A study on visual cryptography and providing a proposed method for color images cryptography

Visual cryptography is a method that makes use of the characteristics of human vision and it requires neither the knowledge of cryptography nor the complex calculations. This method was first proposed by Naor and Shamir، its implementation is simple، it has been extended to an secrect sharing (k، n) in which n shares are made from the image and they are distributed among n participants. Moreover، the image can be retrieved with k shares and their assembling onto each other; however، the image is not retrievable with k-1 shares. Colors are represented using a combination of reflected lights from objects in the subtractive model. A wide range of colors is made with the mixture of cyan (C)، magenta (M) and yellow (Y) and the combination of blue (B)، red (R) and green (G) results in black in this model and also، the combination of these colors with white creates the same colors. This paper presents a visual cryptography proposal for colored images that divides a colored image into some shares after converting it to halftone images based on white and black visual cryptography and their rules are in accordance with the subtractive model of colors.