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The Essential Guide to Image Processing- P21

The Essential Guide to Image Processing- P21:We are in the middle of an exciting period of time in the field of image processing. Indeed, scarcely a week passes where we do not hear an announcement of some new technological breakthrough in the areas of digital computation and telecommunication. | 610 CHAPTER 22 Image Watermarking Techniques and Applications 22.5.1.3 Security The notion of security of watermarking methods has recently attracted the interest of the watermarking community. The distinction between robustness and security is still not well defined and globally agreed upon. A possible somewhat indirect definition and distinction is that attacks to robustness are those that aim at increasing the probability of error of the watermarking channel whereas attacks to robustness try to provide an attacker with knowledge on the secrets of the system e.g. the secret key 59 60 . According to the cryptanalysis point of view on security presented in 61 and inspired bytheworksof Shannon 62 and Diffie-Hellman 63 security refers to the information regarding the secret watermark key that becomes available leaks to an attacker through watermarked data that she possesses. In more detail the attacker is assumed to posses a number of documents watermarked with the same key and different messages. According to Shannon s approach adapted to the case of watermarking the watermarking method is perfectly secure if no information regarding the secret key leaks from these observations. If the method is not perfectly secure then the security level of the method can be defined as the number of watermarked documents that an attacker needs in order to fully discover the key. The authors in 61 proceed in defining measures of information leakage for a watermarking scheme. One such measure is equivocation which has been proposed by Shannon 62 and can be used in methods where the secret key is a binary word. Equivocation measures the uncertainty of an attacker on the key value K when N observations watermarked documents are available and is defined as H K ON H K - I K ON 22.9 where H K ON is the conditional entropy of K given the set of N observations ON H K is the entropy of K uncertainty on the value of the key when no observations are available and I K ON is the mutual .