Securing display of grayscale and multicolored images by use of visual cryptography
Hirotsugu Yamamoto, Yoshio Hayasaki, Nobuo Nishida · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2004
Security has become an important issue as information technology has become increasingly pervasive in our everyday lives. Security risks arise with a display that shows decrypted information. In this paper, we propose a secure information display technique using visual cryptography. Its decryption requires no special computing devices and is implemented using only human vision: the proposed display appears as a random pattern to anyone who looks at it unless the person views the displayed image through a decoding mask. We have constructed code sets to represent grayscale and multicolored images. Each pixel in a secret image is expanded to a group of subpixels. The displayed image consists of black and white subpixels to encrypt a grayscale image. To encrypt a multicolor image, black, red, green, and blue subpixels compose the displayed image. The decoding mask consists of transparent and opaque subpixels. Every pixel is encrypted using a pair that is chosen at random. We have demonstrated the proposed secure display with an LCD panel and a transparency on which a decoding mask was printed. The secret image was visible for a viewer within the viewing zone, although viewers outside the viewing zone perceived it as a random dot pattern.