Physically Unclonable Function–Based Security and Privacy

in RFID Systems

Leonid Bolotnyy and Gabriel RobinsDept. of Computer Science

University of Virginia

www.cs.virginia.edu/robins

Contribution and MotivationContribution• Privacy-preserving tag identification algorithm• Secure MAC algorithms• Comparison of PUF with digital hash functions

Motivation• Digital crypto implementations require 1000’s of gates• Low-cost alternatives

– Pseudonyms / one-time pads– Low complexity / power hash function designs– Hardware-based solutions

PUF-Based Security

• Physical Unclonable Function (PUF) [Gassend et al 2002]• PUF Security is based on

– wire delays– gate delays– quantum mechanical fluctuations

• PUF characteristics– uniqueness– reliability– unpredictability

• PUF Assumptions– Infeasible to accurately model PUF– Pair-wise PUF output-collision probability is constant– Physical tampering will modify PUF

Privacy in RFID

• Privacy

A B C

Alice was here: A, B, C

privacy

Private Identification Algorithm

• Assumptions– no denial of service attacks (e.g., passive adversaries, DoS

detection/prevention mechanisms)– physical compromise of tags not possible

• It is important to have – a reliable PUF– no loops in PUF chains– no identical PUF outputs

ID

Requestp(ID)

ID

Database

ID1, p(ID1), p2(ID1), …, pk(ID1)

...IDn, pn(IDn), pn

2(IDn), …, pnk(IDn)

Improving Reliability of Responses• Run PUF multiple times for same ID & pick majority

μm(1-μ)N-m )kR(μ, N, k) ≥ (1 - ∑

N Nm

N+12

m=

number of runs

chain lengthunreliabilityprobability

overallreliability

R(0.02, 5, 100) ≥ 0.992

• Create tuples of multi-PUF computed IDs &identify a tag based on at least one valid position value

∞expected numberof identifications

S(μ, q) = ∑ i [(1 – (1-μ)i+1)q - (1 – (1-μ)i)q]i=1

tuple size

S(0.02, 1) = 49, S(0.02, 2) = 73, S(0.02, 3) = 90

(ID1, ID2, ID3)

Privacy Model

1. A passive adversary observes polynomially-many rounds of reader-tag communications with multiple tags

2. An adversary selects 2 tags

3. The reader randomly and privately selects one of the 2 tags and runs one identification round with the selected tag

4. An adversary determines the tag that the reader selected

Experiment:

Definition: The algorithm is privacy-preserving if an adversary can notdetermine reader selected tag with probability substantially greater than ½

Theorem: Given random oracle assumption for PUFs,an adversary has no advantage in the above experiment.

PUF-Based MAC Algorithms

• MAC based on PUF– Motivation: “yoking-proofs”, signing sensor data– large keys (PUF is the key)– cannot support arbitrary messages

• MAC = (K, τ, υ)

K

K

• valid signature σ : υ (M, σ) = 1

• forged signature σ’ : υ (M’, σ’) = 1, M = M’

• Assumptions– adversary can adaptively learn poly-many (m, σ) pairs– signature verifiers are off-line– tag can store a counter (to protect against replay attacks)

Large Message Space

σ (m) = c, r1, ..., rn, pc(r1, m), ..., pc(rn, m)

Assumption: tag can generate good random numbers (can be PUF-based)

Signature verification• requires tag’s presence• password-based or in radio-protected environment (Faraday Cage)• learn pc(ri, m), 1 ≤ i ≤ n• verify that the desired fraction of PUF computations is correct

To protect against hardware tampering• authenticate tag before MAC verification• store verification password underneath PUF

Key: PUF

Choosing # of PUF Computations

α < probv ≤ 1 and probf ≤ β ≤ 1

0 ≤ t ≤ n-1

i=t+1

μi(1-μ)n-iprobv(n, t, μ) = 1 - ∑

nni

j=t+1

τj(1-τ)n-jprobf(n, t, τ) = 1 - ∑

nnj

probv(n, 0.1n, 0.02)

probf(n, 0.1n, 0.4)

Theorem

Given random oracle assumption for a PUF, the probability that an adversary could forge a signature for a message is bounded from above by the tag impersonation probability.

Small Message SpaceAssumption: small and known a priori message space

Key[p, mi, c] = c, pc(1)(mi), ..., pc

(n) (mi)

PUFmessage

counter

σ(m) = c, pc(1)(m), ..., pc

(n) (m),

..., c+q-1, pc+q-1

(1)(m), pc+q-1(n)(m)

sub-signature

Verify that the desired number of sub-signatures are valid

PUF reliability is again crucial

Theorem

Given random oracle assumption for a PUF, the probability that an adversary could forge a signature for a message is bounded by the tag impersonation probability times the number of sub-signatures.

Attacks on MAC Protocolsoriginal clone

• Impersonation attacks– manufacture an identical tag– obtain (steal) existing PUFs

• Hardware-tampering attacks– physically probe wires to learn the PUF– physically read-off/alter keys/passwords

• Side-channel attacks– algorithm timing– power consumption

• Modeling attacks– build a PUF model to predict PUF’s outputs

Comparison of PUF With Digital Hash Functions

• Reference PUF: 545 gates for 64-bit input– 6 to 8 gates for each input bit– 33 gates to measure the delay

• Low gate count of PUF has a cost– probabilistic outputs– difficult to characterize analytically– non-unique computation– extra back-end storage

• Different attack target for adversaries– model building rather than key discovery

• Physical security– hard to break tag and remain undetected

MD4

7350

MD5

8400

SHA-256

10868

Yuksel

1701

PUF

545

AES

3400

algorithm

# of gates

PUF Design• Attacks on PUF

– impersonation– modeling– hardware tampering– side-channel

• Weaknesses of existing PUF

• New PUF design– no oscillating circuit– sub-threshold voltage

• Compare different non-linear delay approaches

reliability

Conclusions and Future Work

• Develop theoretical framework for PUF• Design new sub-threshold voltage based PUF• Manufacture and test PUFs

– varying environmental conditions– motion, acceleration, vibration, temperature, noise

• Design new PUF-based security protocols– ownership transfer– recovery from privacy compromise– PUFs on RFID readers

} in progress

• PUF: hardware primitive for RFID security• Identification and MAC algorithms based on PUF• PUFs protect tags from physical attacks• PUFs is the key

Thank You

Questions ?

Leonid Bolotnyylbol@cs.virginia.edu

Dept. of Computer ScienceUniversity of Virginia

PUF-Based Ownership Transfer

• Ownership Transfer

• To maintain privacy we need– ownership privacy– forward privacy

• Physical security is especially important

• Solutions– public key cryptography (expensive)– knowledge of owners sequence– trusted authority– short period of privacy

s2,4

s1,2

s3,9

s2,5

s3,10s3,8

Using PUF to Detect and Restore Privacy of Compromised System

1. Detect potential tag compromise2. Update secrets of affected tags

s1,0

s2,0

s1,1

s2,1

s3,1

s2,2 s2,3

s3,0 s3,4 s3,5s3,2 s3,3 s3,7s3,6

Related Work on PUF

• Optical PUF [Ravikanth 2001]

• Silicon PUF [Gassend et al 2002]– Design, implementation, simulation, manufacturing– Authentication algorithm– Controlled PUF

• PUF in RFID– Identification/authentication [Ranasinghe et al 2004]– Off-line reader authentication using public key cryptography

[Tuyls et al 2006]