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This paper presents an additive watermarking technique for grey-scale pictures. It consists in secretly embedding copyright information (a binary code) into the picture without degrading its quality. Those bits are encoded through the phase of maximal length sequences (MLS). MLS are binary sequences with good correlation properties. The result of the autocorrelation is much greater than crosscorrelations, i.e. correlations made with shifted versions of this sequence. The embedded bits are retrieved from the result of the correlations. The core of the embedding process is underlaid by a masking criterion that guarantees the invisibility of the watermark. It is combined with an edge and texture discrimination to determine the embedding level of the MLS, whose bits are actually spread over 32×8 pixel blocks. Eventually, some results are presented, which analyze the efficiency of the retrieval as well as the resistance of …