physically unclonable function– based security and privacy in rfid systems leonid bolotnyy and...
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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
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
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]