semifragile watermarking

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Technical Seminar onTechnical Seminar on

Comparative Evaluation of

Comparative Evaluation of Semifragile Watermarking AlgorithmsSemifra

gile Watermarking Algorithms

Presented by:Rajesh S.Hegde

4JN05IS042


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Contents:

1. Introduction

2. Methods for Comparison

3. Semifragile Watermarking Algorithms

4. Experimental Results

5. Conclusion

6. References


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Introduction:

Semifragile watermark monitors the integrityof the content of the image but not itsnumerical representation.

Prominent application of watermarkingtechnology is monitoring of the integrity of multimedia documents.


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Attacks on image can occur as follows,

iii.Unintentional or Innocent alterations:Arise as bit errors during transmission and storage

or signal processing operations such as filtering,contrast enhancement, sharpening and compression.

ii. Intentional or Malicious alterations:Due to an explicit forgery attempt by apirate with an explicit purpose of changing

the contents of a document


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Watermarking algorithms:

iii. Fragile watermarking algorithms:

Function as strict tamper detection tool and intolerantof even one bit alteration.

Also called as cryptographic message digests and canvalidate only original copies.

viii. Semifragile Tamper detection methods:

Monitor changes in the content.Capable to differentiate between innocent and

malicious attack categories .


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Classification of tamper detection contentauthentication watermarking techniques:

iv. Visually authenticated (semi)fragile watermarking:

Typically thumbnail or a visual pattern is hidden in theimage.

Tamper detection is based on visual assessment of perceived differences by an operator.

ii. Statistically authenticated (semi)fragile watermarking

An estimate of tampering likelihood is obtained based oncorrelation coefficient

Or Measured mismatch between inserted and recoveredauthentication sequences.


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Proposed algorithm may be,i. Self Authenticating:

Based on the validation of the robust hashwhich was embedded.It is to be extracted again from the test image

itself.

ii. Independently Authenticating:Receive validation based on an image

independent authentication sequence.

Measurement of performance in terms of,i. Probability of miss when there is a forgery

attack ,Pmii. Probability of false alarm when there is no

forgery,Pf


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Methods for comparison: Semifragile watermarking algorithms should not give

false alarm against these permissible alterations:

Mild compression.Histogram equalization.

Sharpening Low-pass filter within a support of 3X3.Median filtering with a support of 3X3.Additive Gaussian noise down to signal to noise ratioof 35dB

Salt-n-pepper noise up to 1%Random bit errors in transmission and storage of theimage in row format, with a 0.001 probability of biterrors


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The nonpermissible alterations are the following:

Image forgeries intended to remove,

substitute, or insert objects in the scene

Image manipulations that modify the geometry of the objects such as rotation, flipping or image

manipulations that change the appearance of object such as color, shape, shadow manipulation,etc

Changes in the scene background.

Cropping


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Lin-Chang Algorithm:

It is conceived to tolerate JPEG-style compressionof watermarked image .

It is based on two properties DCT coefficientquantization,

Order invariance: DCT coefficient pairsremain unaltered after JPEG processing if notset equal.

Coefficient invariance: Coefficient isquantized to an integer multiple of the stepsize, its value is not changed after JPEGcompression with smaller step size.


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1 0 1 0 0 1

C1(p,q) C2(p,q)

BlocksBlocks

DCT DCT

Placed in DCT coefficient of same block pair


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Authentication Data:Consists of ordinal relationship of three pairs of DCT coefficients chosen from 8X8 blocks pair-wisecoupled according to a random mapping.For coupled block the authentication informationat the (p,q) coefficient,

(p,q)= {

Insertion Method:If bits to be inserted complies with leastsignificant bit of carrier DCT coefficient divided by

the quantization step size, then no change is

made.otherwise the coefficient is incremented by a onestep size amount.This procedure should be iterated few times before

DCT coefficients are stabilized.

1 C1(p,q)-C2(p,q)>=00 C1(p,q)-C2(p,q)


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Li P dil h k D l Al i h


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Lin-Podilchuk-Delp Algorithm:Authentication sequence:

It is a pseudo-random zero-mean, unit-varianceGaussian noise sequence.

Its seed is controlled by a key but is otherwiseindependent of the document

Insertion Method:

The Gaussian authentication Sequence is placedin the upper triangular position of an empty DCTmatrixThen the inverse DCT of the matrix is calculatedand resulting 16X16 spatial pattern is mixedadditively with the image DCT block at a givenstrength.Ib(p,q)=Ib(p,q)+Wb(p,q)

Finally block wise inverse DCT yields Semifragilewatermarking image


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The verification is based on correlation of the originaldata with differenced version of the original watermark

pattern.=

< I*b,W*b>

[ ]^1/2


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Performance analysis of Lin-Changs Semifragilewatermarking.

Performance of the Lin-Podilchuk semifragilewatermarking.


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Experimental ResultsThe following comments can be made on theintegrated performance of Semifragile algorithms, on64X64 blocks.

Lin-Changs algorithm: The algorithm performsvery well in the presence of JPEG compression,otherwise very fragile against signal processingattacks.

Lin-Podilchuk-Delps algorithm: It is robust

against nonmalicious signal-processing operations ,With Wiener filtering,to enhance the watermark signal,slightly improves the performance


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ConclusionSemifragile watermarking.Alteration on documents.Types of water marking algorithms.Classification of tamper detection.Types of proposed algorithms.Measurement of the performance.Permissible and non-permissible alterations.Watermarking algorithms.Performace analysis of the studied algoritms.Experimental results.


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September 21, 2009 21 JNNCE,ShivamoggaDept. of ISE


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Special thanks to all staffs of ISEDept.To my friends.

semifragile watermarking

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