Anti-geometric Attacks Digital Watermarking

As an effective way to solve the problem of copyright protection and content authentication, the watermarking technology has been used to protect digital products. It can embed some special message into the digital image, audio, video and others. The watermark could be extracted to prove the ownership of the original author or integrity of the product when it is necessary.This thesis made a complete description of the concept of watermarking, its features, classification, system model, evaluation criterion, classical watermarking algorithms and attack methods. Then two robust watermarking algorithms against geometric attacks are proposed.(1) DCT-based watermarking algorithm against rotation attacks. The signs of DCT coefficients are little changed after the image was attacked. Select a rotation invariant region of an image as the watermarked region can reduce the degree of distortion of the watermarked image. The watermark was embedded into the region by modifying the signs of some coefficients of the DCT. This algorithm can achieve robustness against small-angle rotation attacks. The watermark can be extracted blindly without regulating the watermarked image. As the keys of the watermark, the radius of rotation invariant region, the starting position and the length of coefficients can guarantee its security. The experiment results show that the watermark is invisible and robust against common attacks. When the watermarked image suffers large rotation angle, it needs to be regulated before the watermark is extracted.(2) An adaptive watermarking algorithm based on DWT and HVS. At first, the watermark was scrambled by using Arnold transform, and then the original image was decomposed into wavelet coefficients. To embed the binary watermark, the approximate coefficients were blocked and the just noticeable difference value (JND) of each block was calculated to control the strength. The watermark was embedded by modifying the value of the center coefficient and the average value of its eight neighborhood coefficients. To resist geometric attacks, the watermarked image was regulated with the invariance of unchanged centroid. The experiment results show that the algorithm is invisible as well as robust against common attacks.