VISUAL CRYPTOGRAPHY SCHEME ON GRAPHS WITH m * (G) = 4

Let G = (V;E) be a connected graph and (G) be the strong access structure where obtained from graph G. A visual cryptography scheme (VCS) for a set P of participants is a method to encode a secret image such that any pixel of this image change to m subpixels and only qualied sets can recover the secret image by stacking their shares. The value of m is called the pixel expansion and the minimum value of the pixel expansion of a VCS for * (G) is denoted by m (G). In this paper we obtain a characterization of all connected graphs G with m (G) = 4 and *(G) = 5 which * (G) is the clique number of graph G.