用菲涅耳变换和相位密码板实现盲数字水印

Blind digital watermarking technique based on Fresnel diffraction transform and phase encryption mask

基于菲涅耳衍射变换和相位密码板,结合离散小波变换,设计了一种新的盲数字水印算法。通过提取相位母板的离散菲涅耳衍射变换的相角而获得相位密码板,原始水印图像经离散菲涅耳变换和相位密码板的共同作用形成水印密文,将水印密文嵌入到原始宿主图像的离散小波变换系数矩阵中,再对叠加水印信息的像素用其最邻近的像素均值来替换,从而获得一种新的盲数字水印提取法。数值计算结果表明:该水印算法对JPEG有损压缩、剪切、噪声污染和重采样等攻击具有较强的鲁棒性。由于该算法的密钥空间很大而密钥数据量却不高,所以采用本算法既能灵活选择加密密钥(相位密码母板、衍射距离等),又能确保高安全性,具有很高的实用价值。

The blind digital watermarking algorithm based on Fresnel diffraction transform, phase encryption mask and discrete wavelet transform（DWT） is proposed. The mother phase image is transformed by discrete Fresnel diffraction transform （DFDT） ,then the phase angles are withdrawn to form the phase encryption mask; the original watermark image is encrypted into the cipher text by the phase encrypted mask after DFDT;the original host image is transformed by DWT, the pixels of high frequency parts are modified with the encoded watermark under weight factor,which is replaced by the average of its adjacent four-pixel neighborhoods; finally, the watermarked image is obtained after inverse DWT. Numerical experiments show that the algorithm is robust to the attacks of the JPEG lossy compressing,cropping,superposing noise and resampling from the screen copy. The key space is very large,but the quantity of the keys is not high,so the encryption keys （phase encryption mask,many diffraction distances） can be designed in a large degree of freedom,and the algorithm has high security and practical value.