Decoding

Steps for Decoding

1. Partition the image into 8x8 pixel blocks
2. For each of the 8x8 test blocks Z_r’ two things will happen:
3. Set the LSBs to zero (Hash)
   1. Combine with the entire image X to produce a bitstream of the pixels.
   2. Hash the combined bitstream of pixels together using MD5 to produce P_r.
4. Extract LSBs this will produce C_r”: (Watermark)
   1. Decrypt C_r” with the private-key to produce the decrypted hash.
5. The hash (a) is XORed with the watermark (b) to produce W_r”.
6. Test the difference between the extracted watermark and the original watermark.
7. Mark the block if the watermarks do not match.
8. Recombine all the blocks Z_r’ to reproduce the original image.

Citations

1. J. Pan et al., "Fragile watermarks" in Intelligent Watermarking Techniques, River Edge, N.J.: World Scientific, 2004, ch. 5, sec. 5.2.2, pp. 18-19.