The SFS programming libraries were originally written by David Mazières for the Self-Certifying FileSystem (see www.fs.net). OKWS does not require the whole SFS system (though it is compatible with it); rather, it just requires the SFS programming libraries (made available through sfslite.)
At its core, SFS has libraries for asynchronous networking programming, SUN RPC, and also cryptography.
A white paper on programming in this idiom is available here.