Still in bsd-games, here’s rot13.
rot13 might be the easiest encryption ever made, except for that silly number code you used in primary school to send love notes to the cute kid who sat next to you in class. Don’t be embarrassed; we all did it.
Much as you can see in the screenshot, rot13 spins text through the cipher and sends out the results. Route that same coded text through rot13, and you get the original text. Magic! 😯
I can’t think of what rot13 might be useful for, except as a simple encryption for the weakest possible security. Or maybe to send e-mails to the cute grown-up who sits next to you at the office. 😉