Helium: Scalable MPC among Lightweight Participants and under Churn

We introduce Helium, a novel framework that supports scalable secure multiparty computation (MPC) for lightweight participants and tolerates churn. Helium relies on multiparty homomorphic encryption (MHE) as its core building block. While MHE schemes have been well studied in theory, prior works fall short of addressing critical considerations paramount for adoption such as supporting resource-constrained and unstably connected participants. In this work, we systematize the requirements of MHE-based MPC protocols from a practical lens, and we propose a novel execution mechanism that addresses those considerations. We implement this execution mechanism in Helium, which makes it the first implemented framework to support MPC under network churn based solely on cryptographic assumptions. We show that a Helium network of 30 parties connected with 100Mbits/s links and experiencing a system-wide churn rate of 40 failures per minute can compute the product between a fixed 512x512 secret matrix (e.g., a collectively-trained private model) and a fresh secret vector (e.g., a feature vector) 8.3 times per second. This is ~1500 times faster than a state-of-the-art MPC framework operating under no churn.