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Abstract:
Semifragile watermarking techniques aim to prevent tampering and fraudulent use of modified images. A semifragile watermark monitors the integrity of the content of the image but not its numerical representation. Therefore, the watermark is designed so that the integrity is proven if the content of the image has not been tampered with, despite some mild processing on the image. However, if parts of the image are replaced with the wrong key or are heavily processed, the watermark information should indicate evidence of forgery. We compare the performance of eight semifragile watermarking algorithms in terms of their miss probability under forgery attack, and in terms of false alarm probability under no malicious signal processing operations that preserve the content and quality of the image. We propose desiderata for semifragile watermarking algorithms and indicate the promising algorithms among existing ones

Introduction
One prominent application of watermarking technology is the monitoring of the integrity of the multimedia documents. The specific interest in semi fragile 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. The so-called unintentional or innocent alterations typically arise from such diverse facts as bit errors during transmission and storage, or signal processing operations such as altering contrast enhancement, sharpening, and compression. Intentional or malicious alterations, on the other hand, are assumed to be due to an explicit forgery attempt by a pirate with the explicit purpose of changing the contents of a document. The main distinction then, is whether the content is altered as in malicious and intentional attacks or whether only the representation, but not the content, of the document is altered, as occurs in unintentional, no malicious cases. The line of demarcation between these two attacks categories is, however, not al- ways clear-cut, as it depends very much on the application domain. A case in point is histogram equalization, where, if the representation of the lighting condition is considered informative, it then becomes a malicious manipulation; alternately, it remains simply a well-intentioned contrast enhancement.


Brief:
Prominent application of watermarking technology is monitoring of the integrity of multimedia documents, Semifragile watermark monitors the integrity of the content of the image but not its numerical representation