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Protocol attacks against watermarking schemes pose a threat to modern digital rights management systems; for example, a successful attack may allow to copy a watermark between two digital objects or to forge a valid watermark. Such attacks enable a traitor to hinder a dispute resolving process or accuse an innocent party of a copyright infringement. Secure DRM systems based on watermarks must therefore prevent such protocol attacks. In this paper we introduce a formal framework that enables us to assert rigorously the security of watermarks against protocol attacks. Furthermore, we show how watermarking schemes can be secured against some protocol attacks by using a cryptographic signature of a trusted third party.

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Watermarking schemes embed information imperceptibly in digital objects and are proposed as primitives in various copyright protection applications, such as proofs of authorship, dispute resolving protocols or fingerprinting. In many applications, the presence of watermarks must be provable to any possibly dishonest party. Traditionally, watermark detection requires knowledge of sensitive information like the watermark or the embedding key. This is a major security risk, since this information is in most cases sufficient to remove the watermark and to defeat the goal of copyright protection. Zero-knowledge watermark detection is a promising approach to overcome security issues during the process of watermark detection: cryptographic techniques are used to prove that a watermark is detectable in certain data, without jeopardizing the watermark. This paper presents a formal definition of zero-knowledge watermark detection, discusses zero-knowledge watermark detection protocols and compares their properties.