Evaluation of 90nm 6T-SRAM as Physical Unclonable Function for secure key generation in wireless sensor nodes

Abstract

Due to the unattended nature of WSN (Wireless Sensor Network) deployment, each sensor can be subject to physical capture, cloning and unauthorized device alteration. In this paper, we use the embedded SRAM, often available on a wireless sensor node, for secure data (cryptographic keys, IDs) generation which is more resistant to physical attacks. We evaluate the physical phenomenon that the initial state of a 6T-SRAM cell is highly dependent on the process variations, which enables us to use the standard SRAM circuit, as a Physical Unclonable Function (PUF). Important requirements to serve as a PUF are that the start-up values of an SRAM circuit are uniquely determined, unpredictable and similar each time the circuit is turned on. We present the evaluation results of the internal SRAM memories of low power ICs as PUFs and the statistical analysis of the results. The experimental results prove that the low power 90nm commercial 6T-SRAMs are very useful as a PUF. As far as we know, this is the first work that provides an extensive evaluation of 6T-SRAM-based PUF, at different environmental, electrical, and ageing conditions to representing the typical operating conditions of a WSN.

Performance metrics and empirical results of a PUF cryptographic key generation ASIC

“…Yu [20][21] lacked a 4-corner analysis and NIST randomness data. Selimis in [16] lacked worst-case voltage-temperature analysis, and under single parameter (e.g., temperature only or voltage only) analysis achieved a 24% stability safety margin under the assumption that an error correction scheme can correct up to a quarter of the PUF bits being noisy (flipped), i.e., a fractional Hamming distance of 0.25, which is the theoretical limit for a conventional singlestage error correction scheme such as a BCH code [3] [14]. Paral [13] lacked voltage data, though the results corresponded to an RFID deriving power from its antenna implying some voltage fluctuation.…”

Improved Generation of Identifiers, Secret Keys, and Random Numbers From SRAMs

“…This is known as the bit flipping problem [6]- [12]. In identification applications, some bit flipping is allowable provided that the identifiers generated by genuine and fake memories are sufficiently different.…”

“…For this purpose, Helper Data Algorithms (HDAs) -in particular Code Offset-based HDAs as described in [13] -have been employed. The drawback to this solution is the use of heavy error correction codes (ECCs) [6], [7]. As reported in [7], [9], the percentage of cells showing bit flipping increases if there are variations in the power supply voltage and increases even more if there are variations in the operating temperature.…”

Reducing bit flipping problems in SRAM physical unclonable functions for chip identification

“…First implementations of SRAM PUFs were directed to be used in FPGAs [3]. Lately, their use has been reported in 65-and 90-nm integrated circuits [4] [5]. The behavior of SRAM cells has been studied to generate true random numbers, identifiers, and secret keys for cryptographic algorithms [3]- [6].…”

“…However, the intrinsic noisy nature of PUFs causes that the bits provided by the PUF are not always the same but change under the same challenge from one use to another, which is known as bit flipping effect [4] [5]. This is a problem because it decreases both the intended reliability and uniqueness.…”