Secure Two-Party Computation with Reusable Bit-Commitments, via a Cut-and-Choose with Forge-and-Lose Technique

A

secure two-party computation

(S2PC) protocol allows two parties to compute over their combined private inputs, as if intermediated by a trusted third party. In the malicious model, this can be achieved with a

cut-and-choose of garbled circuits

(C&C-GCs), where some GCs are

verified

for correctness and the remaining are

evaluated

to determine the circuit output. This paper presents a new C&C-GCs-based S2PC protocol, with significant advantages in efficiency and applicability. First, in contrast with prior protocols that require a majority of

evaluated

GCs to be correct, the new protocol only requires that at least one

evaluated

GC is correct. In practice this reduces the total number of GCs to approximately one third, for the same statistical security goal. This is accomplished by augmenting the C&C with a new

forge-and-lose

technique based on bit commitments with trapdoor. Second, the output of the new protocol includes reusable XOR-homomorphic bit commitments of all circuit input and output bits, thereby enabling efficient linkage of several S2PCs in a reactive manner. The protocol has additional interesting characteristics (which may allow new comparison tradeoffs), such as needing a low number of exponentiations, using a 2-out-of-1 type of oblivious transfer, and using the C&C structure to statistically verify the consistency of input wire keys.