1. Hiding Multiple Watermarks in Transparencies of Visual Cryptography
Hao Luo¹, Jeng-Shyang Pan², Zhe-Ming Lu³
¹Harbin Institute of Technology, Harbin , P.R.China
²National Kaohsiung University of Applied Sciences, 807 Kaohsiung, Taiwan
³Sun Yat-sen University, Guangzhou , P.R.China
Intelligent Information Hiding and Multimedia Signal Processing, 2007.
IIHMSP Third International Conference on
Reporter:趙旻玥
May, 20, 2008

2. 1 Introduction
★ The encrypted image can be visible when stacking the transparencies.
Introduction
Digital Watermarking----
embed data in encrypted transparencies.
{2,2} VCS
Scheme
★ Also two extra watermarks can be extracted with simple computations.
Experiment
Summary
DEFINITIONS:
The Secret Image: S
Two Watermarks: W1 and W2
Two Transparencies: T1 and T2

3. 2 Traditional {2,2} VCS Introduction {2,2} VCS Scheme Experiment
White Pixel: {(c1,c1),(c2,c2),(c3,c3),(c4,c4),(c5,c5),(c6,c6)} Black Pixel: {(c1,c2),(c2,c1),(c3,c4),(c4,c3),(c5,c6),(c6,c5)}
Introduction
Figure1. Codebook of {2,2} VCS
{2,2} VCS
Scheme
Experiment
Summary
Figure2. Example of the traditional {2,2} VCS
Secret image (256×256)
The first transparency (512×512）
The second transparency (512×512）
Stacking result (512×512）

4. 3 Proposed Scheme S: m×m pixels T1 and T2 : 2m×2m pixels
W1 : 2m×2m pixels
W2 : a integer sequence with each element is in the range of [1,6]
Introduction
{2,2} VCS
Scheme
3.1 Watermark Embedding and Secret Image Encryption
Experiment
Step 1. Watermark preparation
Step 2. Transparency partition
Step 3. Embedding unit assignment
Step 4. Embedding W1
Step 5. Embedding W2
Step 6. Secret image encryption
Summary
3.2 Watermark Extraction and Secret Image Decryption
Step 1. Secret image recovery
Step 2. Extraction W1
Step 3. Extraction W2

5. 3.1 Watermark Embedding and Secret Image Encryption
2×2blocks
Step 1. Watermark preparation
W PW wb1,wb2,…,wbr
S sb1,sb2,…,sbr
W2: an integer sequence
Introduction
2×2blocks
{2,2} VCS
Scheme
Experiment
Summary
Step 2. Transparency partition
T tb1,tb2,…,tbr
T tb’1,tb’2,…,tb’r
Embedding unit
4×4blocks
4×4blocks
Step 3. Embedding unit assignment
r/4 embedding units
4×4 block partition
Figure 3. Watermark embedding and secret image encryption
Figure 3. Watermark embedding and secret image encryption

6. 3.1 Watermark Embedding and Secret Image Encryption
Step 4. Embedding W1
Introduction
tb1(1,1)=wb1(1,1)
tb1(1,3)=wb1(1,2)
tb1(3,1)=wb1(2,1)
tb1(3,3)=wb1(2,2)
tb’3(1,1)=wb2(1,1)
tb’3(1,3)=wb2(1,2)
tb’3(3,1)=wb2(2,1)
tb’3(3,3)=wb2(2,2)
{2,2} VCS
Scheme
Experiment
Step 5. Embedding W2
tb’2Ul=mod([W2+tb1UL],6)+1
Summary
Embedding unit
Step 6. Secret image encryption
encrypt sb1 in the tb1 and tb’1
tb1(1,1)=1, c(c1,c2,c3)
examine sb1(1,1)
4×4 block partition
Figure 3. Watermark embedding and secret image encryption

7. 3.2 Watermark Extraction and Secret Image Decryption
Step 1. Secret image recovery
Introduction
1 partition
2 exclusive-OR
3 extract
{2,2} VCS
Scheme
Step 2. Extraction W1
Experiment
Directly retrieve the pixels walues of the dark positions of each embedding unit,
And rearrange them to obtain the watermark image PW1.
Summary
Embedding unit
Step 3. Extraction W2
4×4 block partition
UL, UR, DL ,DR of tb1,
Their associated parts og tb’2
Figure 3. Watermark embedding and secret image encryption

8. 4 Experimental Results Introduction {2,2} VCS Scheme S T1 Experiment
Summary T2
Recovered S
Original
watermark W1
Extracted
watermark W1
Figure 4. Experimental results
131072
(512×256)
The watermark W2: an integer sequence in the range of [1,6]

9. 5 Conclusions
This paper propose a scheme to hide two different watermarks in transparencies during encryption of visual cryptography.
The watermarks may be some extra information of the secret image.
Besides, it can be used to transparency authentication.
Introduction
{2,2} VCS
Scheme
Experiment
Summary
Thank you!