| Digital watermarking technique is a whole new mean to guard information security. It has been a strong method for copyright protection and authentication on multimedia. And it involves the theory on communication and information, digital image processing, network security and so on. It is of high academic value and economic value.Semi-fragile watermarking technique is an important branch of digital watermarking. It can tolerate general signal processing, detect any spiteful tamper on marked multimedia data, locate spatial regions within an altered image, and even can estimate what kind of modification had occurred and recover the tampered data. With these attractive characteristics, semi-fragile watermarking has become a research hotspot of multimedia authentication domain in recent years.This paper compares information hiding with digital watermarking and modern cryptography—three ways for information security, reviews the basic theory on digital watermarking, gives an analysis on present researches on digital watermarking techniques, applied to both copyright protection and content integrality protection, and researches on the modern cryptography theory. Then it is proposed that to design watermark algorithms combining these two techniques.From the view of still image authentication, an approach of semi-fragile watermarking, which is based on DWT and capable of tamper location, is presented here. In this method, after the host image was DWT-decomposed, we choose location for watermark insertion by sorting low and intermediate frequency coefficients. By introduction of public-key algorithm in modern cryptography, we insert content-based watermark. Experimental results demonstrate that the algorithm is imperceptible, blind, and robust to JPEG compression, sensitive to spiteful tamper such as cut and cut-replace and so on, precise to tamper localization.Finally, it's the overall summarization of the paper and research expectations of the semi-fragile digital watermarking techniques.
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