On the Composition of Matroids and Ideal Secret Sharing Schemes

Abstract

In an ideal secret sharing scheme, the access structure is uniquely determined by its minimal sets \Delta_s. The purpose of this paper is to characterise \Delta_s. We introduce the concept of strong connectivity and show that under this equivalence relation, an ideal secret sharing scheme decomposes into threshold schemes. We also give a description of the minimal sets that span the strong connectivity classes. As a result we obtain a necessary condition on the types of subsets that are allowed in an ideal access structure as well as an upper bound on the number of such access structures.