Slender PUF Protocol Authentication by Substring Matching
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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 1
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
Physical unclonable functions
(PUFs)
• PUFs based on the inherent, hard to
forge, physical disorders
• Two major types*:
– Weak PUF
– Strong PUF
mair, et al., Book chapter in ‘Intro to Hardware Security and Trust’, Springer’11
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
mair, et al., Book chapter in ‘Intro to Hardware Security and Trust’, Springer’11
Strong PUFs*
• Directly used for challenge response authentication
• Provide large Challenge-Response Pairs (CRPs)
• Often exponential w.r.t. system elements
• Neither an adversary nor manufacturer should
correctly predict the response to a randomly chosen
challenge with a high probability**
mair, et al., Book chapter in ‘Intro to Hardware Security and Trust’, Springer’11
send, et al., CCS’02
Delay-based Strong PUF
1
0 0 1
c-bit *Suh and Devadas, DAC 2007
Challenge 1 1 1
0
1
D Q 1 if top
0 0 0
… path is
faster,
Rising 0 0 0
Edge G else 0
1 1 1
Response
• 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
*Gassend, et al. , SAC’03
**Lee, et al., VLSI Symp’04
Model building
• An arbiter PUF can be modeled
easily*
• Fast modeling compromised
security **
*Majzoobi, Koushanfar, Potkonjak, TRETS’08
**Ruhrmair, et al., CCS’10
Lightweight safeguarding of PUFs
• Protect against machine learning attacks by
• Blocking controllability and observability*
1. Transform
challenges
• Input network
2. Block controllability
3. Block observability
• Output network
* Majzoobi, et al., ICCAD ‘08
XORed delay-based PUF
• Block observability by lossy
compression
• Swapping the challenge order to
improve statistical properties*
*Majzoobi, et al., ICCAD ‘08XORed 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
outputModel 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’10Problem 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’02Desired properties of protocol • Robust against model building attacks • Robust against PUF errors • Ultra low-power – No Hashing – No error correction codes
Slender PUF Protocol
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 ProverXored delay-based PUF model
• PUF secrets
– Set of delays
• The secret sharing is performed
initially
• Electronic fuse burned to disable
Probing here for
model building
access*
*Majzoobi, Koushanfar, Potkonjak, TRETS’08Malicious 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
Slender PUF Protocol Verifier Prover
Slender PUF Protocol Verifier Prover
Slender PUF Protocol Verifier Prover
Slender PUF Protocol The same seed for both sides Random if only one of them is honest Verifier Prover
Slender PUF Protocol
PRNG PRNG
Generate challenge stream from seed
The same challenge for both sides
Verifier ProverSlender PUF Protocol
Slender PUF Protocol
Slender PUF Protocol PUF modeling error
The index is not transmitted
It reveals minimum informationn about original response sequence
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 …
Brute-force modeling attack
• Set Lsub = 500, L = 1024
– 500000/500=1000 rounds of protocol needed
– In each one, ind is unknown
– 1024500000/500 = 10241000 models needed to be built
210000
• Strict avalanche criteria to avoid correlation attacksGuessing attack • Dishonest Prover • Honest Prover – Perr : PUF error rate
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!
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) • ind is generated first – PUF is only challenged when necessary
Slender PUF protocol: System overview
TRNG and PRNG
• TRNG: • PRNG:
– PUF based • Need not to be
– Based on flip-flop cryptographicall
meta-stability y secure
• LFSR is enough
M. Majzoobi, et al., CHES, 2011Slender PUF
Overhead comparison
• Slender PUF Protocol
• Previously known protocol*, just SHA-
2
Gassend, et al., CCS’02Conclusions – 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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