Watermark based access control to copyrighted content

One application of digital watermarks deals with control of copying, playback, transmission and other uses of copyrighted content. In this application, the watermark extractor is typically in user-controlled devices, which means that an attacker could mount the oracle attack, or attempt reverse engineering of the watermarking technology. Furthermore, the watermark extractor has to use limited device resources in order to maintain low implementation cost. On the other hand, in this application the payload is relatively small, and watermarks are typically embedded redundantly. In this paper, we describe methods to utilize the embedding redundancy to maximize robustness and security of the system while maintaining low implementation cost. Some of the proposed techniques include iterative extraction process based on initial sparse and random sampling of the distortion space, followed by partial or tentative watermark extraction that triggers extractor adjustment to estimated distortion parameters. Distortion estimation is based on partial watermark matching to pre-distorted templates which can be preselected using experimental approach based on pilot signals. Upon detection of one or more partial or tentative watermarks a new selection of pre-distorted templates is designed to improve distortion resolution and extraction reliability. Extrapolation of watermark segments in the vicinity of partial or tentative watermark extraction is done in view of estimated distortion and an algorithm for combining results of multiple extrapolation segments is proposed. The techniques proposed here allow reduction in processing load and reduction in false positive rates, which in turn can be used to increase watermark robustness to distortion and improved overall security of the system.