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One of the main properties of digital watermarking methods is their security against attacks. In this paper, a novel attack against an adaptive semi-fragile image watermarking is proposed. In this attack, watermarking key and watermark are estimated by using several watermarked images. In order to improve the watermarking scheme against the proposed attack, the entropy of image blocks is utilized. Using entropy is compatible with Human Visual System (HVS); therefore it is suitable to determine the weight of watermark in image blocks. Since entropy is a sensitive feature, it is used to improve the watermarking method performance against the proposed attack. It is shown that this modification does not affect the fidelity of the watermarked images.