In this paper, the security of Dither Modulation Quantization Index Modulation schemes for digital images is analyzed. Both pixel and DCT coefficient quantization schemes are investigated. The related works that deal with the security of spread spectrum and quantization schemes are presented and their limits are outlined. The use of independent component analysis (ICA) for natural image is introduced. We show that ICA can be an efficient tool to estimate the quantization noise which is by definition independent of the host signal. We present both a method for estimating the carrier, and an attack that relies on the ICA decomposition of patches of images; our attack scheme is also compared with another classical attack. The results reported in this paper demonstrate how changes in natural image statistics can be used to detect watermarks and devise attacks. Such natural image statistics-based attacks may pose a serious threat against watermarking schemes which are based on quantization techniques.
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