Constant-Round Client-Aided Secure Comparison Protocol

Abstract

We present an improved constant-round secure two-party protocol for integer comparison functionality, which is one of the most fundamental building blocks in secure computation.Our protocol is in the so-called client-server model, which is utilized in real-world MPC products such as Sharemind, where any number of clients can create shares of their input and distribute to the servers who then jointly compute over the shares and return the shares of result to the client. In the client-aided client-server model, as mentioned briefly by Mohassel and Zhang (S&P'17), a client further generates and distributes some necessary correlated randomness to servers. Such correlated randomness admits efficient protocols since otherwise servers have to jointly generate randomness by themselves, which can be inefficient.In this paper, we improve the state-of-the-art constant-round comparison protocols by Damgård et al. (TCC'06) and Nishide and Ohta (PKC'07) in the client-aided model. Our techniques include identifying correlated randomness in these comparison protocols. Along the way, we also use tree-based techniques for a building block, which deviate from the above two works. Our proposed protocol requires only 5 communication rounds, regardless of the bit length of inputs. This is at least 5 times fewer rounds than existing protocols. We implement our secure comparison protocol in C++. Our experimental results show that this low-round complexity benefits in low-latency networks such as WAN.

Efficiency and Accuracy Improvements of Secure Floating-Point Addition over Secret Sharing

“…By Table 1, our 2PC protocol (in Truncation mode) has round complexity even smaller than the best known (accurate) 3PC protocol [7]. We note that our protocols (as well as the protocols in [20]) require somewhat complicated correlated randomness and therefore the use of the so-called trusted initializer model, such as the client-aided model (e.g., [18], [19]), in the offline phase suits well.…”

An Efficient Secure Division Protocol Using Approximate Multi-Bit Product and New Constant-Round Building Blocks

“…To the best of our knowledge, our Overflow (and its extensions) are the first constant-round secure protocols that work not over F but over Z 2 . Moreover, in comparison with the state of the art constant-round Comparison protocol in two-party setting [17], our Comparison protocol is better in terms of communication rounds and data transfer. In fact, [17] needs five rounds and ((log ) 3 ) bit data transfer for SS over F , while our protocol needs four rounds and…”

“…In this paper, we adopt the client-aided model [15], [17], [20] for client-server SS-based MPC, which is a kind of trusted dealer setup model. More precisely, in this model, the clients still do not participate in the online computation phase of the protocol, while in the pre-computation phase, the clients send to the servers not only their shared inputs but also certain kinds of auxiliary information (i.e., Beaver triples) we use in the protocol.…”

Recent Advances in Practical Secure Multi-Party Computation

“…We need to O(log n) communication rounds since we calculate the overflow from the lower bits. If we adopt not the shares over Z 2 n but Z p (p: prime), we can construct constant-round Comparison via the different strategy [43], [44].…”