It's dumb. It's something like

perl -le'$_=shift;s/(.)/chr( 13 + ord $1)/ge;print' abcdef
nopqrs


--- [from jargon] ---
rot13 /rot ther'teen/ n.,v. [Usenet: from `rotate alphabet 13 places']
   The simple Caesar-cypher encryption that replaces each English lett
+er
   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 readi
+ng
   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 decodin
+g.
   See also {spoiler space}, which has partly displaced rot13 since
   non-Unix-based newsreaders became common.
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