Achieving subspace or key security for woa using natural or circular watermarking

This paper presents two watermarking schemes that are se-cure when considering the Watermarked content Only At-tack (WOA) framework. The definition of watermark secu-rity is first recalled and the distinction between key-security and subspace-security classes for Spread Spectrum (SS) wa-termarking schemes is presented afterwards. Blind source separation techniques are also recalled as a tool to assess the security of a SS watermarking scheme and prove that clas-sical SS and Improved SS are not secure within the WOA framework. To further illustrate these security issues, we next build on a new watermarking scheme called Natural Watermarking (NW). We prove it to be subspace-secure under specific hypotheses. Natural watermarking does not change the Gaussian natural distributions of the projection of each carriers. Furthermore NW prevents estimations both of the watermark subspace (subspace-security) and the dif-ferent carriers (key-security). We then extend the nice prop-erties of NW to derive the more general family of Circular Watermarking schemes which are key-secure and may offer a better robustness to AWGN attack than NW. An implemen-tation of CW based on ISS is next proposed and comparison of bit error rates for NW, CW, SS and ISS finally draws some conclusions on the robustness cost to achieve security