protected function encipher(&$xl, &$xr)
  {
    for($i = 0; $i < 16; $i++)
    {
      $temp = $xl ^ $this->p[$i];
      $xl = ((($this->s[0][($temp>>24) & 255] + $this->s[1][($temp>>16) & 255]) ^ $this->s[2][($temp>>8) & 255]) + $this->s[3][$temp & 255]) ^ $xr;
      $xr = $temp;
    }

    $xr = $xl ^ $this->p[16];
    $xl = $temp ^ $this->p[17];
  }