Digital Watermarking for Images

Aarathi Raghu

CS 265

Spring 2005

Agenda

1. Motivation

2. What is digital watermarking?

3. DCT

4. A Semi-fragile watermarking algorithm

5. Attacks and countermeasures

6. Conclusion

Motivation

Analog Digital

Photographs JPEG images

Distribution net required

Free to distribute using internet

Hard to modify Easily modifiable

Some level of copyright protection

No copyright protection

Digital Watermarking

Process of embedding information Information embedded is :

• Imperceptible

• Secure

• Robust

Semi-fragile watermarking– Uses:

• Tamper detection

• Image authentication

Scenario

Concepts

Compression is inevitable to accommodate

disk space, bandwidth and transmission time.

Based on:– Redundancy reduction– Irrelevancy reduction

Discrete Cosine Transform(DCT) Divides image into parts based on the visual quality

of the image

Input image is N*M f(i,j) = intensity of pixel in row i and column j F(u,v) is DCT coefficient in DCT matrix Larger amplitudes closer to F(0,0) Compression possible because higher order

coefficients are generally negligible

DCT coding system

image

DCT Transformation

8*8 DCT

Quantization

Entropy encoding

Lossy compressed data

Semi Fragile Watermark (LPD)

Designed by Lin, Podilchuk, Delp Watermark:Pseudo-random zero-mean, unit

variance Gaussian distributed numbers Constructed in DCT domain Watermark embedded in each DCT block

selectively

Semi Fragile Watermark (ctd.)

High frequency coefficients and DC coefficient – unmarked

Inverse DCT produces spatial domain watermark W

Y = X + ßW, where ß is the strength

Watermark Detection

Done block-by-block (col)(B(x,y))=B(x,y)-B(x+1,y) if x E {1,2, ….,

blocksize –1}, 0 otherwise (row)(B(x,y))=B(x,y)- B(x,y+1) if y E {1,2,

……,blocksize –1}, 0 otherwise Tb*= [(col)(Tb(x,y)) | (row)(Tb(x,y))] Wb*= [(col)(Wb(x,y)) | (row)(Wb(x,y))] C = (Tb*.Wb*)

sqrt ((Tb*.Tb*) (Wb*.Wb*))

Block classification

Correlation statistic, C, is compared to a threshold T

C > = T : Block is authentic C < T : Block is altered

Example

Detection

Original image Altered image

Attacks

Removal attacks Geometric attacks Cryptographic attacks Protocol attacks

Precautions

Watermark should be present over more number of pixels

Used keys should be secure Use of collusion-secure watermarks Watermarks should be non-invertible Possible attacks need to be foreseen

References

1. ftp://skynet.ecn.purdue.edu/pub/dist/delp/ei00-water/paper.pdf

2. http://www-nt.e-technik.uni-erlangen.de/~su/seminar/ws99/slides/amon.pdf

3. http://www.lnt.de/~eggers/texte/IEEEcom2.pdf

4. http://www.acm.org/crossroads/xrds6-3/sahaimgcoding.html