The RFID technology has ample growth potential since it can be applied to practically all areas, ranging from retail and logistics sectors to livestock management, home network systems, traffic control and hospital patient management. With the advent of this technology, it is possible to realize ubiquitous sensor network (USN) such that every object is given an RFID tag giving them a digital identity, so-called ubiquitous tag or ubiquitous sensor. However, it is clear that the biggest challenge for RFID technology is to provide benefits without threatening the privacy of information. Many solutions have been suggested for RFID systems, but none of them suits to the USN environment because of their security weakness or non-scalability. In this paper, we propose a secure and scalable transaction (SST) protocol for USN using RFID technology. The SST protocol uses the advanced encryption standard (AES) algorithm for authentication and encryption so that it can be free from tag cloning and privacy attack. Moreover, it is scalable in a sense that an RFID reader does not need to perform a shared key distribution and management. In analyzing SST protocol, we show how forward privacy is guaranteed and demonstrate the results.