Multi-server verifiable delegation of computations: Unconditional security and practical efficiency

Abstract

Outsourcing computation has gained significant popularity in recent years due to the prevalence of cloud computing. There are two main security concerns in outsourcing computation: how to guarantee the cloud server performs the computation correctly and how to keep the client's data secret. The single-server verifiable computation (SSVC) of Gennaro, Gentry and Parno (Crypto'10) enables a client to delegate the computation of a function f on any input x with both concerns highly relieved, but only results in computationally secure schemes that lack practical efficiency.While the SSVC schemes use a single server, in this paper we develop a multi-server verifiable computation (MSVC) model where the client shares both f and x among multiple servers, each server performs a set of computations on its shares, and finally the client reconstructs f(x) from all servers' results. In this MSVC model we propose a generic construction for outsourcing computations of the form Fx, where F is a matrix and x is a vector. Our generic construction achieves information-theoretic security, input privacy and function privacy. By optimizing the parameters, we obtain both a 3-server scheme, which uses the least number of servers, and a 4-server scheme, which incurs the least workload. By decomposing many polynomial computations as a two-stage computation, where the first-stage has the form Fx and the second-stage is fast, and delegating the first-stage computation, we obtain MSVC schemes for these polynomials. We implement our MSVC schemes and show that they are among the most practical ones to date.