Multiple-Image Encryption
by Rotating Random Grids
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 P78011044
許伯誠 P76011242
Date: 2013/01/18
Outline
 Introduction
 Motivation
 Related Work
 Research Methods
• Encryption phase
• Decryption phase
 Research Results
 Conclusion
2
Introduction – Visual Cryptography
 Traditional Cryptography
- Encryption and Decryption by computer
 Visual Cryptography (VC), also called Visual Secret
Sharing (VSS)
- Encrypted by computer, Decrypted by human vision
3
Introduction
Traditional VC and Random Grid
• VSS
Traditional VC-based VSS (Codebook)
RG-based VSS (Random Grid)
• Traditional VC-based VSS has at least two
drawbacks as follows :
1. Codebook design
2. Pixel expansion
4
Introduction
Pixel expansion
Traditional VC-based VSS:
Share1
Secret image
Share2
Share1 + Share2
5
Introduction
Traditional VC and Random Grid
• VSS
Traditional VC-based VSS (Codebook)
RG-based VSS (Random Grid)
• Traditional VC-based VSS has at least two
drawbacks as follows :
1. Codebook design
2. Pixel expansion
6
Motivation
SA
G1
G2
SA
SB
SA’
SA’
G1
G2
SB’
7
Related Work
In 1987, Kafri and Keren propose three different algorithms to encrypt a binary secret image.
Random Grid Algorithm1:
for(i=0 ; i<w ; i++)
for(j=0 ; j<h ; j++)
if(B[i][j] == 0) G2[i][j] = G1[i][j];
else G2[i][j] = G1[i][j];
Output(G1 , G2);
SA
b
g1
g2
g1⊕g2
0
0
1
0
1
0
1
1
0
1
1
1
0
1
G1
G2
O. Kafri, and E. Keren, “Encryption of pictures and shapes by random grids,” Optics Letters, vol. 12, no. 6, 1987, pp. 377-379.
8
Research Methods
9
Research Methods
Encryption phase:
Step 1: SA(i, j) ← f RSP(SA).
 Randomly select a pixel SA(i, j) from the first secret image SA, where the ith row and the j-th column of the matrix SA are in the range of [0,m-1].
Step 2: G1(i, j)||G2(i, j) ← f RG(SA(i, j))
10
Research Methods
11
Research Methods
12
Research Methods
Step 8: G1((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 SA and SB with the size of 512×512
15
Research Results
 Simulation 2: Halftone secrets, 90-degree rotation
• Two gray-level secret images SA and SB 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