private function _closeAgentLaunchProcess()
    {
        if (is_resource($this->_agentProcess)) {
            $this->_debug('CLOSING GPG-AGENT LAUNCH PROCESS');

            // close agent pipes
            foreach ($this->_agentPipes as $pipe) {
                fflush($pipe);
                fclose($pipe);
            }

            // close agent launching process
            proc_close($this->_agentProcess);

            $this->_agentProcess = null;
            $this->_agentPipes   = array();

            $this->_debug('GPG-AGENT LAUNCH PROCESS CLOSED');
        }
    }