对密文进行解密
        :param text: 需要解密的密文
        :param app_id: 微信公众平台的 AppID
        :return: 解密后的字符串

def decrypt(self, text, app_id):
        """
        对密文进行解密
        :param text: 需要解密的密文
        :param app_id: 微信公众平台的 AppID
        :return: 解密后的字符串
        """
        text = to_binary(text)
        decryptor = self.cipher.decryptor()
        plain_text = decryptor.update(base64.b64decode(text)
                                      ) + decryptor.finalize()

        padding = byte2int(plain_text, -1)
        content = plain_text[16:-padding]

        xml_len = socket.ntohl(struct.unpack("I", content[:4])[0])
        xml_content = content[4:xml_len + 4]
        from_appid = content[xml_len + 4:]

        if to_text(from_appid) != app_id:
            raise AppIdValidationError(text, app_id)

        return xml_content