Prof. Amos Beimel

A secret-sharing scheme realizes the forbidden graph access structure determined by a graph G = (V,E) if a pair of vertices can reconstruct the secret if and only if it is an edge in G. Secret-sharing schemes for forbidden graph access structures of bipartite graphs are equivalent to conditional disclosure of secrets protocols, a primitive that is used to construct attributed-based encryption schemes. We study the complexity of realizing a forbidden graph access structure by linear secret-sharing schemes. A secret-sharing scheme is linear if the reconstruction of the secret from the shares is a linear mapping. In many applications of secret-sharing, it is required that the scheme will be linear. We provide efficient constructions and lower bounds on the share size of linear secret-sharing schemes for sparse and dense graphs, closing the gap between upper and lower bounds: Given a sparse graph with n vertices and at most n^1+β edges, for some 0 ≤ β < 1, we construct a linear secret-sharing scheme realizing its forbidden graph access structure in which the total size of the shares is (formula presented). We provide an additional construction showing that every dense graph with n vertices and at least (formula presented) edges can be realized by a linear secret-sharing scheme with the same total share size. We provide lower bounds on the share size of linear secret-sharing schemes realizing forbidden graph access structures. We prove that for most forbidden graph access structures, the total share size of every linear secret-sharing scheme realizing these access structures is Ω(n^3/2), which shows that the construction of Gay, Kerenidis, and Wee [CRYPTO 2015] is optimal. Furthermore, we show that for every 0 ≤ β < 1 there exist a graph with at most n^1+β edges and a graph with at least (formula presented) edges, such that the total share size of every linear secret-sharing scheme realizing these forbidden graph access structures is Ω(n^1+β/2). This shows that our constructions are optimal (up to poly-logarithmic factors).