code: def extract(self, xmltext):
          
          try:
              xml_tree = ET.fromstring(xmltext)
              encrypt = xml_tree.find("Encrypt")
              touser_name = xml_tree.find("ToUserName")
              if touser_name != None:
                  touser_name = touser_name.text
              return WXBizMsgCrypt_OK, encrypt.text, touser_name
          except Exception:
              return WXBizMsgCrypt_ParseXml_Error, None, None
docs: 提取出xml数据包中的加密消息
          @param xmltext: 待提取的xml字符串
          @return: 提取出的加密消息字符串