Improved Anonymity for Key-Trees

Randomized hash-lock protocols for Radio Frequency IDentification (RFID) tags offer forward untraceability, but 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 et al. have defined measures for the resulting loss of anonymity in the system, and approximated their measures by means of simulations. We will further improve upon their trees, and provide a proof of optimality. Finally, an efficient recursive algorithm is presented to compute the anonymity measures.