Pre vs Post State Update: Trading Privacy for Availability in RFID

Designing lightweight RFID protocols that support strong privacy is a major challenge. For anonymity tags use pseudonyms that are refreshed with every interrogation (whether completed or not). For forward secrecy, the state of tags must be updated and it must be hard to reverse updates. Since the interrogating reader can be adversarial, the adversary may control state updates. It follows that it may not be possible for tags to maintain synchrony with authorized readers. In this letter we analyze a recently proposed RFID protocol and show that there is a fundamental trade-off between privacy and availability. We prove that for lightweight RFID applications strong privacy cannot be achieved in the presence of a Byzantine adversary.