Paper 2025/1980

Traceable Secret Sharing Revisited

Abstract

In a secret sharing scheme for a monotone access structure $\mathcal{A}$, one can share a secret among a set of parties such that all subsets of parties authorized by $\mathcal{A}$ can reconstruct the secret while all other subsets learn nothing. However, what if an unauthorized subset of parties collude and offer their shares for sale? Specifically, suppose that the parties pool their shares to create a reconstruction box that reveals the secret upon receiving enough additional shares as input. To deter this behavior, Goyal et al. (CRYPTO'21) introduced the notion of traceable secret sharing (TSS), where it is possible to provably trace reconstruction boxes containing leaked secret shares back to their respective parties. Goyal et al. and subsequent work presented definitions and constructions of TSS for the threshold access structure. In this work, we revisit the notion of TSS. - We identify shortcomings in previous formulations of TSS and present new, strengthened definitions that are not achieved by known constructions. We show that it is easy to build reconstruction boxes for which the tracing guarantees of all previous works fail. - We extend the study of TSS beyond threshold to general access structures. Our new definitions are, in fact, necessary for this setting as natural adaptations of existing definitions become vacuous for general access structures. - We present new constructions of TSS that satisfy our definitions for all access structures captured by monotone circuits. One of our constructions relies solely on one-way functions while the other additionally relies on indistinguishability obfuscation.