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

This paper presents two watermarking schemes that are secure when considering the Watermarked content Only Attack (WOA) framework. The definition of watermark security is first recalled and the distinction between key-security and subspace- security classes for Spread Spectrum (SS) watermarking 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 classical 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 NWprevents estimations both of the watermark subspace (subspace-security) and the different carriers (key-security). We then extend the nice properties 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 implementation 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.