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Analysis by Forrester research revealed that 18% of global exports will flow online in 2004 and that the volume of e-commerce will surpass $400 billion. Digital rights protection is a major issue in the e-commerce of multimedia contents. Watermarking technology has been proposed as a promising enabling technology for the rights protection of multimedia digital contents. A unique watermark is embedded in each piece of multimedia contents before it is distributed to a customer. When unauthorized copies of a piece of contents are found, the customer who owns the contents can be readily identified by means of the embedded watermark. However, the unauthorized copies may also come from the content provider itself. It is therefore a challenging problem to determine whether an unauthorized copy is distributed by an unethical customer or by an unethical content provider. In this paper, we propose a watermarking protocol to address the problem using cryptographic technologies. Our protocol employs a commutative encryption algorithm to protect the privacy of watermarks. Information is doubly locked by two encryption keys kept separately by a customer and a content provider. In the protocol, a customer only gets a piece of watermarked multimedia contents in a transaction and the content provider has no idea how the watermark is formed. This facilitates the authority to determine the unethical party in case of unauthorized distribution of digital contents. We also discuss a couple of common attacks and show that our protocol can defend successfully against them.