1 SRAM PUF Error Correction Because these papers targeted the application of

1 sram puf error correction because these papers

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1) SRAM PUF Error Correction: Because these papers targeted the application of die identification, rather than key generation, this problem could be sidestepped by a statistical analysis of the probability that two independent dies would have IDs that were close enough to be misiden- tified as a result of this noise. Unfortunately, the nature of cryptographic operations is such that not even a single bit can be incorrect. This will require a different approach to error correction. Maes et al. describe a low-overhead approach to imple- menting a soft-decision helper algorithm [18]. They de- scribe a method wherein one collects confidence data in each bit by taking between 10 and 100 measurements of the SRAM PUF with N output bits prior to provisioning. This yields an output estimate vector X 2 f 0 ; 1 g N , and a vector of error probabilities P e , where the i th element of this vector corresponds to the probability that a measure- ment of X i will be erroneous. Going forward, X serves as the ‘‘fuzzy secret,’’ and P e is public information. It has been proven by Maes et al. that revealing P e does not leak any min-entropy of the response X . This work goes on to de- scribe an implementation where the above soft-decision helper algorithm is combined with Reed–Muller codes and a universal hash function to distill the PUF output bits to a full-entropy reproducible secret key. This implementation takes 1536 SRAM PUF response bits (78% min-entropy with an average bit-error probability of 15%), and distills these data down to a 128-bit full-entropy key with a failure rate of µ 10 ³ 6 . This approach demonstrates the feasibility of using SRAM PUFs as cryptographic key sources in spite of the errors inherent in SRAM PUF output bits. 2) Attacks on SRAM PUFs: Because the SRAM PUF provides a secure key (as opposed to providing challenge– response functionality like the strong PUF), it relies on other conventional security primitives to keep that key protected while the chip is powered. As a result, any side channel or other vulnerabilities associated with the cryp- tographic hardware pose a threat to the secret key out- putted by the SRAM PUF. In addition, since this key is kept secret, the modeling attacks used against strong PUFs cannot be used, since no input/output relations of the PUF should ever be revealed. However, there are other ways identified in the liter- ature to attack a SRAM PUF more directly. Many of these depend on the level of access that one has to the SRAM. If one can insert a ‘‘write’’ command, then one could leverage the NBTI to deliberately force individual bits toward ‘‘1.’’ If one could modify the temperature, one could potentially cause the PUF to fail by running the PUF outside its design area. Finally, the ability for a SRAM cell to maintain its state depends on the supply voltage. If, during the turn-on process, the supply voltage is held for some time at a low ( ² 100 mV) voltage, the thermal noise will induce a tran- sition into the cell’s favored state, resulting in higher stability. However, if the voltage turn-on is fast, then cells
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