Spread Spectrum based Invertible Watermarking for Medical Images using RNS and Chaos

In the current paper, we have presented a novel watermarking scheme with making watermark as robust while keeping the image fragile using Residue Number System (RNS) and Chaos. Residues of the image are made to keep it secure since, their sensitive to change is high. Only Region Of Interest (ROI) part of the image is residued. In making residues of ROI,some residues exceed bit size eight so, these residues are converted to eight bits by applying some trick. Two watermarks are embedded in two stages; one to achieve robustness using Spread Spectrum (SS) technique and other to achieve fragility of image by calculating the digest of image. In the first stage, spreaded watermark is embedded in Region Of Non-Interest (RONI) pixels using the chaotic key and in the second stage, hash calculated from the first stage is again embedded in RONI pixels based on the chaotic key. Moreover, the original image is not needed at receiver end, which makes the proposed scheme blind.