*22:17*[Pub][ePrint] How to Hide Circuits in MPC: An Efficient Framework for Private Function Evaluation, by Payman Mohassel and Saeed Sadeghian

We revisit the problem of general-purpose \\emph{private function evaluation} (PFE) wherein a single party $P_1$ holds a circuit $\\C$, while each $P_i$ for $1 \\le i \\leq n$ holds a private input $x_i$, and the goal is for a subset (or all) of the parties to learn $\\C(x_1, \\ldots, x_n)$ but nothing else. We put forth a general framework for designing PFE where the task of hiding the circuit and securely evaluating its gates are addressed independently: First, we reduce the task of hiding the circuit topology to oblivious evaluation of a mapping that encodes the topology of the circuit, which we refer to as \\emph{oblivious extended permutation} (OEP) since the mapping is a generalization of the permutation mapping. Second, we design a subprotocol for private evaluation of a single gate (PFE for one gate), which we refer to as \\emph{private gate evaluation} (PGE). Finally, we show how to naturally combine the two components to obtain efficient and secure PFE.

We apply our framework to several well-known general-purpose MPC constructions, in each case, obtaining the most efficient PFE construction to date, for the considered setting. Similar to the previous work we only consider semi-honest adversaries in this paper.

\\begin{itemize} \\item In the \\emph{multiparty} case with dishonest majority, we apply our techniques to the seminal GMW protocol~\\cite{GMW87} and obtain the first general-purpose PFE with \\emph{linear complexity} in the circuit size.

\\item In the \\emph{two-party} case, we transform Yao\'s garbled circuit protocol~\\cite{yao86} into a constant-round two-party PFE. Depending on the instantiation of the underlying subprotocol, we either obtain a two-party PFE with linear complexity that improves on the only other work with similar asymptotic efficiency (Katz and Malka, ASIACRYPT 2011), or a two-party PFE that provides the best concrete efficiency to date despite not being linear.

\\item The above two constructions are for boolean circuits. In case of \\emph{arithmetic circuits}, we obtain the first PFE with linear complexity based on any additively homomorphic encryption scheme. \\end{itemize}

Though each construction uses different techniques, a common feature in all three is that the overhead of hiding the circuit $\\C$ is essentially equal to the cost of running the OEP protocol on a vector of size $|\\C|$. As a result, to improve efficiency, one can focus on lowering the cost of the underlying OEP protocol. OEP can be instantiated using a singly homomorphic encryption or any general-purpose MPC but we introduce a new construction that we show is significantly more efficient than these alternatives, in practice. The main building block in our OEP construction is an efficient protocol for \\emph{oblivious switching network evaluation} (OSN), a generalization of the previously studied oblivious shuffling problem which is of independent interest. Our results noticeably improve efficiency of the previous solutions to oblivious shuffling, yielding a factor of 25 or more gain in computation and communication.