Current based PUF exploiting random variations in SRAM cells

Physical Unclonable Function (PUF) is a security primitive that has been proven to be effective in diverse security solutions ranging from hardware authentication to on-die entropy generation. PUFs can be implemented in a design in two possible ways: (1) adding a separate dedicated circuit; and (2) reusing an existing on-chip structure for generating random signatures. A large percentage of existing PUFs falls into the first category, which suffers from the important drawback of often unacceptable hardware and design overhead. Moreover, they cannot be applied to legacy designs, which do not allow insertion of additional circuit structures. Intrinsic PUFs, that rely on pre-existing circuit structures, such as static random-access memory (SRAM), fall into the second category. They, however, typically suffer from poor entropy as well as lack of robustness. In this paper, we introduce a novel PUF implementation of the second category that exploits the effect of manufacturing process variations in SRAM read access current. In particular, we note that transistor level variations in SRAM cells cause significant variations in the read current and the variation changes with the stored content in a SRAM cell. We propose a method to transform the analog read current value for an SRAM array into robust binary signatures. The proposed PUF can be easily employed for authentication of commercial SRAM chips without any design modification. Furthermore, it can be realized, with minor hardware modification, into chips with embedded memory, e.g., a processor, for on-die entropy generation. Simulation results at 45nm CMOS process for 1000 chips as well as measurement results based on 30 commercial SRAM chips, show promising randomness, uniqueness and robustness under environmental fluctuations.