3. This method does not specifically protect the DC
value of the pixels covered by an individual block.
In [2], the watermark consists of extended msequences
of length 512 bits added to each pixel in a row
of the image. Extended m-sequences are m-sequences of
order n, with a 0 inserted at the end of the n - 1 run of
zeros. The phase of the extended m-sequence carries the
watermark information. The testing procedure filters the
crosscorrelation function of the possibly forged, watermarked
image row and the extended m-sequence. If a
suitably large crosscorrelation peak is found, the row
passes the watermark test.