Vulnerability of DM watermarking of non-iid host signals to attacks utilising the statistics of independent components

Security is one of the crucial requirements of a watermarking scheme, because hidden messages such as copyright information are likely to face hostile attacks. In this paper, we question the security of an important class of watermarking schemes based on dither modulation (DM). DM embedding schemes rely on the quantisation of a secret component according to an embedded message, and the strategies used to improve the security of these schemes are the use of a dither vector and the use of a secret carrier. In this paper we show that contrary to related works that deal with the security of spread spectrum and quantisation schemes, for non-iid host signals such as images, principal component analysis is not an appropriate technique to estimate the secret carrier. We propose the use of a blind source separation technique called independent component analysis (ICA) to estimate and remove the watermark. In the case of DM embedding, the watermark signal corresponds to a quantisation noise independent of the host signal. An attacking methodology using ICA is presented for digital images; this attack consists first in estimating the secret carrier by an examination of the high-order statistics of the independent components and second in removing the embedded message by erasing the component related to the watermark. The ICA-based attack scheme is compared with a classical attack that has been proposed for attacking DM schemes. The results reported in this paper demonstrate how changes in natural image statistics can be used to detect watermarks and devise attacks. Different implementations of DM watermarking schemes such as pixel, DCT and spread transform-DM embedding can be attacked successfully. Our attack provides an accurate estimate of the secret key and an average improvement of 2 dB in comparison with optimal additive attacks. Such natural image statistics-based attacks may pose a serious threat against watermarking schemes which are based on quantisation techniques