Multiplicative and Verifiably Multiplicative Secret Sharing for Multipartite Adversary Structures

$d$-Multiplicative secret sharing enables $n$ players to locally compute additive shares of the product of $d$ secrets from their shares. Barkol et al. (Journal of Cryptology, 2010) show that it is possible to construct a $d$-multiplicative scheme for any adversary structure satisfying the $Q_d$ property, in which no $d$ sets cover the whole set of players. In this paper, we focus on multipartite adversary structures and propose efficient multiplicative and verifiably multiplicative secret sharing schemes tailored to them. First, our multiplicative scheme is applicable to any multipartite $Q_d$-adversary structure. If the number of parts is constant, our scheme achieves a share size polynomial in the number $n$ of players while the general construction by Barkol et al. results in exponentially large share size in the worst case. We also propose its variant defined over smaller fields. As a result, for a special class of bipartite adversary structures with two maximal points, it achieves a constant share size for arbitrary $n$ while the share size of the first scheme necessarily incurs a logarithmic factor of $n$. Secondly, we devise a more efficient scheme for a special class of multipartite ones such that players in each part have the same weight and a set of players belongs to the adversary structure if and only if the sum of their weights is at most a threshold. Thirdly, if the adversary structure is $Q_{d+1}$, our first scheme is shown to be a verifiably multiplicative scheme that detects incorrect outputs with probability $1$. For multipartite adversary structures with a constant number of parts, it improves the worst-case share and proof sizes of the only known general construction by Yoshida and Obana (IEEE Transactions on Information Theory, 2019). Finally, we propose a more efficient verifiably multiplicative scheme by allowing small error probability $\delta$ and focusing on a more restricted class of multipartite adversary structures. Our scheme verifies computation of polynomials and can achieve a share size independent of $\delta$ while the previous construction only works for monomials and results in a share size involving a factor of $\log\delta^{-1}$

Flaws in a Verifiably Multiplicative Secret Sharing Scheme from ICITS 2017

In this paper, we point out flaws in an existing verifiably multiplicative secret sharing (VMSS) scheme. Namely, we show that a scheme proposed by Yoshida and Obana presented at ICITS 2017 is insecure against an adversary who corrupts a single player. We then show that in the model of ICITS 2017 which restricts the decoder additive, the error-free verification is impossible. We further show that by allowing a general class of decoders which include a linear one, the scheme is error-free

A New Cryptosystem Based On Hidden Order Groups

Let $G_1$ be a cyclic multiplicative group of order $n$. It is known that the Diffie-Hellman problem is random self-reducible in $G_1$ with respect to a fixed generator $g$ if $\phi(n)$ is known. That is, given $g, g^x\in G_1$ and having oracle access to a `Diffie-Hellman Problem' solver with fixed generator $g$, it is possible to compute $g^{1/x} \in G_1$ in polynomial time (see theorem 3.2). On the other hand, it is not known if such a reduction exists when $\phi(n)$ is unknown (see conjuncture 3.1). We exploit this ``gap'' to construct a cryptosystem based on hidden order groups and present a practical implementation of a novel cryptographic primitive called an \emph{Oracle Strong Associative One-Way Function} (O-SAOWF). O-SAOWFs have applications in multiparty protocols. We demonstrate this by presenting a key agreement protocol for dynamic ad-hoc groups.Comment: removed examples for multiparty key agreement and join protocols, since they are redundan

Communication-Efficient MPC for General Adversary Structures

Abstract. A multiparty computation (MPC) protocol allows a set of players to compute a function of their inputs while keeping the inputs private and at the same time securing the correctness of the output. Most MPC protocols assume that the adversary can corrupt up to a fixed fraction of the number of players. Hirt and Maurer initiated the study of MPC under more general corruption patterns, in which the adversary is allowed to corrupt any set of players in some pre-defined collection of sets [6]. In this paper we consider this important direction of research and present significantly improved communication complexity of MPC protocols for general adversary structures. More specifically, ours is the first unconditionally secure protocol that achieves linear communication in the size of multiplicative Monotone Span Program representing the adversary structure in the malicious setting against any Q2 adversary structure, whereas all previous protocols were at least cubic