RFID(Radio Frequency Identification) system will play a critical role providing widespread services in a ubiquitous environment. However, widespread use of RFID tags may create new threats to a consumer privacy such as information leakage and traceability. It is difficult to solve the problems because a tag has the limited computing power that lacks of supporting the general encryption. Although the scheme of  protects a consumer privacy using an external agent, a tag should perform exponential operation requiring high cost. We propose
Self Re-Encryption Protocol
(SREP) which provides strong privacy without help of any external agent. SREP is well suitable for low cost RFID systems since it only needs multiplication and exclusive-or operation.