Stateful multi-client verifiable computation

Abstract

This paper develops an asynchronous cryptographic protocol for outsourcing arbitrary stateful computation among multiple clients to an untrusted server, while guaranteeing integrity of the data. The clients communicate only with the server and merely store a short authenticator to ensure that the server does not cheat. Our contribution is two-fold. First, we extend the recent hash&prove scheme of Fiore et al. (CCS 2016) to stateful computations that support arbitrary updates by the untrusted server, in a way that can be verified by the clients. We use this scheme to generically instantiate authenticated data types. Second, we describe a protocol for multi-client verifiable computation based on an authenticated data type, and prove that it achieves a computational version of fork linearizability. This is the strongest guarantee that can be achieved in the setting where clients do not communicate directly; it ensures correctness and consistency of outputs seen by the clients individually.