code: def _encrypt_message(self, msg, nonce, timestamp=None):
          
          xml = """<xml>
  <Encrypt><![CDATA[{encrypt}]]></Encrypt>
  <MsgSignature><![CDATA[{signature}]]></MsgSignature>
  <TimeStamp>{timestamp}</TimeStamp>
  <Nonce><![CDATA[{nonce}]]></Nonce>
  </xml>"""
          nonce = to_binary(nonce)
          timestamp = to_binary(timestamp) or to_binary(int(time.time()))
          encrypt = self.__pc.encrypt(to_text(msg), self.__id)
          # 生成安全签名
          signature = get_sha1_signature(self.__token, timestamp, nonce, encrypt)
          return to_text(xml.format(
              encrypt=to_text(encrypt),
              signature=to_text(signature),
              timestamp=to_text(timestamp),
              nonce=to_text(nonce)
          ))
docs: 将公众号回复用户的消息加密打包
  
          :param msg: 待回复用户的消息，xml格式的字符串
          :param nonce: 随机串，可以自己生成，也可以用URL参数的nonce
          :param timestamp: 时间戳，可以自己生成，也可以用URL参数的timestamp,如为None则自动用当前时间
          :return: 加密后的可以直接回复用户的密文，包括msg_signature, timestamp, nonce, encrypt的xml格式的字符串