The Paranoid file system is an encrypted, secure, global file system with user managed access control. The system provides efficient peer-to-peer application transparent file sharing. This paper presents the design, implementation and evaluation of the Paranoid file system and its accesscontrol architecture. The system lets users grant safe, selective, UNIX-like, file access to peer groups across administrative boundaries. Files are kept encrypted and access control translates into key management. The system uses a novel transformation key scheme to effect access revocation. The file system works seamlessly with existing applications through the use of interposition agents. The interposition agents provide a layer of indirection making it possible to implement transparent remote file access and data encryption/ decryption without any kernel modifications. System performance evaluations show that encryption and remote file-access overheads are small, demonstrating that the Paranoid system is practical.