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Slender PUF Protocol Authentication by Substring Matching

Slender PUF Protocol Authentication by Substring Matching. M. Majzoobi, M. Rostami , F . Koushanfar, D. Wallach, and S. Devadas* International Workshop on Trustworthy Embedded Devices , San Francisco, May 2012. ACES Lab, Rice University *Computation Structures Group, MIT.

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Slender PUF Protocol Authentication by Substring Matching

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  1. Slender PUF Protocol Authentication by Substring Matching M. Majzoobi, M. Rostami, F. Koushanfar, D. Wallach, and S. Devadas* International Workshop on Trustworthy Embedded Devices, San Francisco, May 2012 ACES Lab, Rice University *Computation Structures Group, MIT

  2. Traditional digital key-based authentication • Keys stored in non-volatile memory • Verifier sends random number (challenge) • Prover signs the number by it’s secret key and sends a response • Limitation • Extra cost of non-volatile memory • Physical and side channel attacks • Intensive cryptographic algorithms Challenge Verifier Prover

  3. Physical unclonable functions(PUFs) • PUFs based on the inherent, hard to forge, physical disorders • Two major types*: • Weak PUF • Strong PUF *Ruhrmair, et al., Book chapter in ‘Intro to Hardware Security and Trust’, Springer’11

  4. Security based on PUFs:Weak PUFs • Also called Physically Obfuscated Keys (POKs) • Limited Challenge-Response Pairs • Based on ring-oscillators • Generate standard digital key for security apps • When challenged by one (or very few) fixed challenge(s) generates Response(s) depending on its physical disorder • Response(s) is used to generate secret key • Intensive cryptographic algorithm is still needed Ruhrmair, et al., Book chapter in ‘Intro to Hardware Security and Trust’, Springer’11

  5. Strong PUFs* • Directly used for challenge response authentication • Provide large Challenge-Response Pairs (CRPs) • Often exponentialw.r.t. system elements • Neither an adversary nor manufacturer should correctly predict the response to a randomly chosen challenge with a high probability** *Ruhrmair, et al., Book chapter in ‘Intro to Hardware Security and Trust’, Springer’11 **Gassend, et al., CCS’02

  6. 1 0 0 0 1 1 Delay-based Strong PUF c-bit Challenge • Compare two paths with an identical delay in design*, ** • Each challenge selects a unique pair of delay paths • Random process variation determines which path is faster • An arbiter outputs 1-bit digital response • Multiple bits can be obtained by either duplicate the circuit or use different challenges *Suhand Devadas, DAC 2007 1 0 1 1 1 D Q 1 if top path is faster, else 0 0 0 0 … RisingEdge 0 0 0 G 1 1 1 Response *Gassend, et al. , SAC’03 **Lee, et al., VLSI Symp’04

  7. Model building • An arbiter PUF can be modeled easily* • Fast modeling  compromised security ** *Majzoobi, Koushanfar, Potkonjak, TRETS’08 **Ruhrmair, et al., CCS’10

  8. Lightweight safeguarding of PUFs • Protect against machine learning attacks by • Blocking controllability and observability* • Transform challenges • Input network • Block controllability • Block observability • Output network * Majzoobi, et al., ICCAD ‘08

  9. XORed delay-based PUF • Block observability by lossy compression • Swapping the challenge order to improve statistical properties* *Majzoobi, et al., ICCAD ‘08

  10. XORed delay-based PUFs • Improvement in randomness of responses • Strict Avalanche Criterion • Any transition in the input causes a transition in the output with a probability of 0.5 • Balances the impact of challenge on output

  11. Model building attack on Xored-PUF • Use XORed PUFs to guard against modeling • Harder, but still breakable * • Logistic regression, evolutionary strategies • Two order of magnitude more CRPs needed *Ruhrmair, et al., CCS’10

  12. Problem with just Xoring • Still breakable • Cannot increase XOR layers indefinitely • Accumulates error • 5%  20% for 4 XOR • A solution* to guard against modeling while robust against errors • Using error correction codes (ECC) and hashing • Computationally intensive! • Not suitable for low-power embedded devices *Gassend, et al., CCS’02

  13. Desired properties of protocol • Robust against model building attacks • Robust against PUF errors • Ultra low-power • No Hashing • No error correction codes

  14. Slender PUF • Protocol

  15. Communicating parties • Prover • Has PUF • Will be authenticated • Verifier • Has a compact soft model of the PUF • Compute challenge/response pairs • Will authenticate the prover Challenge Verifier Prover

  16. Xored delay-based PUF model • PUF secrets • Set of delays • The secret sharing is performed initially • Electronic fuse burned to disable access* Probing here for model building *Majzoobi, Koushanfar, Potkonjak, TRETS’08

  17. Malicious parties • Dishonest prover • Does not have access to the PUF • Wants to pass the authentication • Eavesdropper • Taps the communication between prover and verifier • Tries to learn the secret • Dishonest verifier • Does not have access to the PUF soft model • Tries to actively trick the prover to leak information

  18. Slender PUF Protocol Verifier Prover

  19. Slender PUF Protocol Verifier Prover

  20. Slender PUF Protocol Verifier Prover

  21. Slender PUF Protocol • The same seed for both sides • Random if only one of them is honest Verifier Prover

  22. Slender PUF Protocol PRNG PRNG • Generate challenge stream from seed • The same challenge for both sides Verifier Prover

  23. Slender PUF Protocol

  24. Slender PUF Protocol

  25. Slender PUF Protocol PUF modeling error

  26. The index is not transmitted

  27. It reveals minimum informationn about original response sequence

  28. Model building attacks • Set Lsub = 500, L = 1024 • 99% threshold for authentication  • 99% accuracy in modeling • XORed PUF attack: 500,000 CRPs needed • 500,000 /500=1000 rounds needed • He doesn’t have ind …

  29. Brute-force modeling attack • Set Lsub = 500, L = 1024 • 500000/500=1000 rounds of protocol needed • In each one, ind is unknown • 1024500000/500 =10241000models needed to be built • Strict avalanche criteria to avoid correlation attacks 210000

  30. Guessing attack • Dishonest Prover • Honest Prover • Perr: PUF error rate

  31. Replay attack • Eavesdropping and replying the responses • Nonce scheme prevents it • If prover and verifier nonces are 128-bit: • Size of database for 50%: 2127 • Very low probability!

  32. Implementation • Same challenge streams should not be used • We need : • PRNG (pseudo random number generator) • Challenge stream generation • TRNG (true random number generator) • Nonce • Index of substring (ind) • indis generated first  • PUF is only challenged when necessary

  33. Slender PUF protocol:System overview

  34. TRNG and PRNG • TRNG: • PUF based • Based on flip-flop meta-stability • PRNG: • Need not to be cryptographically secure • LFSR is enough M. Majzoobi, et al., CHES, 2011

  35. Slender PUFOverhead comparison • Slender PUF Protocol • Previously known protocol*, just SHA-2 *Gassend, et al., CCS’02

  36. Conclusions • Authentication protocol based on PUFs • Protect against model building • Revealing a partial section of the PUF responses • Based on string matching • Resilient against PUF error, without: • Error correction • Hashing • Exponentiation

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