Fault attacks have gained particular attention in recent years as they present a severe threat to security in rapidly rising Internet-of-Things (IoT) devices. IoT devices are generally security-critical and resource-constrained. Therefore, any security protocol deployed in these devices has to satisfy several constraints such as small area footprint, low power, and memory consumption. Combinational circuit implementation of S-box is preferable over look-up table (LUT) in terms of memory consumption as the memory operations are usually the costliest part of lightweight cipher implementations. In this work, we analyze the S-box of AES against a novel fault analysis technique, Semi-Permanent Stuck-At (SPSA) fault analysis. We pinpoint hotspots in an optimized implementation of AES S-box that weaken the cryptographic properties of the S-box, leading to key recovery attacks. Our work investigates new vulnerabilities towards fault analysis in combinational circuit implementation.

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