Abstract: To enhance the algorithm's ability of anti-regional attacks of digital images, a secure and highly robust digital watermarking algorithm is proposed. The concept of Random Quaternary Group (RQG) is introduced, and the RQGs of the host image are used to hide the original redundant watermark bits and the mean value of the 8×8 blocks of the RQGs. Before the watermark embedding process, the original watermark is encrypted by chaotic sequence; the performance evaluation function of the proposed algorithm is applied to search the high and low limits of the quantization step of watermark embedding. In watermark embedding process, the algorithm is used to select the optimal quantization step adaptively according to the content of each block. Experiment results show that the RQG strategy is secure; the proposed method has good watermark transparency and high robustness resisting conventional regional attack and other attacks of digital images.

Key words: computer application, regional attacks, random quaternary group, quantization step, watermark transparency, robustness