Department of Information Management
National Central University, 2002
Khaleeque Ansari (2010CS10222)
• General Idea
• Visual Cryptography on binary images
• Visual Cryptography on gray-level images
• Visual Cryptography on color images
• Traditional Cryptography
• Encryption and decryption process need high computation costs
• Need secret key for decoding
• Visual Cryptography
• less computation
• Easy and fast decoding
• No need of secret key
• Encrypt the secret image in n parts.
• All n part can get together and recover secret image.
• Less than n parts cannot recover secret image.
• S1 and S2 individually have no information
About secret number
• S1 XOR S2 = Secret number
• Naor and Shamir proposed the visual cryptography in 1994.
• Used to encrypt written materials like printed text, pictures,
handwritten text etc.
• Naor and Shamir proposed a scheme
• take a pixel from secret image
• extend it onto 2x2 subpixel
• Randomly choose 1 out of 2 share.
• Transform the gray-level image into black-and white halftone image
• Halftone technology uses the density of black dots to simulate the
content of grayscale image
• White area: low density of black dots
• Black area: high density of black dots
• Basic principles of color
• Additive model- primary colors (red, blue and green) are mixed to obtain the
desired color.(eg. Computer screen ).
• Subtractive model- color is represented by combinations of color lights
reflected from the surface of an object.(eg apple absorb green and blue part
of natural light and reflect red light)
• Rijmen and Preneel’s method
• Each color pixel is expanded into a 2x2 block
• Each 2x2 block is filled with red,green,blue and white
• 24 possible combination (permutation of 4 colors)
• Human eye cant detect color of tiny subpixel
• Will treat as an average color
• In additive model, any color mixed with white color gives white.