[WM]: one basic question for fragile watermarking

["Zhao Stephen" <zjunh2002@gmail.com>]

Dear all,
Recently I surveyed some paper on "fragile watermarking" for authentication application.  In these paper, a general scheme is to
first extract a fingerprinting from host image such as "hash", embed the "hash" bits into the host image by watermarking method. In
detection phase, the "hash" bits are re-extracted from the watermarked image for final authentication.
I am puzzled by one question: the extracted "hash" are really identical before and after the "hash"  is watermarked into the host
image, considering watermarking tech is performed by modifying the host image.
Maybe it is contradicting.

Thanks
                            Zhao


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