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From Jargon File (4.3.1, 29 Jun 2001)
  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.
     See also {spoiler space}, which has partly displaced rot13 since
     non-Unix-based newsreaders became common.
  • ROT13(ROT13(x)) = x (for any text x)
  • conversion table for code (decryption and encryption)