This paper deals with the security of the robust zero-bit watermarking technique "Broken Arrows" (BA),<sup>1</sup>
which was invented and tested for the international challenge
BOWS-2.<sup>2</sup> The results of the first episode of the
challenge showed that BA is very robust and we proposed last year an enhancement called "Averaging Wavelet
Coefficients" (AWC),<sup>3</sup> which further strengthens the robustness against the worst attack disclosed during this
BOWS-2's first episode.<sup>4</sup> However, in the second and third episodes of the challenge, during which the pirates
could observe plenty of pictures watermarked with the same secret key, security flaws have been revealed and
discussed.<sup>5</sup> Here we propose counterattacks to these security flaws, investigating BA and its variant AWC. We
propose two counterattack directions: to use the embedding technique AWC instead of BA, and to regulate
the system parameters to lighten the watermarking embedding footprint. We also discuss these directions in
the context of traitor tracing.<sup>6</sup> Experimental results show that following these recommendations is sufficient to
counter these attacks.
This paper presents our recent works on multimedia fingerprinting, which consists in improving both the fingerprinting code and the watermarking scheme. The fingerprinting code is the well known Tardos code. Our contributions only focus on deriving a better accusation process.
It appears that Tardos orginal decoding is very conservative: its performances are guaranteed whatever the collusion strategy. Indeed, major improvements stems from the knowledge of the collusion strategy. Therefore, this paper investigates whether it is possible to learn and adapt to the collusion strategy. This done with an iterative algorithm a la EM, where a better estimation of their strategy yields a better tracing of the colluders, which in return yields a better estimation of their strategy etc.
The second part 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 discloses a flaw in this technique. We just present a counter-measure which blocks this security hole while preserving the robustness of the original technique.