protected function _openCipher()
    {
        $cipher = mcrypt_module_open(
            $this->_encryption['algorithm'],
            $this->_encryption['algorithm_directory'],
            $this->_encryption['mode'],
            $this->_encryption['mode_directory']);

        if ($cipher === false) {

            throw new Zend_Filter_Exception('Mcrypt can not be opened with your settings');
        }

        return $cipher;
    }