Authors: Tzung-Her Chen, Kai-Hsiang Tsao, and Kuo-Chen Wei Source: Proceedings of The 8th International Conference on Intelligent System Design and Applications (ISDA 2008) 學生：張若怡 P 許伯誠 P Date: 2013/01/18 Multiple-Image Encryption by Rotating Random Grids

Outline  Introduction  Motivation  Related Work  Research Methods Encryption phase Decryption phase  Research Results  Conclusion 2

Introduction – Visual Cryptography 3  Traditional Cryptography - Encryption and Decryption by computer  Visual Cryptography (VC), also called Visual Secret Sharing (VSS) - Encrypted by computer, Decrypted by human vision

Introduction Traditional VC and Random Grid 4 V SS Traditional VC-based VSS (Codebook) RG-based VSS (Random Grid) T raditional VC-based VSS has at least two drawbacks as follows : 1.Codebook design 2.Pixel expansion

Introduction Pixel expansion 5 Secret image Share1 Share2 Share1 + Share2 Traditional VC-based VSS:

Introduction Traditional VC and Random Grid 6 V SS Traditional VC-based VSS (Codebook) RG-based VSS (Random Grid) T raditional VC-based VSS has at least two drawbacks as follows : 1.Codebook design 2.Pixel expansion

Motivation 7 SASA G1G1 G2G2 SA’SA’ G1G1 G2G2 SASA SBSB SA’SA’ SB’SB’

Related Work 8 b g 1 g 2 g 1 ⊕ g SASA G1G1 for(i=0 ; i<w ; i++) for(j=0 ; j<h ; j++) if(B[i][j] == 0) G 2 [i][j] = G 1 [i][j]; else G 2 [i][j] = G 1 [i][j]; Output(G 1, G 2 ); G2G2 Random Grid Algorithm1:  O. Kafri, and E. Keren, “Encryption of pictures and shapes by random grids,” Optics Letters, vol. 12, no. 6, 1987, pp In 1987, Kafri and Keren propose three different algorithms to encrypt a binary secret image.

Research Methods 9

Encryption phase:  Step 1: S A (i, j) ← f RSP (S A ).  Randomly select a pixel SA(i, j) from the first secret image SA, where the i- th row and the j-th column of the matrix SA are in the range of [0,m-1].  Step 2: G 1 (i, j)||G 2 (i, j) ← f RG (S A (i, j)) 10

Research Methods 11

Research Methods 12

Research Methods Step 8: G 1 ((m-1)-j, i) ← random(0,1) 13

Research Methods Decryption phase 14

Research Results  Simulation 1: binary secrets, 90-degree rotation Two secret images S A and S B with the size of 512×512 15

Research Results  Simulation 2: Halftone secrets, 90-degree rotation Two gray-level secret images S A and S B with the size of 512×512 16

Research Results Simulation 3: binary secrets, 180-degree rotation Simulation 4: binary secrets, 270-degree rotation 17

Conclusion Property 1: No extra codebook redesigned Property 2: No extra pixel expansion introduced Property 3: Multiple secrets encoded Property 4: Bandwidth and storage saving Property 5: Wide image format 18