The image authenticity confirmation by modifying spectral components

The method for providing digital image authenticity, using the ´digital watermark´ technique, is described. The protection method is based on the modification of the spectral coefficients in the discrete cosine transform (DCT) domain. In order to preserve both the watermark invisibility and its resistance to compression procedure (as JPEG) the low-frequency DCT coefficients of the watermark are inserted into the middle-frequency positions of the original image in every 8/spl times/8 DCT block, satisfying the zig-zag ordering. Our technique also ensures robustness to various image processing algorithms such as common geometric distortion, brightness and contrast modification, JPEG compression, as illustrated on several examples. The protected image and the watermark signal are the same size, providing not only the authenticity confirmation but also some kind of the secure transfer of another image using the same image area.