Securing symmetric watermarking schemes against protocol attacks

With the advent of the web and the creation of electronic distribution channels for multimedia objects, there is an increased risk of copyright infringements. Content providers try to alleviate this problem by using copyright protection facilities that often involve watermarking schemes as primitives. Clearly, the intention of the content provider can be subverted if the watermarking scheme is susceptible to intentional attacks, especially to attacks on the robustness of watermarks. It was noted early during the development of watermarking algorithms that the intention of resolving the copyright situation might be subverted entirely without removing any watermark contained in multimedia objects. Indeed, so-called protocol attacks try to introduce some sort of ambiguity during the copyright resolution process. After providing formal definitions for some common protocol attacks, we discuss the possibility of constructing watermarking schemes that are provably secure against ambiguity and copy attacks. Although there were several previous attempts to secure watermarking schemes against protocol attacks, we provide for the first time a formal security proof of our scheme. The security of the construction is based on a cryptographic primitive, namely an unforgeable public-key signature scheme, that is used to constrain the watermarking bits to have a specific form.