rot13 /rot ther'teen/ n.,v. [Usenet: from `rotate alphabet 13
places'] The simple Caesar-cypher encryption that replaces each English
letter with the one 13 places forward or back along the alphabet
[…] It is used to enclose the text in a sealed wrapper that the
reader must choose to open – e.g., for posting things that might
offend some readers, or spoilers. A major advantage of rot13 over
rot(N) for other N is that it is self-inverse, so the same code can be
used for encoding and decoding.