The present invention relates to a gray-level watermarking system, in which sub-band and codebook technique are applied, and the feature of progressive transformation is utilized so as to randomly and evenly distribute the watermark to the entire data with the robustness and the visibility reduced. A four-fold key system is developed with encrypted key to exact the watermark, thus forming a system capable of protecting internet-transmitted multimedia signals such as audio, video ones that are combined with the watermark from being detected or attacked.