A blind watermarking algorithm based on the relationship of coefficients in the Discrete Cosine Transform (DCT) domain is proposed. Firstly

the original image is divided into non-overlapping 8×8-pixels blocks

and each block is independently transformed to frequency domain through 2D-DCT. Secondly

according to the relationship of DC and middle-frequency coefficients

a polarity matrix is obtained; and then compare the polarity matrix with binary watermark image after scrambling. Finally

the middle-frequency coefficients of the original image are modified in terms of the comparation result to get thewatermarked image. The binarywatermark is extracted from the watermarked image without the original image. The experimental results show that the algorithm is invisible and robust against the general image processing methods.