1. 隱 像 術
張真誠
國立中正大學資訊工程研究所

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. Vector Quantization Encoder

7. Vector Quantization Decoder

8. 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

9. 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}

11. 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

12. Decryption

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

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

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

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

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

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

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

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

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