Hiding Multiple Watermarks in Transparencies of Visual Cryptography

1. Hiding Multiple Watermarks in Transparencies of Visual Cryptography Hao Luo¹, Jeng-Shyang Pan², Zhe-Ming Lu³ ¹Harbin Institute of Technology, Harbin 150001, P.R.China ²National Kaohsiung University of Applied Sciences, 807 Kaohsiung, Taiwan ³Sun Yat-sen University, Guangzhou 501725, P.R.China luohao723@hotmail.com, jspan@cc.kuas.edu.tw,zhemingl@yahoo.com Intelligent Information Hiding and Multimedia Signal Processing, 2007. IIHMSP 2007. Third International Conference on Reporter:趙旻玥 May, 20, 2008

2. 1 Introduction ★ Theencrypted 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 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 W1 PW 1 wb1,wb2,…,wbr S sb1,sb2,…,sbr W2: an integer sequence Introduction 2×2blocks {2,2} VCS Scheme Experiment Summary Step 2. Transparency partition T1tb1,tb2,…,tbr T2 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!