Automatic recovery of invisible image watermarks from geometrically distorted images

One method of attacking an imbedded invisible watermark is to create a derivative image that is geometrically distorted relative to the original. One attack, developed at Cambridge University, is called StirMark.' Image-distorting methods modify images so subtly that the changes are essentially unnoticeable to a viewer. However, their effect on invisible watermarks can be devastating, rendering them unextractable. In this paper, an automated countermeasure to image-distorting attacks will be described. Employing an unmarked copy of the original image as a reference, the possible distortion in a suspect image is first detected by the method, then measured, and finally reversed, producing a restored image approximately geometrically aligned with the original. Using a robust invisible watermarking method presented previously by one of the authors to produce a watermarked image, StirMark' to distort the watermarked image, and a copy of the original unmarked image for reference, the restoration method is demonstrated to be sufficient by showing successful extraction of the imbedded watermark from a restored image.