protected function sign($operation) {
	
		$params = array(
			'AWSAccessKeyId' => $this->accesskey,
			'Timestamp' => gmdate('Y-m-d\TH:i:s.000\Z'),
		);

		$sign_str = 'AmazonS3' . $operation . $params['Timestamp'];
		
		$params['Signature'] = base64_encode(hash_hmac('sha1', $sign_str, $this->secretkey, TRUE));
		
		return $params;
	}