WiP Paper: BUSted second stop! A first step for breaking cryptographic applications on MCU-based IoT devices

Abstract

IoT devices have become ubiquitous in modern society. However, the security of the newly connected devices, typically powered by microcontrollers (MCUs), has not been thoroughly investigated. One of the overlooked types of attacks on MCU-base IoT devices are microarchitectural side-channel attacks. Until recently, microarchitectural side-channel attacks on MCUs were deemed unlikely due to their perceived simplicity. The BUSted attack has challenged this assumption by unveiling a novel timing-based microarchitectural side channel on the bus interconnect, impacting all mainstream MCUs. BUSted introduces two variants, single-run and multi-run, however only the former was implemented against a simple Smart Lock application. This paper takes the first steps toward exploiting more advanced targets, such as cryptographic applications, by exploring the BUSted multi-run variant. We introduce practical methods for profiling cryptographic applications and address the limitations of the single-run variant, particularly the impractical high profiling time. We propose a new BUSted interrupt-based multi-run methodology to trace the victim on a per-clock basis. Our approach includes a trampoline and sliding window technique to reduce stack interference and enhance profiling granularity. We demonstrate the effectiveness of this solution by successfully profiling the TinyCrypt implementation of AES-CBC.