Privacy for Key-Trees with Adaptive Adversaries⋆

Hash-lock authentication protocols for Radio Frequency IDentification (RFID) tags incur heavy search on the server. Key-trees have been proposed as a way to reduce search times, but because partial keys in such trees are shared, key compromise affects several tags. Buttyán [4] and Beye & Veugen [3] devised trees to withstand such attacks, but assumed adversaries to be non-adaptive, without access to side-channel information. We argue that side-channel information is in practice readily available, and study its implications on anonymity in key trees. Theoretical analysis leads to new requirements and a new tree construction. Simulation is used to test its performance and shows an improved resistance to adaptive adversaries.