Control-Flow Attestation (CFA) aims to precisely verify execution paths to a remote verifier. However, existing solutions are fundamentally incompatible with resource-constrained, microcontroller-based systems due to the following reasons: (1) the overhead of transmitting measurement and trace data scales poorly (i.e., often exponentially) with the number of basic blocks or linearly with the length of execution traces, making such approaches impractical even on high-end microprocessor-based systems and entirely infeasible on microcontrollers; (2) cryptographic keys and measurement data are typically stored in memory, rendering them vulnerable to cold boot and memory corruption attacks, which is a serious threat for field-deployed devices; and (3) reliance on software-based measurements, combined with frequent context switches between the Rich Execution Environment (REE) and the Trusted Execution Environment (TEE), introduces significant performance overhead. In this paper, we present ENOLA, a linear-space and low-overhead control-flow attestation solution for microcontroller-based systems. ENOLA achieves linear transmission complexity with basic blocks, guaranteeing its scalability for larger programs. Moreover, ENOLA uses hardware-assisted measurement computation present in off-the-shelf devices, and avoids key storage in memory. ENOLA also allocates general-purpose registers for measurements to thwart memory corruption attacks. ENOLA significantly reduces context switching overhead by eliminating transitions from REE to TEE for backward-edge measurements. We developed the ENOLA compiler using LLVM passes and a custom attestation engine targeting the ARMv8.1-M architecture. Our evaluation shows that ENOLA reduces data transmission overhead by an average of 52x on the Embench, while maintaining performance comparable to (or exceeding) that of existing approaches.

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