Slender PUF Protocol Authentication by Substring

MatchingM. Majzoobi, M. Rostami,

F. Koushanfar, D. Wallach, and S. Devadas* International Workshop on Trustworthy Embedded

Devices, San Francisco, May 2012

1ACES 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 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

6

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

GResponse

*Suh and Devadas, DAC 2007

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

7

• An arbiter PUF can be modeled easily*

• Fast modeling compromised security **

Model building

*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*

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

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 PUFProtocol

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 ProtocolVerifier Prover

Noncev(1)

Verifier Prover

19

Slender PUF Protocol

Verifier Prover

Verifier Prover

Noncep(2)

Noncev(1)

20

Slender PUF Protocol

Verifier Prover

Verifier Prover

Noncep

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

(2)

Noncev(1)

21

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

22

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

23

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)

24

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)

25

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

26

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

27

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

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 = 10241000 models 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)

• ind is 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

Control

MonitorD

C

Q

PDLBinary Sequence

M. Majzoobi, et al., CHES, 2011

• PRNG:• Need not to be

cryptographically secure

• LFSR is enough

35

• Slender PUF Protocol

• Previously known protocol*, just SHA-2

Slender PUFOverhead comparison

*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