Paper 2025/168

Revisiting Beimel-Weinreb Weighted Threshold Secret Sharing Schemes

Miquel Guiot, Rovira i Virgili University

Abstract

A secret sharing scheme is a cryptographic primitive that allows a dealer to share a secret among a set of parties, so that only authorized subsets of them can recover it. The access structure of the scheme is the family of authorized subsets. In a weighted threshold secret sharing scheme, each party is assigned a weight according to its importance, and the authorized subsets are those in which the sum of their weights is at least the threshold value. For these access structures, Beimel and Weinreb [IPL 2006] presented the best general constructions: The scheme with computational security has a total share size polynomial in $n$, while the scheme with perfect security has a total share size $n^{O(\log n)}$. However, these constructions require the use of shallow monotone sorting networks, which limits their practical use. In this work, we revisit weighted threshold secret sharing from a circuit-based perspective. By considering alternative circuits and formulas that avoid monotone sorting networks, we obtain substantial improvements in two directions. First, in the computational setting, we provide a computational scheme that is feasible in practice. Assuming the existence of one-way functions with security parameter $\lambda$, we show that any weighted threshold access structure over $n$ parties with threshold $\sigma$ admits a computational secret sharing scheme with share size $\lambda$, public information of size $O(\lambda n^2 \log \sigma)$, and a reconstruction procedure in which any authorized subset needs to download only $O(\lambda n \log \sigma)$ bits of public information. Notably, for weight distributions that arise in the most widely deployed blockchain networks, our construction reduces the total share size ranging from $300\times$ to $6700\times$ compared to the best previously known schemes. Second, extending these techniques, we obtain improved information-theoretic ramp weighted threshold secret sharing schemes. For any weights, threshold $\sigma$, and $\epsilon < 1$, we construct a $(\sigma,(1+\epsilon)\sigma)$-ramp weighted threshold scheme with share size $O\left((n/\epsilon) \log n\right)$, reducing the share size with respect to the state-of-the-art solutions.