Designing Fully Secure Protocols for Secure Two-Party Computation of Constant-Domain Functions.

In a sense, a two-party protocol achieves fairness if the output from the computation is obtained simultaneously by both parties. A seminal result by Cleve (STOC 1986) states that fairness is impossible, in general. Surprisingly, Gordon et al. (JACM 2011) showed that there exist interesting functions that are computable with fairness. The two results give rise to a distinction between fair functions and unfair ones. The question of characterizing these functions has been studied in a sequence of works leading to the complete characterization of (symmetric) Boolean functions by Asharov et al. (TCC 2015). In this paper, we design new fully secure protocols for functions that were previously unknown to be fair. To this end, our main technical contribution is a generic construction of a fully secure (fair) protocol, starting with a constant-round protocol satisfying limited security requirements. Our construction introduces new conceptual tools for the analysis of fairness that apply to arbitrary (constant-domain) functions. While the characterization remains open, we believe that our results lay the foundation for a deeper understanding of fairness.