var normall = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
var rotall  = "nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM";
function rottext(testo) {
  risp = "";
  for(i=0; i<testo.length; i++) {
    thechar = testo.substr(i,1);
    x = normall.indexOf(thechar);
    risp += x > -1 ? rotall.substr(x,1) : thechar;
  }
  return risp
}

function rot() {
  document.getElementById("testo").value = rottext(document.getElementById("testo").value);
  document.getElementById("testo").focus();
  document.dati.testo.select();
}