Based on the strongSwan Open Source project:

  http://www.strongswan.org

The App and the charon-xpc privileged helper tool are licensed under the General Public License Version 2. The strongSwan source code is available for download at:

  http://download.strongswan.org/

The App and the charon-xpc helper sources are not part of the main strongSwan distribution, but available separately under:

  http://download.strongswan.org/osx/

The osx-sources tarball should get extracted into the strongSwan sources root directory. The README.md in src/frontends/osx has more information about building charon-xpc and the App.