viper slide 1 detection of image alterations using semi-fragile watermarks eugene t. lin †,...

VIPER Slide 1

Detection of Image Alterations Using Semi-fragile Watermarks

Eugene T. Lin†, Christine I. Podilchuk‡ and Edward J. Delp†

†Purdue University School of Electrical and Computer Engineering

Video and Image Processing Laboratory (VIPER)West Lafayette, Indiana

‡Bell Laboratories, Lucent TechnologiesMurray Hill, New Jersey

VIPER Slide 2

Overview

• Introduction

– Image authentication

– Fragile watermarks

– Robust watermarks

– Semi-fragile watermarks

• Description of proposed technique

• Results

• Conclusion

VIPER Slide 3

Image Authentication

• Identify the source of an image

• Determine if the image has been altered

• If so, locate regions where alterations have occurred

• Authentication watermark

– watermark is imperceptible under normal observation

– allows user to determine if image has been altered after mark embedding

VIPER Slide 4

Fragile Watermarks

• Watermark is rendered undetectable after slightest modifications to marked content

• Typically able to localize alterations with high degree of precision

• Sensitivity achieved through use of hash functions

• Problem: if lossy compression is applied to marked image, mark is destroyed even though compressed image remains perceptually similar

VIPER Slide 5

Robust Watermarks

• Resists removal attempts

• Examines large regions of image, limited localization of alterations

• Robustness typically achieved through spread-spectrum techniques

• Problem: robust watermark may remain even after alterations that change the visual message conveyed by the image

VIPER Slide 6

Semi-Fragile Watermarks

• Able to detect and localize significant “information altering” transformations (feature replacement)

• Able to tolerate some degree of “information preserving” transformations (lossy compression)

• Suitable in authentication applications where legitimate use includes lossy compression or other image adjustment by users

VIPER Slide 7

Semi-Fragile Watermarks

• Challenges for fragile watermark semi-fragile watermark:

– LSB plane embedding not tolerant to compression

– Cryptographic hash functions too sensitive

• Challenges for robust watermark semi-fragile watermark:

– Reduce region size used in mark detection but retain enough SNR to achieve reliable detection

– Boundary effects

VIPER Slide 8

Description of Proposed Technique

• Watermark construction

– DCT construction, spatial embedding

• Watermark detection

– Based on differences of adjacent pixel values

– Most natural images contain large regions of relatively smooth features

VIPER Slide 9

Watermark Construction

DCDC

1 2 3 4 5 6 7

1234567

= Mark coefficient is set tozero.

= Mark coefficientsampled from PRNG(zero mean, 2 variance)

8x8 DCT Block

DCT Watermark Generation

VIPER Slide 10

Watermark Construction

• After watermark is constructed in DCT domain, it is transformed to spatial domain and embedded

DCT watermark Generation

IDCT

Original Image

+Marked Image

W

X

Y=X+W

VIPER Slide 11

• Independent detection performed on each block, for localizing altered blocks

• Define two operators:

Watermark Detection

Blocksize x if

} 1-Blocksize,1,2, { xif

0

),1(),()),((

yxByxByxBCOL

Blocksize y if

} 1-Blocksize,1,2, {y if

0

)1,(),()),((

yxByxByxBROW

VIPER Slide 12

Example of Differential Operators

3541

3415

1533

7411

),( yxB

0215

0134

0620

01132

)(BCOL

0000

6954

4122

6124

)(BROW

VIPER Slide 13

Watermark Detection

• Tb = Block of image being tested

• Wb = Corresponding block of watermark image

• Detector uses both row and column differences:

)),(()),((

)),(()),((

*

*

yxWyxWW

yxTyxTT

bROWbCOLb

bROWbCOLb

VIPER Slide 14

Block Test Statistic

• Tb* and Wb* are correlated to compute block test statistic b:

))(( ****

**

bbbb

bbb

WWTT

WT

b T: Block is likely authenticb < T: Block is likely altered.

VIPER Slide 15

Results - Gradient

Original “Gradient” Altered “Gradient”

Total Blocks: 682, Altered:300 (44%)

Detector Block size:16x16, embedding =5.0

VIPER Slide 16

Results - Gradient

0.00.10.20.30.40.50.60.70.80.91.0

0.0 0.5 1.0 1.5 2.0 2.5Image Bit Rate (bits/pixel)

Dete

cti

on

Sta

tisti

c V

alu

e

Mean Unaltered Mean Altered

VIPER Slide 17

Results - Gradient

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

Threshold Value

Pe

rce

nt

Co

rre

ct

De

tec

tio

ns

JPEG-90 JPEG-50 JPEG-30

VIPER Slide 18

Results - Sign

Original “Sign” Altered “Sign”



VIPER Slide 19

Results - Sign

0.00.10.20.30.40.50.60.70.80.91.0


Det

ecti

on

Sta

tist

ic V

alu

e


VIPER Slide 20

Results - Sign

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

Threshold Value

Pe

rce

nt

Co

rre

ct

De

tec

tio

ns


VIPER Slide 21

Results - Money

Original “Money” Altered “Money”



VIPER Slide 22

Results - Money

0.00.10.20.30.40.50.60.70.80.91.0


Det

ecti

on

Sta

tist

ic V

alu

e


VIPER Slide 23

Results - Money

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

Threshold Value

Pe

rce

nt

Co

rre

ct

De

tec

tio

ns


VIPER Slide 24

Results - Girls

Original “Girls”

Altered “Girls”



VIPER Slide 25

Results - Girls

0.00.10.20.30.40.50.60.70.80.91.0


Det

ecti

on

Sta

tist

ic V

alu

e


VIPER Slide 26

Results - Girls

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

Threshold Value

Pe

rce

nt

Co

rre

ct

De

tec

tio

ns


VIPER Slide 27

Detection Performance

Embed: =5.0

Detection:T=0.1

blocksize=16x16JPEG-60

bitrate=0.90 bpp

93% correct detection4% false positive

17% misses

VIPER Slide 28

Conclusions

• A semi-fragile watermarking technique was proposed which classifies about 70%of blocks correctly for moderate JPEG compression, 90% for light JPEG compression

• Detector has problems with edges and textures

• Future work:

– Integrate a visual model to embed mark at higher strengths in textured areas

viper slide 1 detection of image alterations using semi-fragile watermarks eugene t. lin †,...

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