of multimedia data. However, this technology is still in its infancy, and no standard has been proposed yet. In this paper, interoperability between different watermarking techniques used in the context of Electronic Copyright Management is addressed. The results of experiments regarding the interoperability between different techniques are reported, in particular for applications to copyright protection of Cultural Heritage and Contemporary Art.
1. Introduction The day by day increasing availability of multimedia contents and the rapid development of communication networks, raised the need for new systems dedicated to the management of electronic transactions of digital data. Research and commercial institutions have proposed different applications, involving the use of Electronic Copyright Management Systems. These systems are designed in such a way that the protection of the copyright of multimedia data is accomplished at several levels: as a first step, control over data access is usually provided. Next, cryptography algorithms are used to make data unreadable to nonauthorized users. Finally, multimedia works are usually marked to allow their distribution to be tracked, in such a way to permit content-owners to make evident any possible copyright infringement. This last protection level is achieved by means of digital watermarking technology: a digital watermark is an electronic code tightly embedded into multimedia data. According to the particular application scenario, the inserted signal can bear information about the content owner, the distributor of the work, the authorized purchaser, a legal notice, and so on. Digital watermarking technology is a very recent research field, so no standard has been proposed yet: instead, many algorithms have been developed trying to solve the problem of copyright protection of digital data, but each technique has its own particular requirements and characteristics. Since copyright protection has to be granted in open environments where many solutions are likely to exist, how different schemes can effectively be integrated must be carefully investigated. In this paper, the interoperability between different watermarking techniques used in the context of Electronic Copyright Management is addressed. Even if general principles can not be given without taking into account the particular scenario where a system is to be used, some guidelines are proposed to understand whether different watermarking schemes can be used together. The results of experiments regarding the interoperability between different watermarking techniques for digital images (some of the above developed in the Laboratorio Comunicazioni e Immagini - LCI Lab - of the Department of Electronic Engineering of the Florence University) are reported, in particular regarding applications to copyright protection of Cultural Heritage and Contemporary Art.
2. A classification of digital watermarking algorithms The watermarking process consists of two main steps. The first step is watermark casting, in which the watermark is inserted into the original image; the second step is watermark detection, in which the code is extracted from the watermarked image. Since many techniques have been proposed so far, to understand how different watermarking schemes can be used together, it is important to define some criteria to classify them. In this section, three characteristics of watermarking tools have been chosen from the point of view of watermark detection, since the way the code is recovered from the watermarked content plays a major role in assessing its effectiveness in a given application scenario. 2.1. Blind vs. non blind techniques Two main algorithm classes can be distinguished: that of the algorithms requiring the original image for watermark detection (non-blind techniques) [1,2], and that of the algorithms that do not require it (blind techniques) [3,4,5]. Algorithms relying on the original data for watermark detection are usually more robust but the constraint they impose is very strong. For example, they do not permit to a purchaser to read/detect the watermark to be sure that the work has been sold legally to him/her. 2.2. Public vs. non private techniques A watermark is said private if only authorized readers can detect it. In other words, in private watermarking a mechanism is conceived that makes it impossible for unauthorized users to extract the watermark. For instance, privateness may be achieved by means of a user-defined pseudo-random key to be used in the embedding step, whose knowledge is necessary to extract the watermark from the host data. On the contrary, techniques allowing anyone to read the watermark are referred to as public. Private watermarking techniques are likely to be significantly more robust than public ones, in that, once the embedded code is known, it is much easier for an attacker to remove it or to make it unreadable. 2.3. Readable vs. detectable watermarks Another important distinction can be made between readable algorithms, that embed a code which can be read, and detectable ones, which insert a mark that can only be detected. While the former kind of algorithms, if public, allows anybody to read the code that is inserted into the data, the latter only permits to check if a given code is present or not. In this second case, then, it is required that the inserted code is known in advance if its presence has to be checked. 3. Watermarking systems interoperability Interoperability between different watermarking techniques is of major importance when copyright protection has to be granted in open environments where many solutions are likely to exist. The definition of standard specifications could allow to define an environment where different schemes can be used together or easily interchanged, without problems for the users [6]. Though general rules can not be stated without taking into account a particular application scenario, some guidelines can be given to understand whether two or more techniques can be used together. Ultimately, we can say that, given an application scenario, two watermarking techniques can be used interchangeably if both of them satisfy the requirements the scenario imposes. To be more specific, regardless of the framework they are used in, the compatibility of different watermarking systems may be addressed at two different levels: intrinsic compatibility and format compatibility. Intrinsic compatibility refers to the basic requisites watermarking algorithms have to fulfil to be used in the framework of a given transaction model. For instance, if the application at hand requires a public watermarking scheme, interoperability is not achievable with private algorithms.
With regard to format compatibility, it mainly refers to the watermark format, which, in principle, is not rigidly fixed by the application scenario. In many cases, in fact, only some loose requirements are specified, so that some freedom is available to accommodate different watermark formats. 3.1. Intrinsic compatibility By referring to the analysis carried out in the previous section, we can say that two different watermarking algorithms can coexist in the same environment only if the following conditions are satisfied: they allow a predefined minimum number of bits to be inserted into the data, in such a way that the same information can be carried out by all the involved watermark; watermarking algorithms must be homogenous, in that public and private algorithms can not be used together, nor blind/non-blind (readable/detectable) techniques can be mixed together. In contrast, no special requirement has to be satisfied with respect to robustness, in that watermarks characterized by a different resistance to possible attacks can be used together without any particular problem. It should also be noted that interoperability may hold only in a direction. For example, may be a blind technique can be adopted in a framework where non-blind techniques are supposed to be used, but it is surely impossible that a non-blind technique is used in an environment where blind watermarking is required. 3.2. Format compatibility If intrinsic compatibility is granted, some minor aspects must be taken into consideration before concluding that two different watermark technologies are interoperable in a given scenario. A major aspect to be considered here is whether the watermark formats can bear the same kind of information. From a quantitative point of view, this is granted by the first requirement of intrinsic interoperability, when saying that in order to be intrinsically compatible two techniques must allow a predefined minimum number of bits to be inserted into the data. On the other hand, even if the number of bits which can be hidden in the data is approximately the same, the format the information is casted into is likely to be different. Therefore some adjustments must be made before the different systems can be used together. Other aspects that could need a common standard are the format of the watermark key, if any, and the format of the data defining the energy of the watermark to be embedded [6]. Given that the technologies under examination are already intrinsically compatible, is very likely that such adjustments are possible, however, each particular situation should be analyzed separately. 4. An application scenario Aiming at demonstrating how the proposed guidelines can be applied to decide whether different watermarking techniques can work together, we present now a simple application scenario where watermarking is used to protect Cultural Heritage contents. Let us suppose a gallery has decided to develop a Web site containing a set of digital images of the works contained in it, in order to attract new visitors. Such a scenario imposes a set of requirements onto the watermarking method: first, the algorithm has to be private, since the gallery is only interested in verifying the presence of the watermark in a possibly corrupted image, without allowing anyone to extract the embedded code; the scheme should be blind, so that a web crawler or an automatic agent could easily look for watermarked images in the web, without the need to access to the original images. A detectable watermark could be used, since in this scenario the agent should only verify the presence or the absence of the digital code identifying the museum. Finally, the watermark should be very robust, in order to resist to the several possible attacks non-authorized users could perform in order to remove it.
4.1. Our proposed system An image watermarking system satisfying the previous requirements, i.e. a blind, private, detectable watermarking technique, has been developed in the LCI Lab. Experimental results demonstrate that the scheme is robust to several signal processing techniques, including JPEG compression, low pass and median filtering, histogram modifications, dithering, addition of noise, resizing, and multiple watermarking. The embedded code has the following format: A string, maximum 22 characters long, used to insert the name of the museum; A string, maximum 8 digits long, used to insert some additional information. The images are marked according to the scheme proposed in [3]. The algorithm encodes the two strings the mark consists of, by generating a set of 4 integers. This set is then used as a seed for a pseudo-random generator which produces a sequence of real numbers having a Gaussian distribution. The sequence obtained in this way is the watermark, that is then used to modify a selected set of the full-frame DCT coefficients of the to-be-marked image. After watermark embedding, the modified DCT coefficients are reinserted in their position, and the inverse DCT is applied, obtaining a preliminary watermarked image. To enhance watermark robustness, without compromising its invisibility, the characteristics of the Human Visual System are then exploited by blending the original image and the preliminary watermarked one based on a spatial masking image, thus obtaining a final watermarked image. In Figure 1 (left) an image representing a particular of the world-wide known painting The birth of Venus by S.Botticelli is depicted. On the right of the figure, the same image after watermarking is shown. As it can be seen, the two images are not distinguishable, thus proving that the requirement of watermark invisibility is satisfied. In the detection stage, the image to be tested is DCT-transformed, and the same set of DCT coefficients used in the marking step is selected. The correlation between the extracted coefficients and the mark itself is computed and compared to a threshold: if the correlation is larger than the threshold, it is decided that the given watermark is embedded in the image, otherwise it is assumed that the watermark is not present.
Figure 1: The original image “La Nascita di Venere” (Left) and the watermarked copy (Right).
4.2. Interoperability with other systems The technical requirements imposed by the application scenario reject all the algorithms that need the original image in detection step, such as [1,2]; moreover, the system has to be private and detectable: many commercial systems do not have these characteristic. For example the SureSign Writer/Detector, from Signum Technologies, they allows to any user to read the embedded watermark. Indeed, Signum Technologies recently developed a new
system, SureSign X, based on a private-key algorithm: in this case only the owner of the correct key can embed and read the watermark. The algorithm, originally public, has thus become private, but it still embeds a readable watermark. However, it is evident that for the proposed application, the requirement for a detectable algorithm is not compulsory, but only preferable. The intrinsic compatibility between the system proposed by the LCI Lab and SureSign X is then demonstrated. As a second step, the format compatibility has to be studied. The SureSign system embeds into the image two strings, a unique registered user's 6-digit alpha-numeric Fingerprint ID, and an user-defined 7-digit alpha-numeric Image ID. The two systems, then, embed two strings, but the SureSign can embed shorter strings than our system. Finally, we can conclude that the two systems can be interchangably used in the proposed application scenario, provided that the length of the two strings to be embedded is chosen equal to that of the SureSign algorithm. 5. Conclusions Digital watermarking technology is a very recent research field, so no standard has been proposed yet. Since multimedia data copyright protection has to be granted in open environments, where many solutions are likely to exist, how different schemes can effectively be integrated must be carefully investigated. In this paper, interoperability between different watermarking techniques used in the context of Electronic Copyright Management has been addressed, and some guidelines have been proposed in the attempt to understand whether different schemes can be used together in the same application scenario. The results of experiments regarding the interoperability between different watermarking techniques for digital images have also been reported. 6. Acknowledgements This work was partially supported by the Italian National Research Council (CNR) in the framework of the “Progetto Finalizzato Beni Culturali” and by MURST. References [1] I.J. Cox, , J. Kilian, T. Leighton and T. Shamoon, “Secure Spread Spectrum Watermarking for Multimedia”, IEEE Transactions on Image Processing, Vol. 6, No. 12, pp. 1673-1687, December 1997. [2] M.D. Swanson, B. Zhu and A.H. Tewfik, “Transparent Robust Image Watermarking”, Proc. of IEEE International Conference on Image Processing (ICIP'96), Lausanne, Switzerland, September 16-19, 1996, vol. III, pp. 211-214. [3] M. Barni, F. Bartolini, V. Cappellini and A. Piva, “A DCT-domain system for robust image watermarking”, Signal Processing, 66 (3), pp.357-372, May 1998. [4] E. Koch, J. Rindfrey, and J. Zhao, “Copyright Protection for Multimedia Data”, Proc. of the International Conference on Digital Media and Electronic Publishing, December 6-8, 1994, Leeds, UK. [5] N. Nikolaidis and I. Pitas, “Robust image watermarking in the spatial domain”, Signal Processing, 66 (3), pp.385-403, May 1998. [6] F. Mintzer, G. Braudaway and A. Bell, “Opportunities for Watermarking Standards”, Communications of the ACM, vol. 41. No.7, pp. 56-64, July 1998.
