Aging-Resilient SRAM-based True Random Number Generator for Lightweight Devices

A random number generator (RNG) is an important building block for cryptographic operations primarily to generate random nonces and secret keys. The power-up value of an SRAM array has been widely accepted as an entropy source for generating random numbers. However, only a few cells of the SRAM are truly random upon repeated power-ups; the vast majority of cells display a distinct bias from manufacturing process variations. Consequently, a relatively large SRAM array is required to obtain sufficient entropy for generating random numbers. Earlier research has proposed the use of controlled device aging at pre-deployment stage to enhance the initial entropy of an SRAM array. However, aging in the field can adversely affect the entropy and degrade randomness; we show here that any initial aging to increase SRAM entropy can even be counterproductive. Instead, we propose an SRAM-based random number generation approach, which continually manipulates device aging during operation to constantly maximize entropy for the entire deployment period. The key idea is to continually stress the SRAM cells in their power-up states at regular intervals. This helps counteract the aging caused by the random memory states that occur during operation. Silicon results are presented to validate our proposed approach.