International Association
for Cryptologic Research

Homomorphic encryption (HE) is a foundational technology in privacy-enhancing cryptography, enabling non-interactive computation over encrypted data. Recently, generalized HE primitives designed for multi-party applications, such as multi-group HE (MGHE), have gained significant research interest. While constructing secure multi-party protocols from (MG)HE in the semi-honest model is straightforward, zero-knowledge techniques are essential for ensuring security against malicious adversaries.

In this work, we design practical proof systems for MGHE to guarantee the well-formedness of public keys and ciphertexts. Specifically, we develop and optimize a polynomial interactive oracle proof (PIOP) for MGHE, which can be compiled into zk-SNARKs using a polynomial commitment scheme (PCS).

We compile our PIOP using a lattice-based PCS, and our implementation achieves a 5.5x reduction in proof size, a 70x speed-up in proof generation, and a 343x improvement in verification time compared to the previous state-of-the-art construction, PELTA (ACM CCS 2023). Additionally, our PIOPs are modular, enabling the use of alternative PCSs to optimize other aspects, such as further reducing proof sizes.