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There is an increased interest, by content creators and owners, in content protection systems that provide the ability to control or restrict the content that can be shared on peer-to-peer file sharing systems. Some content protection systems have been proposed for centralized peer-to-peer systems (such as Napster) where a central authority controls all indexing and querying. These systems cannot be applied to decentralized peer-to-peer systems since they rely on a central server. Also, such systems limit the ability of end-users to effectively share content and can make the peer-to-peer distribution model resemble a client-server model in many respects. In this paper, we propose CITADEL, a novel content protection architecture designed to operate in decentralized peer-to-peer systems (such as Gnutella). CITADEL enforces a range of protection policies while maintaining an open peer-to-peer distribution model. CITADEL builds a protected file sharing environment over a normal peer-to-peer network using secured content objects and file sharing software enhanced to perform protection operations. A flexible content importation system that is part of CITADEL allows all users to insert new content as well as additional copies of protected content.