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 with- out introducing serious distortions into the watermarked works. In some image watermarking schemes, the detection boundaries are the patchworks of certain numbers of hyper- planes. Here the existing sensitivity attacks are not suitable anymore for the detection functions have numbers of differ- ent gradients. The letter proposed a new sensitivity attack in which the attacking directions were calculated with a cer- tain number of gradients estimated separately with the old sensitivity attack. Our experiments show the new attack can remove the watermarks successfully without seriously tam- pering the fidelity.
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