public static String rot13(String cleartext)
  {
    StringBuffer sb = new StringBuffer(cleartext);
    for (int i = 0; i < sb.length(); i++)
    {
      char ch = sb.charAt(i);
      if (ch >= 'a' && ch <= 'z')
      {
        ch += 13;
        if (ch > 'z') ch -= 26;
      }
      else if (ch >= 'A' && ch <= 'Z')
      {
        ch += 13;
        if (ch > 'Z') ch -= 26;
      }
      sb.setCharAt(i, ch);
    }
    return sb.toString();
  }
