Constant Round Maliciously Secure 2PC with Function-independent Preprocessing using LEGO

Secure two-party computation (S2PC) allows two parties to compute a function on their joint inputs while leaking only the output of the function. At TCC 2009 Orlandi and Nielsen proposed the LEGO protocol for maliciously secure 2PC based on cut-and-choose of Yao’s garbled circuits at the gate level and showed that this is asymptotically more efficient than on the circuit level. Since then the LEGO approach has been improved upon in several theoretical works, but never implemented. In this paper we describe further concrete improvements and provide the first implementation of a protocol from the LEGO family. Our protocol has a constant number of rounds and is optimized for the offline/online setting with function-independent preprocessing. We have benchmarked our prototype and find that our protocol can compete with all existing implementations and that it is often more efficient. As an example, in a LAN setting we can evaluate an AES-128 circuit with online latency down to 1.13ms, while if evaluating 128 AES-128 circuits in parallel the amortized cost is 0.09ms per AES-128. This online performance does not come at the price of offline inefficiency as we achieve comparable performance to previous, less general protocols, and significantly better if we ignore the cost of the function-independent preprocessing. Also, as our protocol has an optimal 2-round online phase it is significantly more efficient than previous protocols when considering a high latency network. Keywords—Secure Two-party Computation, Implementation, LEGO, XOR-Homomorphic Commitments, Selective OT-Attack