Buyer-seller watermarking (BSW) protocols allow copyright protection of digital content. The protocol is anonymous when the identity of buyers is not revealed if they do not release pirated copies. Existing BSW protocols are not provided with a formal analysis of their security properties. We employ the ideal-world/real-world paradigm to propose a formal security definition for copyright protection protocols, and we analyze an anonymous BSW protocol and prove that it fulfills our definition. Additionally, we implement the protocol and measure its efficiency.