## CryptoDB

### Reo Eriguchi

#### Publications

**Year**

**Venue**

**Title**

2021

CRYPTO

Non-Interactive Secure Multiparty Computation for Symmetric Functions, Revisited: More Efficient Constructions and Extensions
📺
Abstract

Non-interactive secure multiparty computation (NIMPC) is a variant of secure computation which allows each of $n$ players to send only a single message depending on his input and correlated randomness.
Abelian programs, which can realize any symmetric function, are defined as functions on the sum of the players' inputs over an abelian group and provide useful functionalities for real-world applications.
We improve and extend the previous results in the following ways:
\begin{itemize}
\item We present NIMPC protocols for abelian programs that improve the best known communication complexity.
If inputs take any value of an abelian group $\mathbb{G}$, our protocol achieves the communication complexity $O(|\mathbb{G}|(\log|\mathbb{G}|)^2)$ improving $O(|\mathbb{G}|^2n^2)$ of Beimel et al. (Crypto 2014).
If players are limited to inputs from subsets of size at most $d$, our protocol achieves $|\mathbb{G}|(\log|\mathbb{G}|)^2(\max\{n,d\})^{(1+o(1))t}$ where $t$ is a corruption threshold.
This result improves $|\mathbb{G}|^3(nd)^{(1+o(1))t}$ of Beimel et al. (Crypto 2014), and even $|\mathbb{G}|^{\log n+O(1)}n$ of Benhamouda et al. (Crypto 2017) if $t=o(\log n)$ and $|\mathbb{G}|=n^{\Theta(1)}$.
\item We propose for the first time NIMPC protocols for linear classifiers that are more efficient than those obtained from the generic construction.
\item We revisit a known transformation of Benhamouda et al. (Crypto 2017) from Private Simultaneous Messages (PSM) to NIMPC, which we repeatedly use in the above results.
We reveal that a sub-protocol used in the transformation does not satisfy the specified security.
We also fix their protocol with only constant overhead in the communication complexity.
As a byproduct, we obtain an NIMPC protocol for indicator functions with asymptotically optimal communication complexity with respect to the input length.
\end{itemize}

2021

ASIACRYPT

Homomorphic Secret Sharing for Multipartite and General Adversary Structures Supporting Parallel Evaluation of Low-Degree Polynomials
📺
Abstract

Homomorphic secret sharing (HSS) for a function $f$ allows input parties to distribute shares for their private inputs and then locally compute output shares from which the value of $f$ is recovered. HSS can be directly used to obtain a two-round multiparty computation (MPC) protocol for possibly non-threshold adversary structures whose communication complexity is independent of the size of $f$. In this paper, we propose two constructions of HSS schemes supporting parallel evaluation of a single low-degree polynomial and tolerating multipartite and general adversary structures. Our multipartite scheme tolerates a wider class of adversary structures than the previous multipartite one in the particular case of a single evaluation and has exponentially smaller share size than the general construction. While restricting the range of tolerable adversary structures (but still applicable to non-threshold ones), our schemes perform $\ell$ parallel evaluations with communication complexity approximately $\ell/\log\ell$ times smaller than simply using $\ell$ independent instances. We also formalize two classes of adversary structures taking into account real-world situations to which the previous threshold schemes are inapplicable. Our schemes then perform $O(m)$ parallel evaluations with almost the same communication cost as a single evaluation, where $m$ is the number of parties.

#### Coauthors

- Koji Nuida (2)
- Kazuma Ohara (1)
- Shota Yamada (1)