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:

    https://www.scs.stanford.edu/nyu/02fa/lab/cfs.pdf