Differential loop physically unclonable function

An electronic system for generating a secret key comprising a Physically Unclonable Function (PUF) circuit, wherein the PUF circuit comprises two identical loops, arranged in parallel, comprising a first loop and a second loop, each loop comprising a number of delay elements arranged in series, each loop forming a Free-Running Oscillator configured to oscillate between two states,

• wherein the system is configured to apply one or more challenges to the PUF circuit, each challenge forming a twofold challenge comprising a first challenge and a second challenge, the first challenge being applied to the first loop and the second challenge being applied to the second loop,
• wherein the PUF circuit comprises a subtractor configured to receive as inputs the number of oscillations in the first loop and in the number of oscillations in the second loop, for each applied challenge, and determine a difference value corresponding to the difference between the received numbers of oscillations,
• wherein the system is configured to determine a set of enrolled challenges, during an enrollment phase, by evaluating at least one set of candidate challenges, said evaluation comprising applying each candidate challenge of said set of candidate challenges to the PUF circuit, which provides a difference value, for each applied candidate challenge, at the output of the subtractor, a candidate challenge being selected as an enrolled challenge if the difference meets a first reliability condition,
• the system further comprising a secret key generator configured to generate a secret key, comprising a set of bits, during a key generation phase, in response to the application of one or more enrolled challenges to the PUF circuit, each bit of the secret key corresponding to an applied enrolled challenge and corresponding to the sign of the difference value provided by the subtractor in response to said applied enrolled challenge, if the difference meets a second reliability condition.