code: def _decrypt_message(self, msg, msg_signature, timestamp, nonce):
          
          timestamp = to_binary(timestamp)
          nonce = to_binary(nonce)
          if isinstance(msg, six.string_types):
              try:
                  msg = xmltodict.parse(to_text(msg))['xml']
              except Exception as e:
                  raise ParseError(e)
  
          encrypt = msg['Encrypt']
          signature = get_sha1_signature(self.__token, timestamp, nonce, encrypt)
          if signature != msg_signature:
              raise ValidateSignatureError()
          return self.__pc.decrypt(encrypt, self.__id)
docs: 检验消息的真实性，并且获取解密后的明文
  
          :param msg: 密文，对应POST请求的数据
          :param msg_signature: 签名串，对应URL参数的msg_signature
          :param timestamp: 时间戳，对应URL参数的timestamp
          :param nonce: 随机串，对应URL参数的nonce
          :return: 解密后的原文