Impact of Aging on the Reliability of Delay PUFs

Physically Unclonable Functions (PUFs) are mainly used for generating unique keys to identify electronic devices. These entities mainly benefit from the process variations occurring during the device manufacturing. To be able to use PUFs to identify electronic devices or to utilize them in cryptographic applications, the reliability of PUFs needs to be assured under a wide variety of environmental conditions and aging mechanisms, including the switching activity of the PUFs’ internal signals. In practice, it is important to evaluate aging effects as early as possible, preferentially at design time. In this paper, we evaluate the impact of aging on two types of delay-PUFs (arbiter-PUFs and loop-PUFs) with different switching activities. This work takes advantage of both simulation tool and silicon tests on a 65nm ASIC implementation. To expedite the simulation process and get rid of conducting simulations of multiple delay-element PUFs, we propose an extrapolation method to evaluate the effect of BTI (Bias Temperature-Instability) and HCI (Hot Carrier Injection) aging under different switching activities on PUFs with multiple delay elements using the aging effects on single delay-element PUFs. The results show that switching activity (expressed in terms of transitions/time) has a limited impact on delay chains of considered delay-PUFs, while it has a greater impact on the arbiter (RS latch) of the arbiter-PUF. The simulation results show that the aging-related Bit Error Rate in an arbiter-PUF with high switching activity can be 11 times worse than the Bit Error Rate in the same PUF when there is no activity in 20 months.