Tiny-CFA: Minimalistic Control-Flow Attestation Using Verified Proofs of Execution

The design of tiny trust anchors attracted much attention over the past decade, to secure low-end MCU-s that cannot afford more expensive security mechanisms. In particular, hardware/software (hybrid) co-designs offer low hardware cost, while retaining similar security guarantees as (more expensive) hardware-based techniques. Hybrid trust anchors support security services (such as remote attestation, proofs of software update/erasure/reset, and proofs of remote software execution) in resource-constrained MCU-s, e.g., MSP430 and AVR AtMega32. Despite these advances, detection of control-flow attacks in low-end MCU-s remains a challenge, since hardware requirements for the cheapest mitigation techniques are often more expensive than the MCU-s themselves. In this work, we tackle this challenge by designing Tiny-CFA - a Control-Flow Attestation (CFA) technique with a single hardware requirement - the ability to generate proofs of remote software execution (PoX). In turn, PoX can be implemented very efficiently and securely in low-end MCU-s. Consequently, our design achieves the lowest hardware overhead of any CFA technique, while relying on a formally verified PoX as its sole hardware requirement. With respect to runtime overhead, Tiny-CFA also achieves better performance than prior CFA techniques based on code instrumentation. We implement and evaluate Tiny-CFA, analyze its security, and demonstrate its practicality using real-world publicly available applications.