A POCS-based algorithm to attack image watermarks embedded in DFT amplitudes

During recent years, digital watermarking has drawn a lot of attention as a means to protect the copyright of image data. Numerous algorithms have been proposed in different domains. Particularly, a set of these algorithms embed the watermark information in the amplitudes of the discrete Fourier transform (DFT) coefficients. However, studies have shown that the image can be reconstructed by using only the DFT phase information. It is a common belief that watermarking algorithms can be improved by evaluating possible attacking methods and modifying the algorithms accordingly. We propose an algorithm to attack the watermarks embedded in DFT amplitudes. The proposed attacking algorithm uses projection onto convex sets (POCS). The results show that it may be necessary to embed the watermark information in both amplitudes and phases of the DFT coefficients.