1. Physically Unclonable Function– Based Security and Privacy in RFID Systems Leonid Bolotnyy and Gabriel Robins Dept. of Computer Science University of Virginia www.cs.virginia.edu/robins

2. Contribution and Motivation Contribution Privacy-preserving tag identification algorithm Secure MAC algorithms Comparison of PUF with digital hash functions Motivation Digital crypto implementations require 1000’s of gates Low-cost alternatives –Pseudonyms / one-time pads –Low complexity / power hash function designs –Hardware-based solutions

3. PUF-Based Security Physical Unclonable Function (PUF) [Gassend et al 2002] PUF Security is based on –wire delays –gate delays –quantum mechanical fluctuations PUF characteristics –uniqueness –reliability –unpredictability PUF Assumptions –Infeasible to accurately model PUF –Pair-wise PUF output-collision probability is constant –Physical tampering will modify PUF

4. Privacy in RFID Privacy ABC Alice was here: A, B, C privacy

5. Private Identification Algorithm Assumptions –no denial of service attacks (e.g., passive adversaries, DoS detection/prevention mechanisms) –physical compromise of tags not possible It is important to have –a reliable PUF –no loops in PUF chains –no identical PUF outputs ID Request p(ID) ID Database ID 1, p(ID 1 ), p 2 (ID 1 ), …, p k (ID 1 )... ID n, p n (ID n ), p n 2 (ID n ), …, p n k (ID n )

6. Improving Reliability of Responses Run PUF multiple times for same ID & pick majority μ m (1-μ) N-m ) k R( μ, N, k ) ≥ (1 - ∑ N NmNm N+1 2 m= number of runs chain length unreliability probability overall reliability R(0.02, 5, 100) ≥ 0.992 Create tuples of multi-PUF computed IDs & identify a tag based on at least one valid position value ∞ expected number of identifications S( μ, q ) = ∑ i [(1 – (1- μ ) i+1 ) q - (1 – (1-μ) i ) q ] i=1 tuple size S(0.02, 1) = 49, S(0.02, 2) = 73, S(0.02, 3) = 90 (ID 1, ID 2, ID 3 )

7. Privacy Model 1.A passive adversary observes polynomially-many rounds of reader-tag communications with multiple tags 2.An adversary selects 2 tags 3.The reader randomly and privately selects one of the 2 tags and runs one identification round with the selected tag 4.An adversary determines the tag that the reader selected Experiment: Definition: The algorithm is privacy-preserving if an adversary can not determine reader selected tag with probability substantially greater than ½ Theorem: Given random oracle assumption for PUFs, an adversary has no advantage in the above experiment.

8. PUF-Based MAC Algorithms MAC based on PUF –Motivation: “yoking-proofs”, signing sensor data –large keys (PUF is the key) –cannot support arbitrary messages MAC = (K, τ, υ) K K valid signature σ : υ (M, σ) = 1 forged signature σ’ : υ (M’, σ’) = 1, M = M’ Assumptions –adversary can adaptively learn poly-many (m, σ) pairs –signature verifiers are off-line –tag can store a counter (to protect against replay attacks)

9. Large Message Space σ (m) = c, r 1,..., r n, p c (r 1, m),..., p c (r n, m) Assumption: tag can generate good random numbers (can be PUF-based) Signature verification requires tag’s presence password-based or in radio-protected environment (Faraday Cage) learn p c (r i, m), 1 ≤ i ≤ n verify that the desired fraction of PUF computations is correct To protect against hardware tampering authenticate tag before MAC verification store verification password underneath PUF Key: PUF

10. Choosing # of PUF Computations α < prob v ≤ 1 and prob f ≤ β ≤ 1 0 ≤ t ≤ n-1 i=t+1 μ i (1-μ) n-i prob v (n, t, μ) = 1 - ∑ n nini j=t+1 τ j (1-τ) n-j prob f (n, t, τ) = 1 - ∑ n njnj prob v (n, 0.1n, 0.02) prob f (n, 0.1n, 0.4)

11. Theorem Given random oracle assumption for a PUF, the probability that an adversary could forge a signature for a message is bounded from above by the tag impersonation probability.

12. Small Message Space Assumption: small and known a priori message space Key[p, m i, c] = c, p c (1) (m i ),..., p c (n) (m i ) PUF message counter σ(m) = c, p c (1) (m),..., p c (n) (m),..., c+q-1, p c+q-1 (1) (m), p c+q-1 (n) (m) sub-signature Verify that the desired number of sub-signatures are valid PUF reliability is again crucial

13. Theorem Given random oracle assumption for a PUF, the probability that an adversary could forge a signature for a message is bounded by the tag impersonation probability times the number of sub-signatures.

14. Attacks on MAC Protocols originalclone Impersonation attacks –manufacture an identical tag –obtain (steal) existing PUFs Hardware-tampering attacks –physically probe wires to learn the PUF –physically read-off/alter keys/passwords Side-channel attacks –algorithm timing –power consumption Modeling attacks –build a PUF model to predict PUF’s outputs

15. Comparison of PUF With Digital Hash Functions Reference PUF: 545 gates for 64-bit input –6 to 8 gates for each input bit –33 gates to measure the delay Low gate count of PUF has a cost –probabilistic outputs –difficult to characterize analytically –non-unique computation –extra back-end storage Different attack target for adversaries –model building rather than key discovery Physical security –hard to break tag and remain undetected MD4 7350 MD5 8400 SHA-256 10868 Yuksel 1701 PUF 545 AES 3400 algorithm # of gates

16. PUF Design Attacks on PUF –impersonation –modeling –hardware tampering –side-channel Weaknesses of existing PUF New PUF design –no oscillating circuit –sub-threshold voltage Compare different non-linear delay approaches reliability

17. Conclusions and Future Work Develop theoretical framework for PUF Design new sub-threshold voltage based PUF Manufacture and test PUFs –varying environmental conditions –motion, acceleration, vibration, temperature, noise Design new PUF-based security protocols –ownership transfer –recovery from privacy compromise –PUFs on RFID readers } in progress PUF: hardware primitive for RFID security Identification and MAC algorithms based on PUF PUFs protect tags from physical attacks PUFs is the key

18. Thank You Questions ? Leonid Bolotnyy lbol@cs.virginia.edu Dept. of Computer Science University of Virginia

19. PUF-Based Ownership Transfer Ownership Transfer To maintain privacy we need –ownership privacy –forward privacy Physical security is especially important Solutions –public key cryptography (expensive) –knowledge of owners sequence –trusted authority –short period of privacy

20. s 2,4 s 1,2 s 3,9 s 2,5 s 3,10 s 3,8 Using PUF to Detect and Restore Privacy of Compromised System 1.Detect potential tag compromise 2.Update secrets of affected tags s 1,0 s 2,0 s 1,1 s 2,1 s 3,1 s 2,2 s 2,3 s 3,0 s 3, 4 s 3,5 s 3,2 s 3,3 s 3,7 s 3,6

21. Related Work on PUF Optical PUF [Ravikanth 2001] Silicon PUF [Gassend et al 2002] –Design, implementation, simulation, manufacturing –Authentication algorithm –Controlled PUF PUF in RFID –Identification/authentication [Ranasinghe et al 2004] –Off-line reader authentication using public key cryptography [Tuyls et al 2006]