Physically Unclonable Function-based Security And Privacy In RFID Systems

1. Leonid Bolotnyy and Gabriel Robins Department of Computer Science University of Virginia Presented by Jeffery Barton Physically Unclonable Function-based Security And Privacy In RFID Systems

2. Outline • Introduction • Related Work • PUF-Based Tag Identification Algorithm • PUF-Based MAC Protocols • PUF Vs. Digital Hash Functions • Building PUFs • Conclusion

3. Purpose • What problem are we solving? • Privacy and Security in RFID Systems • Current cryptographic solutions are too expensive • Privacy-preserving tag identification • Secure message authentication codes • Comparisons • Directions for future research Introduction

4. Introduction A Familiar Subject… • What is RFID? • <Insert last two presentations here> • In general  uses radio signals for identity verification • Low-cost  Analogous to sensor networks • What is a PUF? • “Not easy to find random generator”

5. Physically Unclonable Functions • “Random number function that can only be evaluated by a specific instance of the underlying hardware” • Hardware based function • Easy evaluation • Hard characterization • Reliable and unpredictable • What makes it unclonable? Introduction

6. Unclonability • Physical • Inherent random components • Wire/gate delays, manufacturing variations • Hard to define  Even with identical hardware • Challenges mapped to responses = Unpredictable • Mathematical • Hard to compute responses given exact parameters/CRPs • Response = Complex interactions of random components • Modeling with known random values  Oodles of computational effort • Combination of the two = extremely unclonable Introduction - PUFs

7. Related Work • Physical one-Way Functions [16] • Origination – optical PUFs • Controlled Physical Random Functions [7] & Extracting Secret Keys From Integrated Circuits [12] • Silicon prototype • Reliable, can tolerate varying environmental conditions • Variability  PUF circuits across multiple chips • Accurate model difficult (w/polynomially-many i/o pairs) • RFID-Tags for Anti-Counterfeiting [17] • Off-line reader authentication algorithm based on PUFs using public key cryptography • Still too much for low-cost RFID tags Related Work

8. More Related Work • Security and Privacy: Modest Proposals for Low-Cost RFID Systems [15] • Identification/authentication algo based on Silicon Physical Random Functions [8] • No state maintenance/random responses = easy tracking • No access control = easy identification by adversaries • Abundant challenges  more ID time/power consumption • Therefore • Only use challenge-response algos for authentication • Send ID to reader first  less communication & query more challenges • Tag tracking still possible Related Work

9. General Assumptions • Cannot recover PUF model given polynomial # of i/o pairs • τ (op1 = op2) is constant and independent of the # of identical responses from other tags • Hardware tampering = new function • Secure against side-channel attacks • Random function Assumptions

10. PUF-Based Tag Identification Algorithm • Single-use 1-step identification algo to maintain privacy in face of passive adversaries • Pseudonyms and one-time-pads • Privacy-preserving PUF-Based Tag Id Algo

11. Other Tag ID Algorithms • “Minimalist” approach • Uses readers to generate pseudonyms • Using PUFs requires fewer updates • Hash-chains • Tags must compute 2 expensive cryptographic hash functions • PUF = only 1 PUF-Based Tag Id Algo

12. Database ID1, p(ID1), p2(ID1), …, pk(ID1) ... IDn, pn(IDn), pn2(IDn), …, pnk(IDn) Authors’ Tag ID Algorithm ID ID • Interrogation by reader  response with ID from tag  tag updates ID with p(ID) • Back-end keeps list of ID values • Pseudonyms exhausted  new seed ID • Multiple executions and Parallel PUFs • Why? p(ID) Request PUF-Based Tag Id Algo

13. Multiple Executions & Parallel PUFs • Reason  increase reliability of output • Parallel PUFs  each produces sub-signature • Sub-signatures contain many PUF compositions • Early invalid results reflect heavily on later compositions • Multiple Executions  PUF is run several times for each input in each sub-signature • Number of valid sub-signatures must be above a threshold

14. Multiple Executions • Averages values for greater reliability • R Reliability of last value where: • μ = .02 probability of unreliable value • k = 100 compositions • Nexecutions at each stage • For 1 execution, R= .49 • For 5 executions, R= .992268 PUF-Based Tag Id Algo – Author’s

15. Parallel PUFs • Tuple response, any one accepted, also increases reliability • S Successful consecutive identifications where: • q tuple size • For q = 2, S ≈ 73 • For q = 3, S ≈ 90 • More PUFs = few gates • One PUF can simulate many • Combination possible PUF-Based Tag Id Algo – Author’s

16. Tag ID Specific Assumptions and Requirements • No DOS attacks (only passive) • ID not overwritable by adversary w/o altering PUF circuits • Back-end must contain significantly more i/o values than # of tags • PUF must be able to produce many unique IDs • Tags should not yield same outputs • If ID repeats, new ID is sent along with power to perform write operations PUF-Based Tag Id Algo – Author’s

17. Adversarial Model • Observe reader communication with multiple tags, single out two of them • Randomly select one and runs ID algo • Adversary is successful if they can determine which tag was selected with much greater accuracy than ½ (better than guessing) PUF-Based Tag Id Algo – Author’s

18. Theorem 3.1 • **Given a random oracle assumption for PUFs, and adversary has no advantage in attempting to compromise a tag’s privacy • Proof sketch: • Observe output of two tags • Obtain next output from one • Adversary cannot determine which tag it came from b/c PUF is assumed to be random PUF-Based Tag Id Algo – Author’s

19. PUF-Based MAC Protocols • Three-tuple (K, T, V) • K = generation algo  generates key used in T and V • T = tagging algo  takes input message m and outputs signature σ • V = verification algo  verifies signature σ for message m is authentic • Secure if resistant to forgeries • Adversary is successful if they can determine signature from message PUF-Based MAC Protocols

20. Other MAC Protocols • Various implementations: • Standard cryptographic hash function • Block cipher • One-time signature scheme • List of secrets that are 0 or 1 • Oodles of memory usage • “Minimalistic” approach • Each secret is a single bit • Longer message size and shorter message space PUF-Based MAC Protocols

21. Authors’ MAC Protocols • PUF acts like a public key: • PUF computation algo (schematic) is known • Private key (PUF’s i/o behavior) remains unknown • Seller possesses a tag, but cannot predict PUF computations • Resistant to forgery even when verifier is offline • Defense against hardware alterations • Physically locating tag’s verification password storage circuitry under PUF’s circuitry/wires • Multiple executions/Parallel PUFs can be used PUF-Based MAC Protocols

22. Comparisons • Vs. tag authentication • Tag signs/authenticates message instead of reader • Signed message is input, output is signature/MAC • Key used to sign is PUF itself • Vs. standard cryptographic MAC algos • Keys are larger • Physical presence of tag required • Cannot sign arbitrary messages • Back-end computation  keeps tag costs down PUF-Based MAC Protocols – Author’s

23. Components of the Protocol • Key Generation • Verifier creates table of values • Occurs before deployment • Can be disabled/passworded • Large key required for verification w/o tag presence • Tagging algo signs message • Verification algo verifies signature PUF-Based MAC Protocols – Author’s

24. Key GenerationAlgorithm • Input: Message set M; tag/PUF identifiers set P; # of needed signatures k; # of sub-signatures q for each PUF p ∈ Pdo for i = 1 to |M| do for c = 1 to k · qdo Key[p,mi, c] = {c, pc(mi), . . . , p(n)c(mi)} end end end PUF-Based MAC Protocols – Author’s - Components

25. Tagging Algorithm • Input: Message m; # of sub-signatures q • Side effect: c = c + q PUF-Based MAC Protocols – Author’s - Components

26. Verification Algorithm • Input: Key K; PUF p; # of needed signatures k; # of sub-signatures q; allowed number t of incorrect PUF responses; verify that 1 ≤ c ≤ k ∙ q v = 0 for each sub-signature σcdo σ* = K[p, m, c] if σc agrees with σ* in at least n − t terms then v = v + 1 if v ≥ threshold then accept else reject PUF-Based MAC Protocols – Author’s - Components

27. Large Message Spaces • Signature verification only possible when tag is in range • b/c of size of key • Unique token c (counter) • Substitute for timestamp in passive tags • Natural total ordering • Info leak possible  tells state of tag • Multiple executions  forgery resistance PUF-Based MAC Protocols – Author’s

28. Quantifying Auth. Reliability and Forgery Difficulty • probv valid signature detection probability • probf  forgery non-recognition probability • τ = .4  PUF1 output = PUF2 output probability • µ = .02  output deviation probability • n = 30  # of responses • t = 3  # of deviations allowed • probv = .997107 • probf = .000313 • Tweak n and t to get better results if necessary PUF-Based MAC Protocols – Author’s – Large Msg Spaces

29. Theorem 4.1 • Given a random oracle assumption for PUF p, the probability that an adversary can forge a signature σ for a message m is bounded from above by β. • Proof sketch: • To forge a signature: • Find n distinct numbers r1, . . . , rn • Find unused counter value c • Compute correct PUF values pc(ri ,m) for at least n – tof them • p is assumed to be random and c was never inputted into p  adversary must rely on the tag(s) in their possession PUF-Based MAC Protocols – Author’s

30. Small Message Spaces • Outputs can be computed ahead of time • Can verify signature w/o tag’s presence • Tokens generated on tag ≠ random • Counters can be used just like large MS PUF-Based MAC Protocols – Author’s

31. Theorem 4.2 • Given a random oracle assumption for a PUF p, the probability that an adversary could forge a signature σfor a message m is bounded from above by q · β. • Proof sketch: • Adversary finds next counter value c • PUF is random  accurate modeling not possible • Must use other tags for impersonation • Success of forging a sub-signature  bounded by β • Success of forging whole signature  bounded by q · β PUF-Based MAC Protocols – Author’s

32. Attacks on MAC Protocols - Impersonation original clone • Manufacture tag duplicate  forge signatures • Obtain multiple tags  use responses to impersonate • PUF = random  duplicating or selecting equivalent tag = improbable (“unclonable”) • Tweaking n and t • Raise valid signature detection probability probv • Lower forgery non-recognition probability probf • Makes impersonation more improbable PUF-Based MAC Protocols - Attacks

33. Attacks on MAC Protocols - Modeling • Attempt to model PUF using signature/message pairs • PUFs determined by unreliable factors  modeling is very difficult • Attempt to measure wire delays • This in itself will alter wire delays • Likely disrupt/damage overlying circuitry • Alters functionality of PUF PUF-Based MAC Protocols - Attacks

34. Attacks on MAC Protocols – Side-channel • Attempt to learn secret info using timing and power analyses attacks • PUF-based secrets are difficult to represent correctly in digital form • Therefore hard to model PUF-Based MAC Protocols - Attacks

35. Attacks on MAC Protocols – Hardware Tampering • Attempt to physically probe wires • High risk of altering/destroying PUF’s behavior • Attempt to physically read-off or alter digital key/password • Likely damage overlying wires and alter tag behavior • Detection is possible by precompiling information about tag PUF-Based MAC Protocols - Attacks

36. algorithm # of gates MD4 MD5 SHA-256 AES Yuksel PUF 7350 8400 10868 3400 1701 545 PUF Vs. Digital Hash Functions • Much less hardware required • Drawbacks to low hardware complexity: • Probabilistic consistency with expected output • Tag copies = similar computational behavior • Back-end must store all challenge/response pairs for each tag PUF Vs. Digital Hash Functions

37. More Comparisons to DHF • Modeling PUF vs. determining key • Difficult to represent accurately in concise form • Difficult to model  random components • More resistant to side-channel attacks/physical tampering • Even with physical measurements, PUF is difficult to duplicate • Reliance upon physical characteristics makes security difficult to guarantee/characterize analytically PUF Vs. Digital Hash Functions

38. Building PUFs • First prototype of silicon PUF: • Silicon Physical Random Functions • B. Gassend, D. Clarke, M. van Dijk, and S. Devadas • Oscillating counter circuit used to measure intrinsic delays • Slow counting mechanism  slowed manufacturing process  increased overall cost Building PUFs

39. More Building of PUFs • Delay values for different challenges tend towards Gaussian distribution • Certain challenges should be avoided • Identical/similar outputs even when signals travel different paths • Filtered out of database at creation • Response reliability is low • More computation rounds • Still risking producing noise Building PUFs

40. Avoiding Drawbacks • Use sub-threshold voltage techniques to compare gate polarizations • Fast w/o using oscillating counter • Separates PUF values better and avoids highly skewed distributions of responses • Still preserves reliability/unpredictability • Variable non-linear delays can be added to keep modeling difficult Building PUFs

41. Future Research • Characterization of security of PUFs • Thorough testing of RFID tags with PUFs satisfying current RFID standards • Sub-threshold voltage-based PUFs • Conditional testing  environmental and operational • Behavior testing under varying levels of motion, acceleration, vibration, temperature, noise, etc. • τ and μ should be characterized as functions of operational environment Conclusion - Future Research

42. More Future Research • Adaptations for various applications • Multi-tag regimes • Ownership transfer algos • Tree-based identification protocols • PUFs in readers can be used to combat rogue readers Conclusion - Future Research

43. Conclusion • Full-fledged cryptographic security mechanisms are too costly for low-cost RFID tags  enter PUF approach • Exponential # of keys  no key distribution problem • Protects from cloning, even with physical access to tags and circuit schematics • Valuable in access control and authenticity verification • MAC protocols require few hardware resources  keeps tag costs down • Comparison to digital counterparts • Possible improvements in PUF design • Outline of future research Conclusion

44. Questions? Are you still reading these?

45. GO HOME!! Seriously, go home

46. Reference(s) • Bolotnyy, Leonid, and Gabriel Robins. “Physically Unclonable Function-Based Security and Privacy in RFID Systems.” University of Virginia. 15 April 2008 <http://www.cs.virginia.edu/papers/2007_3_nd_L_Bolotnyy.pdf>.