This paper presents our recent works on multimedia fingerprinting, improving both the fingerprinting code and the watermarking scheme. Our first contribution focuses on deriving a better accusation process for the well known Tardos codes. It appears that Tardos orginal decoding is very conservative: its performances are guaranteed whatever the collusion strategy. Indeed, major improvements stem from the knowledge of the collusion strategy. Therefore, the first part of this paper investigates how it is possible to learn and adapt to the collusion strategy. Our solution is based on an iterative algorithm a la EM, where a better estimation of the collusion strategy yields a better tracing of the colluders, which in return yields a better estimation of the collusion strategy etc.The second part of this paper focuses on the multimedia watermarking scheme. In a previous paper, we already used the 'Broken Arrows' technique as the watermarking layer for multimedia fingerprinting. However, a recent paper from A. Westfeld disclosed a flaw in this technique. We present here a counter-measure which blocks this security hole while preserving the robustness of the original technique.
This paper considers the security aspect of the robust zero-bit watermarking technique 'Broken Arrows'(BA), 1 which was invented and tested for the international challenge BOWS-2. The results of the first episode of the challenge showed that BA is very robust. Last year, we proposed an enhancement so-called AWC, 2 which further strengthens the robustness against the worst attack disclosed during the challenge. However, in the second and third episodes of the challenge, when the pirate observes plenty of watermarked pictures with the same secret key, some security flaws have been discovered. They clearly prevent the use of BA in multimedia fingerprinting application, as suggested in.3 Our contributions focus on finding some counter-attacks. We carefully investigate BA and its variant AWC, and take two recently published security attacks 4 as the potential threats. Based on this, we propose three countermeasures: benefiting from the improved embedding technique AWC; regulating the system parameters to lighten the watermarking embedding footprint; and extending the zero bit watermarking to multi-bits for further increase the security level. With this design, experimental results show that these security attacks do not work any more, and the security level is further increased.
This paper presents yet another attempt towards robust and secure watermarking. Some recent works have looked at this issue first designing new watermarking schemes with a security oriented point of view, and then evaluating their robustness compared to state-of-the-art but unsecure techniques. Our approach is, on contrary, to start from a very robust watermarking technique and to propose changes in order to strengthen its security levels. These changes include the introduction of a security criterion, an embedding process implemented as a maximization of a robustness metric under the perceptual and the security constraints, and a watermarking detection seen as a contrario decision test.Our experimentations lead to, once again, a trade-off between security and robustness. The technique is now perfectly secure against attacks mounted during the second edition of the BOWS challenge, but the price to pay is either a lower robustness against common image processing, either a bigger probability of false alarm.
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