Circuit-Free General-Purpose Multi-Party Computation via Co-Utile Unlinkable Outsourcing
This work addresses the usability and flexibility challenges in MPC for developers and users, though it appears incremental as it builds on existing MPC concepts with a new outsourcing approach.
The paper tackles the problem of making general-purpose multiparty computation (MPC) more accessible by eliminating the need for circuit-based compilers, which require significant programming effort and limit code expressiveness. It presents a method that allows MPC on arbitrary ordinary code, using co-utile outsourcing and decentralized reputation, and empirically shows that reputation effectively ensures correct results for high-reputation parties.
Multiparty computation (MPC) consists in several parties engaging in joint computation in such a way that each party's input and output remain private to that party. Whereas MPC protocols for specific computations have existed since the 1980s, only recently general-purpose compilers have been developed to allow MPC on arbitrary functions. Yet, using today's MPC compilers requires substantial programming effort and skill on the user's side, among other things because nearly all compilers translate the code of the computation into a Boolean or arithmetic circuit. In particular, the circuit representation requires unrolling loops and recursive calls, which forces programmers to (often manually) define loop bounds and hardly use recursion. We present an approach allowing MPC on an arbitrary computation expressed as ordinary code with all functionalities that does not need to be translated into a circuit. Our notion of input and output privacy is predicated on unlinkability. Our method leverages co-utile computation outsourcing using anonymous channels via decentralized reputation, makes a minimalistic use of cryptography and does not require participants to be honest-but-curious: it works as long as participants are rational (self-interested), which may include rationally malicious peers (who become attackers if this is advantageous to them). We present example applications, including e-voting. Our empirical work shows that reputation captures well the behavior of peers and ensures that parties with high reputation obtain correct results.