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Since lower capacity of least significant bit (LSB) of image for watermark information, many fragile watermark algorithms have drawbacks on tamper location and security. Based on the thought of secret key sharing, a novel extended channel fragile watermarking scheme and its algorithm case for color image are presented. The extended channels help to increase the watermark capacity dramatically. The new algorithm for color image employs chaos iteration to create multiple extended channels for storing original image data and its watermark information. Simple combination of all extended channels' data can produce the original image. But extended channel's data can not be created from tampered image without secret key. Compared with other fragile watermark algorithms, simulation experiments reveal that the new algorithm can authenticates each color channel of image in any cropping shape and can localize tampered pixel precisely. Meanwhile, the new algorithm has proved security limit by use of the chaos iteration and pseudorandom code in extended channel.

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