ROT13 one ASCII character
C (77 64 chars)

char f(char c){return(c&gt;64&amp;&amp;c&lt;91)?65+(c-52)%26:(c&gt;96&amp;&amp;c&lt;123)?97+(c-84)%26:c;}

char f(char c){return(isalpha(c))?65+(c&amp;32)+(c-52-(c&amp;32))%26:c;}