protected function _putencryption()
    {
        $this->_put('/Filter /Standard');
        $this->_put('/V ' . ($this->revision === 3 ? '2' : '1'));
        $this->_put('/R ' . ($this->revision === 3 ? '3' : '2'));
        if ($this->revision === 3) {
            $this->_put('/Length 128');
        }
        $this->_put('/O (' . $this->_escape($this->oValue) . ')');
        $this->_put('/U (' . $this->_escape($this->uValue) . ')');
        $this->_put('/P ' . $this->pValue);
    }