Analysis on the sensitivity attack to watermarking schemes with patchwork detection boundaries

The sensitivity attack is a main threat to the security of watermarking schemes with open detectors. By the attempts across the detection boundary, the attackers gain adequate information of the embedded watermark to remove it without introducing serious distortions into the watermarked works. In some image watermarking schemes, the detection boundaries are the patchworks of certain numbers of hyperplanes. Here the existing sensitivity attacks are not suitable anymore for the detection functions have numbers of different gradients. The letter proposed a new sensitivity attack in which the attacking directions were calculated with a certain number of gradients estimated separately with the old sensitivity attack. Our experiments show the new attack can remove the watermarks successfully without seriously tampering the fidelity.