I don't think it was an either or question though. Crypto.rot13(x) would of course also be a possiblity.
+1, if it's an alias for your implementation, only with a fix key==13. (That way, it works a bit like the hash functions, and leaves kind of an implementation paper trail, presuming someone wanted to use another key. By which I mean rot13 is good, and a magic rot%d is not good. :-)