This is CFS, Matt Blaze's Cryptographic File System. It provides transparent encryption and decryption of selected directory trees. It is implemented as a user-level NFS server and thus does not require any kernel modifications. For an overview of how to use it, read "${PREFIX}/share/doc/cfs/notes.ms" and the manual pages. There is a paper describing CFS at: http://www.crypto.com/papers/cfs.pdf