On top of another
One opensource (OpenSource?) project builds on top of another. EncFS is an encrypted file system for linux, runs in userspace (wow), can share an existing filesystem (files that aren't encrypted, double wow). It is built on top of
fuse, Filesystem in Userspace.
http://www.linuxdevcenter.com/pub/a/linux/2005/04/14/encfs.html
Convoluted it is!
That's a weird way to point to
20q.net. Oh well. There is actually a ton of other links to stuff that's just as time wasting.
http://www.pcmag.com/article2/0,1759,1782166,00.asp