Verifying the Consistency of Remote Untrusted Services with Commutative Operations

A group of mutually trusting clients outsources a computation service to a remote server, which they do not fully trust and that may be subject to attacks. The clients do not communicate with each other and would like to verify the correctness of the remote computation and the consistency of the server’s responses. This paper presents the Commutative-Operation verification Protocol (COP) that ensures linearizability when the server is correct and preserves fork-linearizability in any other case. Fork-linearizability ensures that all clients that observe each other’s operations are consistent in the sense that their own operations and those operations of other clients that they see are linearizable. COP strictly generalizes previous protocols by modeling abortable services and by supporting waitfree client operations for sequences of arbitrary commutative operations. When combined with authenticated data structures, COP also adds distributed consistency verification to computations with outsourced state, where the clients do not store the whole state of the computation.