PUFs

Question 1

PUFs

Answer 1

• Physically Unclonable Function
  -> Is an inherent feature of a physical object, e.g., an integrated circuit
  -> Takes an input (challenge) and generates an output (response) based on the physical characteristics of the embedding IC
  -> Its challenge-response behavior is instance-specific since it leverages process variations during ICs manufacturing (unique IC identifier)
  -> Is a noisy function embedded in an IC (affected by operating conditions)

Question 2

Properties of ideal PUFs: Reproducibility (intra-distance)

Answer 2

With high probability, responses resulting from evaluating the same challenge on the same PUF instance should be similar. Reproducibility can be measured by Hamming distance (also known as intra-distance), ideally HDintra(R,R’) = 0

Question 3

Properties of ideal PUFs: Uniqueness (inter-distance)

Answer 3

With high probability, responses resulting from evaluating the same challenge on different PUF instances should be dissimilar. Uniqueness can be measured by Hamming distance also known as inter-distance, ideally HDinter(Rpufi,Rpufj) = 50%

Question 4

Properties of ideal PUFs: Identifiability

Answer 4

A PUF class exhibits both reproducibility and uniqueness, it follows that its PUF instances can be identified based on their challenge-response behavior.

Question 5

Properties of ideal PUFs: Physical Unclonability

Answer 5

Assuming an adversary with control over the creation procedure of a PUF class, i.e., he can influence the conditions, parameters and randomness sources to a certain (feasible) extent. It should be infeasible for the adversary to create two PUF instances that exhibit similar challenge-response behavior.

Question 6

Properties of ideal PUFs: Unpredictability

Answer 6

For a single PUF instance, unobserved responses remain sufficiently random, even after observing responses to other challenges.

Question 7

Properties of ideal PUFs: Mathematical Unclonability

Answer 7

Assuming an adversary with unlimited physical access to a PUF instance and its CRPs within a timeframe, a PUF exhibits mathematical unclonability if it is unpredictable
-> Mathematical unclonability is hence the extension of unpredictability to an adversary with unlimited access to a PUF CRPs
-> A direct condition for a PUF to be mathematically unclonable is having a very large challenge set, preferably exponential

Question 8

Properties of ideal PUFs: True Unclonability

Answer 8

A PUF exhibits true unclonability if it is both physically and mathematically unclonable

Question 9

Properties of ideal PUFs: Tamper Evident

Answer 9

Tampering represents a powerful attack against security implementations. It is used to remove or bypass protection mechanisms to obtain information about sensitive values and parameters.
-> Some PUF classes, which rely on sensitive measurements of random physical features, cannot be physically tampered without significantly changing their challenge-response behavior