Source code on github: https://github.com/jedisct1/dsvpn
This came up when I was browsing through the trending page on github. It is a project that aims to ship all core functionalities of a virtual private network in as few lines as possible. The whole code repository is about 25kb only, and the steps to use it can be explained within one paragraph of text.
The code seems to be fairly straightforward and easy to understand. It is entirely written in C. I think this would be a good starting point for me to contribute towards open-source projects.
Bo 