Ideal Secret Sharing Schemes: Yet Another Combinatorial Characterization, Certain Access Structures, and Related Geometric Problems

An ideal secret sharing scheme is a method of sharing a secret key in some key space among a finite set of participants in such a way that only the authorized subsets of participants can reconstruct the secret key from their shares which are of the same length as that of the secret key. The set of all authorized subsets of participants is the access structure of the secret sharing scheme. In this paper, we derive several properties and give a new combinatorial characterization of ideal secret sharing schemes. We propose two practical models, namely the parallel model and the hierarchical model, for access structures, and then, by the new characterization, we discuss sufficient conditions on finite geometries for ideal secret sharing schemes to realize these access structure models. Several series of ideal secret sharing schemes realizing special parallel or hierarchical access structure models are constructed from finite projective planes.