rot13

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, 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 {spoiler}s.  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.