  Recently, a self-embedding fragile watermark scheme based on reference-bits
interleaving and adaptive selection of embedding mode was proposed. Reference
bits are derived from the scrambled MSB bits of a cover image, and then are
combined with authentication bits to form the watermark bits for LSB embedding.
We find this algorithm has a feature of block independence of embedding
watermark such that it is vulnerable to a collage attack. In addition, because
the generation of authentication bits via hash function operations is not
related to secret keys, we analyze this algorithm by a multiple stego-image
attack. We find that the cost of obtaining all the permutation relations of
$l\cdot b^2$ watermark bits of each block (i.e., equivalent permutation keys)
is about $(l\cdot b^2)!$ for the embedding mode $(m, l)$, where $m$ MSB layers
of a cover image are used for generating reference bits and $l$ LSB layers for
embedding watermark, and $b\times b$ is the size of image block. The simulation
results and the statistical results demonstrate our analysis is effective.
