Universally verifiable multiparty computation from threshold homomorphic cryptosystems

Multiparty computation can be used for privacy-friendly outsourcing of computations on private inputs of multiple parties. A computation is outsourced to several computation parties; if not too many are corrupted (e.g., no more than half), then they cannot determine the inputs or produce an incorrect output. However, in many cases, these guarantees are not enough: we need correctness even if /all/ computation parties may be corrupted; and we need that correctness can be verified even by parties that did not participate in the computation. Protocols satisfying these additional properties are called ``universally verifiable''. In this paper, we propose a new security model for universally verifiable multiparty computation, and we present a practical construction, based on a threshold homomorphic cryptosystem. We also develop a multiparty protocol for jointly producing non-interactive zero-knowledge proofs, which may be of independent interest. Keywords: multiparty computation, verifiability, Fiat-Shamir heuristic, threshold homomorphic cryptosystem