[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, so that The butler did
it!
becomes Gur ohgyre qvq vg!
Most Usenet news
reading and posting programs include a rot13 feature. 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. See
also spoiler space, which has partly displaced rot13
since non-Unix-based newsreaders became common.