function rot13 (string) {
  var aCode = 'a'.charCodeAt();
  var zCode = 'z'.charCodeAt();
  var ACode = 'A'.charCodeAt();
  var ZCode = 'Z'.charCodeAt();
  var result = '';
  for (var c = 0; c < string.length; c++) {
    var charCode = string.charCodeAt(c);
    if (charCode >= aCode && charCode <= zCode)
      charCode = aCode + (charCode - aCode + 13) % 26;
    else if (charCode >= ACode && charCode <= ZCode)
      charCode = ACode + (charCode - ACode + 13) % 26;
    result += String.fromCharCode(charCode);
  }
  return result;
}

function write_rot13(string){ document.write(rot13(string)); }