Advanced Analysis of Cell Stability for Reliable SRAM PUFs

A Physically Unclonable Function (PUF) is a structure that when issued a challenge, it produces a unique and reliable response which can be used as an identifier or a cryptographic key. SRAM PUFs create unique responses upon power up as certain SRAM cells output a ´1´ or ´0´ with high probability due to uncontrollable process variations. A current challenge in SRAM PUFs is their sensitivity to temperature and voltage variations as well as aging. By creating algorithms that isolate stable bits quickly and with minimal testing, the use of SRAM PUF should become more practical. In this paper, we explore the selection of stable bits through enrollment under different conditions (temperature, voltage, and aging) and also by exploiting previously undiscovered interactions between neighboring SRAM cells. We develop metrics that analyze the impact of each neighboring cell and each enrollment condition. Our metrics can be used to identify the best cells and conditions for stable bit selection. We have analyzed data from Spartan 3 FPGA and our metrics identify the best neighborhood size (16 stable neighbors) and best enrollment condition pair (high temperature, high voltage and low temperature).