Targeted dewatermarking of two non-blind SVD-based image watermarking schemes

Digital watermarking has become an important technique for copyright protection of multimedia contents. In recent years, Singular Value Decomposition (SVD) has been used as a valuable transform technique for robust digital watermarking. Designers mostly use dewatermarking tools such as Stirmark to prove the robustness of watermarking schemes. Distortions available in these tools usually degrade the quality of images and resistance against these distortions does not imply the scheme is secure. Despite of distortions, there are many types of attacks which violate security. There is a kind of attack in which the attacker needs to know the watermarking algorithm to perform the attack. In this paper, two attacks of this kind have been designed against two specific non-blind SVD-based watermarking schemes. These attacks try to subtract the added watermark or change the exact spaces where watermarks have been added. The experiments show our attacks make fewer changes to remove embedded watermarks than other distortions available in Stirmark, so the quality of attacked images is better than processed images by using Stirmark in terms of PSNR.