Paper 2024/194

Helium: Scalable MPC among Lightweight Participants and under Churn

Abstract

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 $100$Mbits/s links and experiencing a system-wide churn rate of $40$ failures per minute can compute the product between a fixed $512\times512$ 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 $\sim\!\!1500$ times faster than a state-of-the-art MPC framework operating under no churn.