## CryptoDB

### Paper: Upslices, Downslices, and Secret-Sharing with Complexity of $1.5^n$

Authors: Oded Nir , Tel Aviv University Benny Applebaum , Tel Aviv University DOI: 10.1007/978-3-030-84252-9_21 (login may be required) Search ePrint Search Google CRYPTO 2021 A secret-sharing scheme allows to distribute a secret $s$ among $n$ parties such that only some predefined authorized'' sets of parties can reconstruct the secret, and all other unauthorized'' sets learn nothing about $s$. The collection of authorized/unauthorized sets is be captured by a monotone function $f:\{0,1\}^n\rightarrow \{0,1\}$. In this paper, we focus on monotone functions that all their min-terms are sets of size $a$, and on their duals -- monotone functions whose max-terms are of size $b$. We refer to these classes as $(a,n)$-\emph{upslices} and $(b,n)$-\emph{downslices}, and note that these natural families correspond to monotone $a$-regular DNFs and monotone $(n-b)$-regular CNFs. We derive the following results. \begin{enumerate} \item (General downslices) Every downslice can be realized with total share size of $1.5^{n+o(n)}<2^{0.585 n}$. Since every monotone function can be cheaply decomposed into $n$ downslices, we obtain a similar result for general access structures improving the previously known $2^{0.637n+o(n)}$ complexity of Applebaum, Beimel, Nir and Peter (STOC 2020). We also achieve a minor improvement in the exponent of linear secrets sharing schemes. \item (Random mixture of upslices) Following, Beimel and Farr{\{a}}s (TCC 2020) who studied the complexity of random DNFs with constant-size terms, we consider the following general distribution $F$ over monotone DNFs: For each width value $a\in [n]$, uniformly sample $k_a$ monotone terms of size $a$, where $\vec{k}=(k_1,\ldots,k_n)$ is an arbitrary vector of non-negative integers. We show that, except with exponentially small probability, $F$ can be realized with share size of $2^{0.5 n+o(n)}$ and can be linearly realized with an exponent strictly smaller than $2/3$. Our proof also provides a candidate distribution for the exponentially-hard'' access structure. \end{enumerate} We use our results to explore connections between several seemingly unrelated questions about the complexity of secret-sharing schemes such as worst-case vs. average-case, linear vs. non-linear, and primal vs. dual access structures. We prove that, in at least one of these settings, there is a significant gap in secret-sharing complexity.
##### BibTeX
@inproceedings{crypto-2021-31128,
title={Upslices, Downslices, and Secret-Sharing with Complexity of $1.5^n$},
publisher={Springer-Verlag},
doi={10.1007/978-3-030-84252-9_21},
author={Oded Nir and Benny Applebaum},
year=2021
}
`