A visual cryptography scheme for a set P of n participants is a method to encode a secret image SI into n shadow image called shares, where each participant in P receives one share. In this paper, access structures based on graphs is studied. We consider graph-based access structure, i.e., access structures in which any qualified set of participants contains at least an edge of a given graph whose vertices represent the participants of the scheme and use one technique to compute lower bound on the pixel expansion of matching access structures.

كلمات كليدي:

Visual Cryptography, Secret Sharing, Pixel Expansion, Graph-based Access Structure.