Abstract: Aiming at two important issues in the watermark system,public watermark detection and geometric attack, a public-key watermark algorithm is proposed, which is resistant to geometric attacks. The embedment and extraction of watermark are based on the normalized image. The normalized image is unique under a certain size when the image is under affine geometric attacks. The private key is used to generate embedding watermarks. The public key is generated by the private key through phase-shift, and used in watermark detection. Through the correlation estimation, the watermark is extracted bit by bit. The results from Stirmark software show that this algorithm is robust to a wide range of geometric attacks. With the use of the public key to the watermark extraction, the public verification of a watermark can be achieved.

Key words: public watermark detection, normalized image, geometric attack, phase-shift