Visual Cryptography on Color Images

Young-Chang HouDepartment of Information Management

National Central University, 2002

Mukesh Kumar(2010CS50288)Khaleeque Ansari (2010CS10222)

Outline

• General Idea• Introduction• Visual Cryptography on binary images• Visual Cryptography on gray-level images• Visual Cryptography on color images

Visual Cryptography

• 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

General Idea

• 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.• Example- • S1 and S2 individually have no information About secret number• S1 XOR S2 = Secret number

Secret: 1011

S1:0110 S2:1101

Introduction

• Naor and Shamir proposed the visual cryptography in 1994. • Used to encrypt written materials like printed text, pictures,

handwritten text etc.

Visual cryptography

Visual cryptography for Binary Image• Naor and Shamir proposed a scheme• take a pixel from secret image• extend it onto 2x2 subpixel• Randomly choose 1 out of 2 share.

Visual Cryptography for gray-level image

(The halftone technology )• 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

Visual cryptography for color images

• 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)

Visual cryptography for color images (cont.)• 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

• Disadvantage- • In additive model, any color mixed with white color gives white.

• Color Image Decomposition to C,Y & M

Visual cryptography for color images (cont.)

Visual cryptography for color images (cont.)• Method 1• 1 secret image is converted to 4 shares C,M,Y & Mask

Visual cryptography for color images (cont.)• Method 1• 50% lost in contrast• Two level security control

Visual cryptography for color images (cont.)• Method 2

Visual cryptography for color images (cont.)• Method 2• 25% contrast of Original Image

Visual cryptography for color images (cont.)• Method 3

Visual cryptography for color images (cont.)• Method 3• 50% Contrast Loss• 2 Shares

Conclusions:

• First paper to exploit color decomposition and halftone technology to generate visual cryptography for both gray-level and color images

References:

• Shyong jian Shyu, Efficient visual secret sharing scheme for color images, Pattern Recognition Volume 39, Issue 5, May 2006• Wikipedia page on Visual Cryptograpy

http://en.wikipedia.org/wiki/Visual_cryptography

Thank You

Questions ?