Smallest graphs achieving the Stinson bound

Abstract: Perfect secret sharing scheme is a method of distribute a secret information $s$ among participants such that only predefined coalitions, called qualified subsets of the participants can recover the secret, whereas any other coalitions, the unqualified subsets cannot determine anything about the secret. The most important property is the efficiency of the system, which is measured by the information ratio. It can be shown that for graphs the information ratio is at most $(\delta+1)/2$ where $\delta$ is the maximal degree of the graph. Blundo et al. constructed a family of $\delta$-regular graphs with information ratio $(\delta+1)/2$ on at least $c\cdot 6^\delta$ vertices. We improve this result by constructing a significantly smaller graph family on $c\cdot 2^\delta$ vertices achieving the same upper bound both in the worst and the average case.