Hardening the Security of Server-Aided MPC Using Remotely Unhackable Hardware Modules

Garbling schemes are useful building blocks for enabling secure multi-party computation (MPC), but require considerable computational resources both for the garbler and the evaluator. Thus, they cannot be easily used in a resource-restricted setting, e.g. on mobile devices. To circumvent this problem, server-aided MPC can be used, where circuit garbling and evaluation are performed by one or more servers. However, such a setting introduces additional points of failure: The servers, being accessible over the network, are susceptible to remote hacks. By hacking the servers, an adversary may learn all secrets, even if the parties participating in the MPC are honest. In this work, we investigate how the susceptibility for such remote hacks in the server-aided setting can be reduced. To this end, we modularize the servers performing the computationally intensive tasks. By using data diodes, air-gap switches and other simple remotely unhackable hardware modules, we can isolate individual components during large parts of the protocol execution, making remote hacks impossible at these times. Interestingly, this reduction of the attack surface comes without a loss of efficiency.