Analysis of Communication Channels Related to Physical Unclonable Functions

TL;DR

This paper introduces the Varying Binary Symmetric Channel (VBSC) model for PUF responses, accounting for unequal bit reliabilities, and analyzes its capacity under different channel state information scenarios.

Contribution

It proposes a more realistic PUF channel model (VBSC) and evaluates its capacity considering various levels of channel state information at encoder and decoder.

Findings

VBSC provides a better approximation of PUF behavior than BSC.

Channel capacity varies significantly with different CSI scenarios.

The model helps improve error correction strategies for PUF-based key generation.

Abstract

Cryptographic algorithms rely on the secrecy of their corresponding keys. On embedded systems with standard CMOS chips, where secure permanent memory such as flash is not available as a key storage, the secret key can be derived from Physical Unclonable Functions (PUFs) that make use of minuscule manufacturing variations of, for instance, SRAM cells. Since PUFs are affected by environmental changes, the reliable reproduction of the PUF key requires error correction. For silicon PUFs with binary output, errors occur in the form of bitflips within the PUFs response. Modelling the channel as a Binary Symmetric Channel (BSC) with fixed crossover probability $p$ is only a first-order approximation of the real behavior of the PUF response. We propose a more realistic channel model, refered to as the Varying Binary Symmetric Channel (VBSC), which takes into account that the reliability of different PUF response bits may not be equal. We investigate its channel capacity for various scenarios which differ in the channel state information (CSI) present at encoder and decoder. We compare the capacity results for the VBSC for the different CSI cases with reference to the distribution of the bitflip probability according a work by Maes et al.