Sensing environmental variables is an important step for autonomous devices which operate in different conditions. For this purpose, the digital chips embed sensors, preferably digital for portability reasons, to ensure that they are not operating out-of-specifications. However, a digital sensor is exposed to various stress as well as aging during its lifetime. Aging makes the sensors' output deviate from the original intended one, resulting in an incorrect report of the operating condition and in turn improper actions based on such reports. This paper presents a thorough study on the effect of aging on digital sensors. We first propose two low-overhead characterization schemes for these sensors. Then, we investigate the accuracy of such characterizations on different processes, voltages, temperatures and aging conditions. Our results show that sensors' ability to detect attacks (as well as false detections) varies with aging, notably in the first 5 years of usage. Indeed, comparing new versus aged sensor shows that 3% to 12% of the alarms that would raise if the sensor were new, will not be fired for a 5 year old device, while 15%-25% false alarms are raised for the device after 5 years of usage. This brings a great concern to the reliability as well as the security of the digital sensors which can not be used as is after aging. It is also shown that these results stay the same whatever the variation of the process. Consequently, it is recommended to associate an aging correction to digital sensors, such as in-field calibration techniques, and/or sensor replication.
One way for an attacker to break a system is to perturb it. Expected effects are countermeasure deactivation or data corruption to disclose sensitive information. The prevention of such actions relies on detection of abnormal operating conditions. Digital sensors can play this role. A digital sensor is built out of the very same standard cells as the user logic to be protected. This ensures the advantage that the sensor and the user logic are exposed to the same stress. Balancing True positives and False negatives is a tough question in field of sensors. This is a general issue, and the best way to mitigate this paradox is to thoroughly investigate their properties, through simulations and real experiments. This results in characterizations, which in turn allows for intuitions on how to handle sensing values. In this paper, we exhibit the complex relationships between propagation times in logic and environmental conditions. Those results reinforce the relevance of the digital sensor versus the adversarial manipulation of environmental conditions: fewer false alarms are raised even if temperature (resp. voltage) is extreme, provided the effect is balanced by voltage (resp. temperature). Owing to the complex relationship between propagation delays, temperature and voltage, this cannot happen with a set of independent temperature and voltage sensors.
As an outstanding cell-level countermeasure to defeat power analysis attacks, dual-rail pre-charge logics rely on balanced complementary paths. During the circuit lifetime, the gates undergo unavoidable changes due to the so-called device aging, hence imbalancing the dual rails. Here, we focus on Sense Amplifier Based Logic (SABL), and highlight the vulnerability of corresponding circuits when the device is aged. By integrating gate-level masking, we introduce a modified variant of SABL, maintaining its resistance in presence of device aging. The corresponding results, covering both dynamic and static power profiles, show the prominent impact of our construction on extending the protection of circuits for their entire lifetime.
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