1. A Virtual Image Cryptosystem Based upon Vector Quantization
Chair Professor Chin-Chen Chang
National Tsing Hua University
National Chung Cheng University
Feng Chia University

2. Introduction Images have been widely used in our daily life.
The image security has become an important issue in current computer world.
Image cryptology is a very useful tool to defend the information security.

3. Apply the Traditional Cryptosystem on Images

4. Problems The cipherimage is meaningless. Image Camouflage(影像偽裝)
Image size is huge
Image Compression(影像壓縮)
The decrypted image containing a small distortion is usually acceptable.
Vector Quantization (向量量畫編碼法)

5. Virtual Image Cryptosystem

6. VQ Compression Index table Original Image Codebook (16, 200, …, 90) 1
(16, 200, …, 90) 1
(35, 22, …, 100) 2
(40, 255, …, 59) .
254
(90, 102, …, 98)
255
(145, 16, …, 99) 1 60 61
175 …
100 95
203 .
. . .
Index table
Original Image
Codebook

7. Vector Quantization (VQ) Codebook Training
Codebook Generation 1 2 .
N-1 N
Training Images
Training Set
Separating All Training Images to Vectors

8. Vector Quantization (VQ) Codebook Training
Codebook Generation (Ex: Codebook Size = 256) 1 . 1 .
254
255
N-1 N
Initial Codebook
Training Set
Codebook Initiation

9. Vector Quantization (VQ) Codebook Training
LBG Algorithm
Training Set
Training 256 codewords each time
K times
Until the difference between every two times is smaller than the threshold

11. Example
Codebook
To encode an input vector, for example, v = (150,145,121,130)
(1) Compute the distance between v with all vectors in codebook
d(v, cw1) = d(v, cw2) = d(v, cw3) = 112.3
d(v, cw4) = d(v, cw5) = d(v, cw6) = 235.1
d(v, cw7) = d(v, cw8) = 63.2
(2) So, we choose cw8 to replace the input vector v.

12. VQ Decompression Index Table VQ Coded Image Codebook (16, 200, …, 90)
(16, 200, …, 90) 1
(35, 22, …, 100) 2
(40, 255, …, 59) .
254
(90, 102, …, 98)
255
(145, 16, …, 99) 1 60 61
175 …
100 95
203 .
. . .
Index Table
VQ Coded Image
Codebook

13. The Principle of the Virtual Image Cryptosystem
Separate O into a set of vectors {O1, O2, O3,…, Ono}.
Separate V into another set of vectors {V1, V2, V3, … , Vnv}
Let O be the original image
Let {V1, V2, V3, …, Vnv} be the codebook

14. Encryption
Randomly generate the transformed-origin G and the project-direction D.
Project {V1, V2, V3, …, Vnv} to D based on G
Sort the projected results, and obtain {{V’1, V’2, V’3, …, V’nv}

16. Encrypt w, h, no, G, and D into wc, hc, noc, Gc, and Dc by DES-like, respectively.
Encrypt I into Ic, where Ic=IXORX and X is the bit-string containing G, D, G, D,… only.
Hide wc, hc, noc, Gc, Dc, and Ic into the pixels of V.
Cipher Image Vc

17. Decryption

18. Original Image Airplane 512 X 512
Empirical Tests
Test1:
Original Image Airplane 512 X 512

19. Virtual Image
Lena
256 X 256
Cipher Image
Lena
256 X 256
PSNR=37.87dB

20. Decrypted Image Airplane 512 X 512
PSNR=30.22dB

21. Original Image Airplane 512 X 512
Test2:
Original Image Airplane X 512

22. Virtual Image
Lena
360 X 360
Cipher Image
Lena
360 X 360
PSNR=45.13dB

23. Decrypted Image Airplane 512 X 512
PSNR=31.36dB

24. Original Image Peppers 512 X 512
Test3:
Original Image Peppers 512 X 512

25. Virtual Image
Lena
256 X 256
Cipher Image
Lena
256 X 256
PSNR=37.60dB

26. Decrypted Image Peppers 512 X 512
PSNR=29.91dB