We propose a lightweight RFID authentication protocol that supports forward and backward security. The only cryptographic mechanism that this protocol uses is a pseudo-random number generator (PRNG) that is shared with the backend Server. Authentication is achieved by exchanging a few numbers (3 or 5) drawn from the PRNG. The protocol is optimistic with constant lookup time, and can be easily ... [Show full abstract] adapted to prevent online man-in-the-middle relay attacks. Security is proven in the UC security framework.