Author Topic: Performance Comparison of Semifragile Watermarking Methods for Image Au-thentica  (Read 2609 times)

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Author : Mrs. Swati V. Sankpal, Dr. Vishram Bapat
International Journal of Scientific & Engineering Research Volume 2, Issue 10, October-2011
ISSN 2229-5518
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Abstract- Data authentication is one of the primary requisites in present day communication systems. In image processing, data authentication is implemented by using watermarking techniques. The specific interest in semifragile watermarking algorithms arises from the multitude of practical and commercial applications, where content needs to be strictly protected, but the exact representation during exchange and storage need not be guaranteed. The alterations on the documents can occur unintentionally or can be implanted intentionally. Semifragile watermarks are more robust and less sensitive to classical user modifications such as JPEG compression, content-preserving operations and content altering manipulations while sensitive to content integrity verification. In present paper the performance of seven semifragile watermarking methods are compared in terms of their PSNR, robustness and temper sensitivity properties.

Key words: semi-fragile watermarking, PSNR, Robustness, Tamper detection.

To facilitate the authentication and content-integrity verification for multimedia applications where content-preserving operations are a com-mon practice, semi-fragile watermarking scheme have been proposed in the last few years [1]. This class of watermarks is intended to be fragile only when the manipulations on the watermarked media are deemed malicious by the schemes [2]. Semifragile watermarking is a potential solution to the image content authentication problem which seeks to verify that the content of the multimedia has not been modified by any of a predefined set of illegitimate distortions, while allowing modification by legitimate distortions.

 To be an effective image authentication system, it must satisfy the following criteria [8]:

a. Sensitivity: The system must be sensitive to malicious manipulations.

b. Tolerance: The system must tolerate some loss of
information and more generally non-malicious

c. Localization of altered regions: The system should be able to locate precisely any malicious alteration made to the image and verify other areas as authentic.

d. Reconstruction of altered regions: The system may need the ability to restore, even partially, altered or destroyed regions in order to allow the user to know what the original content of the ma-nipulated areas was.

In this work focus is on different authentication based semifragile watermarking techniques, thus paper is organized as follows: Section 2 discusses methods of comparison used for comparing different algorithms, Section 3 gives details on attacks and their countermeasures, Section 4 outlines different semifragile watermarking algorithms and Sections 5 & 6 presents analysis and conclusion.

2  Methods for Comparison

The semifragile watermarking method should be moderately robust to discriminate between mali-cious manipulations, such as the addition or re-moval of a significant element of the image, and global operations preserving the semantic content of the image. The semifragile watermarking methods should be moderately robust to differentiate between malicious and non malicious attacks. However, the line of demarcation between the benign and malicious attacks is application and document dependent [3]. In this comparison for given algorithms values of PSNR(Peak Signal to Noise Ratio), robustness and tamper sensitivity properties are given, no specific application context is considered so depending on application particular methods can be used .Analysis of available watermarking scheme is done on basis of-

I. PSNR: High value of PSNR shows the water-marked image has a better quality, the difference between the original image and the watermarked image is imperceptible.
II. Robustness:  Robustness depends on the infor-mation capacity of the watermark, the watermark strength / visibility, and the detection statistics (threshold). Robustness is also influenced by the choice of images (size, content, color depth). The minimal required robustness is highly application dependent. It may not make sense to compare techniques intended for different applications.
III. Tamper detection- Tamper detection aims to monitor modifications on digital documents where a distinction needs to be made between innocent and malicious alterations. When multimedia content is used for legal purposes, medical applications, news reporting, and commercial transactions, it is important to ensure that the content was originated from a specific source and that it had not been changed, manipulated or falsified. This can be achieved by embedding a watermark in the data.

IV. Recovery of content-Lossless recovery of con-tent especially reversible recovery is taken into consideration.
The main content-altering manipulations that must generate tamper alarm, hence, the non permissible alterations, are the following: Image forgeries intended to remove, substitute, or insert objects in the image. Image manipulations that modify the geometry of objects such as their rotation, flipping, translation, and scaling or image manipulations that change the appearance of objects such as color, shade, shadow manipulation, etc.

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