Watermark detection based on the properties of error control codes

Watermark detection is a topic which is seldom addressed in the watermarking literature. Most authors concentrate on developing novel watermarking algorithms. In a practical watermarking system, however, one must be able to distinguish between watermarked and unwatermarked documents. Many existing systems belong to the class of so called ´yes/no´ watermarks, where the detector correlates the candidate image with some known sequence to determine whether a mark is present. Unfortunately, these watermarks often carry no extra information and are not very useful. On the other hand, multi-bit watermarking schemes typically use a separate reference watermark and the payload of the watermark is decoded only when this reference watermark is successfully detected in the received image. It is shown that it is not necessary to use a reference watermark for detection purposes if the watermark payload is encoded with an error control code. One can then put all the energy into the payload watermark and increase its robustness. The turbo code is used as an example of error control codes in the work presented, and simulation results using an algorithm based on the authors´ previous work verifies their theory.