- https://help.github.com/articles/generating-a-new-gpg-key/ (and follow other links)
- http://blog.ghostinthemachines.com/2015/03/01/how-to-use-gpg-command-line/
- http://harryrschwartz.com/2014/11/01/automatically-signing-your-git-commits.html
Sum up:
brew install gnupg