A robust type-III data hiding technique against cropping & resizing attacks

We propose an oblivious information hiding method which allows watermark recovery for signals subjected to cropping and resampling consecutively. We employ multiple embedding of a watermark signal using a Type - III data hiding method which has a better robustness vs. rate trade off than the conventional methods under mean square error distortion measure. Cyclic auto-correlation features of the cropped-resampled signal are used to estimate the amount of cropping. We analytically showed that within a small error range, it is possible to restore the cropped-resampled signal to the cropped signal. Synchronization of watermark detection in the cropped stego-signal is achieved by designing white noise like watermark signals that are uncorrelated with their shifted replicas. For this purpose we use all-pass filters which are orthogonal to all their cyclic shifts. Freedom to hide data is obtained by modulating the phase of the cyclic all-pass filters. Further, we use Reed-Solomon error correcting codes both for introducing redundancy and achieving synchronization. We also address the issue of data hiding under multiple cropping attacks.