Security Analysis of Public-Key Watermarking Schemes

摘要

Traditional watermarking systems require the complete disclosure of the watermarking key in the watermark verification process. In most systems an attacker is able to remove the watermark completely once the key is known, thus subverting the intention of copyright protection. To cope with this problem, public-key watermarking schemes were proposed that allow asymmetric watermark detection: while a private key is used to insert watermarks in digital objects, a separate, public key is used to verify the marks' presence. We descibe two public-key watermarking schemes which are similar in spirit to zero-knowledge proofs. The key idea of one system is to verify a watermark in a blinded version of the document, where the scrambling is determined by the private key. A probabilistic protocol is constructed that allows public watermark detection with probability of 1/2; by iteration, the verifier can get any degree of certainty that the watermark is present. The second system is based on watermark attacks, using controlled counterfeiting to conceal real watermark data safely amid data useless to an attacker. We discuss the security of these schemes against protocol attacks like inversion, watermark forgery or ambiguity attacks. Furthermore, we we show that public-key watermarks in general can be vulnerable to certain protocol attacks which do not pose a threat in the private-key case. Finally, we discuss the practicality of public-key watermarking, raising the thorny issue of their ultimate usefulness relative to simpler, private-key based schemes.