Forgery prevention and detection are of great social importance.This paper attempts to apply digital watermarking into forgery prevention. The digital watermarks are embedded into the most important DCT coefficients except the DC coefficient of the original image through linearly additive operation.The corresponding watermarks are extracted throught threshold judgement and the threshold is determined through the difference of two cycle print&scan and one cycle print&scan of the images.To improve the robustness of the algorithm

the simple and effective repetition is applied to watermark configuration.And the CIELab color space is chosen to guarantee the repetition of the results and minimize the error brought by the transference of images between differnt equipments.Experimental results show that the proposed algorithm can satisfy the requirements of forgery prevention for ordinary images

that is to say the digital watermark is robust to one cycle print&scan and fragile to two cycle print&scan. Compared with other forgery prevention and detection techniques

the algorithm described in this paper reduced the computation cost considerably as no other special material or equipment is required and ordinary user can verify the authentication of printing materials. This algorithm can also be used in printing materials copyright protection

such as the owner verification in certificates

passports

ect.