In this paper, a secure blind watermarking scheme is proposed. In the scheme, an embedding function matrix (EFM) is generated based on a serial number which is unique to each owner or owner group, then the cellular automata system embeds the scrambled watermark into the 3-level DWT coefficients of the original image based on the EFM. In the procedure of extracting, the original image is not needed, however even if the algorithm is known, without the correct function matrix, watermark can not be extracted. This shows that the proposed scheme can enhance the security of watermark. Experimental results also show that the proposed scheme is robust to geometrical attacks and common image processing attacks.