隱 像 術

張真誠張真誠國立中正大學資訊工程研究所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.

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, … , Vn

v}

•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}

10

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

Empirical Tests

Original Image Airplane 512 X

512

Test1:

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:

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:

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