Feistel networks are a central element of DES and other encryption algorithms. In 2006, I published a (German) tutorial on Feistel networks in the German developer forum Entwickler-Ecke (formerly “Delphiforum”), together with two simple implementations in Delphi and C++. The original topic can be found in the Delphi Library. Alternatively, a PDF version of it is made available below.
Erläuterung und Implementation eines einfachen Feistel-Netzwerks