Abstract
Most digital watermarking algorithms are not robust against geometric attacks. In this paper several improvements of watermarking algorithms are proposed. It may present additional advantages in terms of detection and recovery from geometric attacks by exploiting chaotic spread spectrum and synchronization techniques. These improvements come from three aspects. First, based on the randomness, noise like and extreme sensitivity to initial conditions of chaotic signal, chaotic sequence is exploited as spread spectrum code in the watermarking algorithms. The code spread from chaotic sequence has better performance than pseudo random code because it is massive, arbitrary length and high security. The robustness of the watermarking is improved greatly due to the original watermarking signal is modulated in broadband chaotic signal. Second, a synchronization code generated from chaotic sequence is inserted into the watermarking signal at specified intervals. A watermark cannot be detected correctly after geometric attacks because the synchronization information is lost in watermarking extracting. By locating the synchronization code, re-synchronization can be achieved. Hence, the watermark can be recovered furthest from geometric attacks. Third, the extracted watermark will be post processed with noise reduction filter in order to improve the accuracy of watermarking verification. Based on the characteristics of chaotic sequence, the noise signal can be separated from chaotic background by exploiting wavelet multi-scaling decomposition method. These improvements, which take full advantage of the characteristic of chaotic sequences, are feasible to resist geometric attacks. Experiment results demonstrate its effectiveness.
