The open source code is available for inspection.
The fundamental techniques are described in Sliding Block Ciphers.
The fpwdman file format is described in the section on importing and exporting password files.
Gathering and computing entropy has a few subtleties.
A library to support Sliding Block Ciphers is provided in the sbc subdirectory. A reference implementation, sbcref, is provided and documented in that subdirectory, along with the demo-sbc.sh script that demonstrates various uses of SBC.