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

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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

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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 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**

*Ruhrmair, et al., Book chapter in ‘Intro to Hardware Security and Trust’, Springer’11**Gassend, et al., CCS’02

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Delay-based Strong PUF

• 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

…

c-bitChallenge

RisingEdge

1 if toppath is faster,else 0

D Q1

1

0

0

1

1

0

0

1

1

0

0

1 0 10 0 1

01

G

Response

*Suh and Devadas, DAC 2007

*Gassend, et al. , SAC’03**Lee, et al., VLSI Symp’04

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• An arbiter PUF can be modeled easily*

• Fast modeling compromised security **

Model building

*Majzoobi, Koushanfar, Potkonjak, TRETS’08**Ruhrmair, et al., CCS’10

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Lightweight safeguarding of PUFs

• Protect against machine learning attacks by• Blocking controllability and observability*

PUF

PUFInput

Net. (G)

PUF

Input Net. (G)

Input Net. (G)

...

...

Output Network (Z)

...

Interconnect Network

Input Output

1. Transform challenges • Input network

2. Block controllability3. Block observability

• Output network

* Majzoobi, et al., ICCAD ‘08

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XORed delay-based PUF

• Block observability by lossy compression• Swapping the challenge order to improve

statistical properties*

*Majzoobi, et al., ICCAD ‘08

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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

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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

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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

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Desired properties of protocol

• Robust against model building attacks • Robust against PUF errors• Ultra low-power

– No Hashing – No error correction codes

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Slender PUFProtocol

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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

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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

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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

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Slender PUF ProtocolVerifier Prover

Noncev(1)

Verifier Prover

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Slender PUF Protocol

Verifier Prover

Verifier Prover

Noncep(2)

Noncev(1)

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Slender PUF Protocol

Verifier Prover

Verifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}(3)

(2)

Noncev(1)

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Slender PUF Protocol

Verifier Prover

Verifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}(3)

(2)

Noncev(1)

The same seed for both sides Random if only one of them is honest

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Slender PUF Protocol

Verifier Prover

Verifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}

C = G(Seed) C = G(Seed)

(3)

(4)

(2)

Noncev(1)

PRNG PRNG

Generate challenge stream from seedThe same challenge for both sides

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Slender PUF ProtocolVerifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}

C = G(Seed) C = G(Seed)

R’ = PUF_model(C) R = PUF(C)

(3)

(4)

(2)

(5)

Noncev(1)

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Slender PUF Protocol

1 0 0 1 1 1 0 0 1 1 0 1 1 0 0 0 1 1 1 0 0 1 0 0 1 1R:

R’: 1 0 0 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 1

Verifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}

C = G(Seed) C = G(Seed)

R’ = PUF_model(C) R = PUF(C)

(3)

(4)

(2)

(5)

Noncev(1)

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Slender PUF Protocol

1 0 0 1 1 1 0 0 1 1 0 1 1 0 0 0 1 1 1 0 0 1 0 0 1 1R:

R’: 1 0 0 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 1

Verifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}

C = G(Seed) C = G(Seed)

R’ = PUF_model(C) R = PUF(C)

(3)

(4)

(2)

(5)

Noncev(1)

PUF modeling error

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Verifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}

C = G(Seed) C = G(Seed)

R’ = PUF_model(C) R = PUF(C)

W = sub-seq (ind,Lsub,R)

(3)

(4)

(2)

(6)

(5)

Noncev(1)

1 0 0 1 1 1 0 0 1 1 0 1 1 0 0 0 1 1 1 0 0 1 0 0 1 1

Lsub = 11

ind = 10

R:

1 0 1 1 0 0 0 1 1 1 0 0W:

The index is not transmitted

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Verifier Prover

Noncep

Seed ={Noncev,Noncep} Seed = {Noncev,Noncep}

C = G(Seed) C = G(Seed)

R’ = PUF_model(C) R = PUF(C)

W = sub-seq (ind,Lsub,R)

(3)

(4)

(2)

(6)

(5)

Noncev(1)

T =match(R’,W,e)

Auth. pass: T = true?

(7)

1 0 1 1 0 0 0 1 1 1 0 0W:

R’: 1 0 0 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 0 1 1

10errors:

It reveals minimum informationn about original response sequence

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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 …

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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

• Strict avalanche criteria to avoid correlation attacks

210000

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Guessing attack

• Dishonest Prover

• Honest Prover – Perr : PUF error rate

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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!

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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

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Slender PUF protocol:System overview

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TRNG and PRNG

• TRNG:– PUF based– Based on flip-flop meta-

stability

Control

MonitorD

C

Q

PDLBinary Sequence

M. Majzoobi, et al., CHES, 2011

• PRNG:• Need not to be

cryptographically secure

• LFSR is enough

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• Slender PUF Protocol

• Previously known protocol*, just SHA-2

Slender PUFOverhead comparison

*Gassend, et al., CCS’02

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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