Paper 2019/1300 Original: eprint.iacr.org/2019/1300 ↗

Actively Secure Setup for SPDZ

Abstract

We present an actively secure, practical protocol to generate the distributed secret keys needed in the SPDZ offline protocol. The resulting distribution of the public and secret keys is such that the associated SHE `noise' analysis is the same as if the distributed keys were generated by a trusted setup. We implemented the presented protocol for distributed BGV key generation within the SCALE-MAMBA framework. Our method makes use of a new method for creating doubly (or even more) authenticated bits in different MPC engines, which has applications in other areas of MPC-based secure computation. We were able to generate keys for two parties and a plaintext size of 64 bits in around five minutes, and approximately eighteen minutes for a 128 bit prime. This version contains differences from the published version due to an attack on the method in the published version discovered by Victor Treinsoutrot. In this revised version we describe a fix (due to Carsten Baum) which would result in a roughly two fold performance loss in our protocol. This revised version does not update the running times of the protocol precisely, but estimates the expected running time from extrapolating micro-benchmarks. The new expected run times are roughly two times slower than those in the previous version. This file contains, to aid the reader, first the new version and then a diff with the published version which appeard in Journal of Cryptology.