Transcript Paper #32 - University of Michigan

Reliable and Efficient PUFBased Key Generation Using
Pattern Matching
Srini Devadas and Zdenek Paral (MIT), HOST 2011
Thomas Chen, Anup Jadhav
UNIVERSITY OF MICHIGAN
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Outline
 Motivation & Security Challenges
 Problem & Previous Approaches
 Physical Unclonable Functions (PUF)
 PUF-based Key Generation Using Pattern Matching
 Results
 Conclusion
 References
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Motivation


Secure computing
 Devices are becoming:
 Distributed
 Unsupervised
 Physically exposed
Prone to physical tampering
 Need protection at the hardware level
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Problem & Previous Approaches


Making a device tamper proof is difficult and expensive
 IBM 4758 cryptographic coprocessor ($3000)
 Battery powered sensors
 Anti-tamper package
Attackers can
 Extract keys from NVM while processor is off
 Depackage,etch, and polish down to poly to read off fuse bits
ROM
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Fuses
Flash
Anti-fuses
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Physical Unclonable Function (PUF)



Silicon “fingerprint”
 Unique per instance
 Reproducible/repeatable
Challenge
Variability
Sensitive Circuit
Usefulness
 Random key generation
 Low-cost key “storage”
 Tamper resistant
Extract keys from complex physical system
C
R1
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Response
!=
R2
!=
R3
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PUF-based Key Generation


Use PUF to generate fixed size of secret bits
 Can use as symmetric key bits or seed for asymmetric key
But…
 Some bits may be “noisy”- need error correction
 Need to use helper data/syndrome to correct
Response
PUF
Key
Key Generator
Path-swapping switch
Arbiter
…
D Q
C
C0
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C1
C2
Cn
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Reproducibility

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Intra-distance metric (use fractional Hamming distance)
 Ideally HDintra=0
Mean intra-distance varies with voltage, temperature
 Can reduce unstable bits by:
 pre/post selection, temporal majority voting, compensation, etc.
Typically >5%, <20% over region of operation (before corr.)
PUF A
2 bits -> 6.25%
Stored PUF A response
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Uniqueness
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Inter-distance metric
Use fractional Hamming distance
Ideally, HDinter of 50% -> no correlation between chips
15 bits ->
46.875%
PUF A
PUF B
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Error Correction & Entropy
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Key must be 100% reproducible (HDintra=0)
 Often use BCH codes
Increase reproducibility
 But helper data leaks information, reduces unpredictability
Need bigger response then compress
 Extracted key length <= Total accumulated entropy
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Correction
1 0 0 1 0 1 1 0
Helper
Data
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Pattern Matching Key
Generator(PMKG) Architecture
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Key Generation Scheme

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Major Difference
 Instead of making challenge public, make response public
Provisioning and Regeneration
 Happens over a number of rounds
Regeneration
 Involves matching the patterns provisioned to recreate key
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Pattern Matching
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Provisioning
 In each round select an index I
 Starting at that index store a pattern of length W
Regeneration
 Match against known patterns to obtain index
Index=sub-key
X X 7
PUF generated
bit stream:
1
1110100110
10 bits
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Pattern Storage
011
000
101
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Key Generator Architecture
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Security

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Public helper data does not leak information about key
 Index based key
Key mixer
 Post process key bits
LFSR forking
 Fork the next round of challenge generator based on key index
 Fixed number of comparisons against helper patterns
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Key Generation Parameters
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Intra-distance and Inter-distance
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Matching threshold and FAR,FRR
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Tolerance match detector
 Causes false positives and false negatives
 Requires appropriate matching threshold
 Requires sufficiently wide pattern
Otherwise use error correction scheme
For small pattern, additional logic required to prevent collision
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False Negatives and False Positives
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Trials Required For Key Regeneration
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Conclusion

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Main contribution
 Expose PUF response, keep challenge hidden
 Key regeneration via pattern matching
 Key bits are not directly stored
 Subkeys are indices of PUF responses
Avoid heavy error correction logic
 But need to choose good threshold and pattern width
 False positives, false negatives
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Questions & Discussion Points
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Is there enough process variation to identify between ICs?
Is setting a threshold a good enough approach?
Is the arbiter PUF a good choice?
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References

[1] Paral, Z., and Srinivas Devadas. "Reliable and efficient
PUF-based key generation using pattern
matching." Hardware-Oriented Security and Trust (HOST),
2011 IEEE International Symposium on. IEEE, 2011.
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