Transcript Physically Unclonable Function– Based Security and Privacy in RFID Systems Leonid Bolotnyy and Gabriel Robins Dept.

Physically Unclonable Function–
Based Security and Privacy
in RFID Systems
Leonid Bolotnyy and Gabriel Robins
Dept. of Computer Science
University of Virginia
www.cs.virginia.edu/robins
Contribution and Motivation
Contribution
• 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
Alice was here: A, B, C
privacy
C
Private Identification Algorithm
Database
ID
ID
p(ID)
Request
ID1, p(ID1), p2(ID1), …, pk(ID1)
...
IDn, pn(IDn), pn2(IDn), …, pnk(IDn)
• It is important to have
– a reliable PUF
– no loops in PUF chains
– no identical PUF outputs
• Assumptions
– no denial of service attacks (e.g., passive adversaries, DoS
detection/prevention mechanisms)
– physical compromise of tags not possible
Improving Reliability of Responses
• Run PUF multiple times for same ID & pick majority
number of runs
unreliability
chain length
probability
N
overall
reliability
R(μ, N, k) ≥ (1 - ∑
m=
R(0.02, 5, 100) ≥ 0.992
N+1
2
N μm(1-μ)N-m )k
m
• Create tuples of multi-PUF computed IDs &
identify a tag based on at least one valid position value
tuple size
expected number
of identifications
(ID1, ID2, ID3)
∞
S(μ, q) = ∑
i q
i [(1 – (1-μ)i+1)q - (1 – (1-μ) ) ]
i=1
S(0.02, 1) = 49, S(0.02, 2) = 73, S(0.02, 3) = 90
Privacy Model
Experiment:
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
Definition: The algorithm is privacy-preserving if an adversary can not
determine 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 = (K, τ, υ)
• valid signature σ : υ K(M, σ) = 1
• forged signature σ’ : υ K(M’, σ’) = 1, M = M’
• MAC based on PUF
– Motivation: “yoking-proofs”, signing sensor data
– large keys (PUF is the key)
– cannot support arbitrary messages
• 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
Assumption: tag can generate good random numbers
(can be PUF-based)
Key: PUF
σ (m) = c, r1, ..., rn, pc(r1, m), ..., pc(rn, m)
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
Choosing # of PUF Computations
probv(n, 0.1n, 0.02)
n
probv(n, t, μ) = 1 - ∑
i=t+1
probf(n, 0.1n, 0.4)
n μi(1-μ)n-i
i
n
n τj(1-τ)n-j
probf(n, t, τ) = 1 - ∑
j
j=t+1
α < probv ≤ 1 and probf ≤ β ≤ 1
0 ≤ t ≤ n-1
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 Space
Assumption: small and known a priori message space
message
PUF
counter
Key[p, mi, c] = c, pc(1)(mi), ..., pc(n) (mi)
PUF reliability is again crucial
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
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 Protocols
• Impersonation attacks
– manufacture an identical tag
– obtain (steal) existing PUFs
• Modeling attacks
– build a PUF model to predict PUF’s outputs
• Side-channel attacks
– algorithm timing
– power consumption
• Hardware-tampering attacks
– physically probe wires to learn the PUF
– physically read-off/alter keys/passwords
original
clone
Comparison of PUF With Digital
Hash Functions
algorithm
MD4
MD5
SHA-256
AES
Yuksel
PUF
# of gates
7350
8400
10868
3400
1701
545
• 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
PUF Design
•
Attacks on PUF
–
–
–
–
•
impersonation
modeling
hardware tampering
side-channel
Weaknesses of existing PUF
reliability
•
New PUF design
–
–
•
no oscillating circuit
sub-threshold voltage
Compare different non-linear delay approaches
Conclusions and Future Work
•
•
•
•
PUF: hardware primitive for RFID security
Identification and MAC algorithms based on PUF
PUFs protect tags from physical attacks
PUFs is the key
• 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
Thank You
Questions ?
Leonid Bolotnyy
[email protected]
Dept. of Computer Science
University 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
Using PUF to Detect and Restore
Privacy of Compromised System
s1,0
s2,0
s3,0
s1,1
s2,1
s3,1
s3,2
s2,2
s3,3 s3,
s3,5
s1,2
s2,3
s3,6
s2,4
s3,7
4
1. Detect potential tag compromise
2. Update secrets of affected tags
s3,8
s3,9
s2,5
s3,10
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]