EMMA: Hardware/Software Attestation Framework for Embedded Systems Using Electromagnetic Signals

Establishing trust for an execution environment is an important problem, and practical solutions for it rely on attestation, where an untrusted system (prover) computes a response to a challenge sent by the trusted system (verifier). The response typically is a checksum of the prover's program, which the verifier checks against expected values for a "clean" (trustworthy) system. The main challenge in attestation is that, in addition to checking the response, the verifier also needs to verify the integrity of the response computation. On higher-end processors, this integrity is verified cryptographically, using dedicated trusted hardware. On embedded systems, however, constraints prevent the use of such hardware support. Instead, a popular approach is to use the request-to-response time as a way to establish confidence. However, the overall request-to-response time provides only one coarse-grained measurement from which the integrity of the attestation is to be inferred, and even that is noisy because it includes the network latency and/or variations due to micro-architectural events. Thus, the attestation is vulnerable to attacks where the adversary has tampered with response computation, but the resulting additional computation time is small relative to the overall request-to-response time. In this paper, we make a key observation that execution-time measurement is only one example of using externally measurable side-channel information, and that other side-channels, some of which can provide much finer-grain information about the computation, can be used. As a proof of concept, we propose EMMA, a novel method for attestation that leverages electromagnetic side-channel signals that are emanated by the system during response computation, to confirm that the device has, upon receiving the challenge, actually computed the response using the valid program code for that computation. This new approach requires physical proximity, but imposes no overhead to the system, and provides accurate monitoring during the attestation. We implement EMMA on a popular embedded system, Arduino UNO, and evaluate our system with a wide range of attacks on attestation integrity. Our results show that EMMA can successfully detect these attacks with high accuracy. We compare our method with the existing methods and show how EMMA outperforms them in terms of security guarantees, scalability, and robustness.