关键词: 数字水印, 离散余弦变换（DCT）, 正负量化, 盲检测

Abstract: A blind watermark algorithm of gray image is proposed based on DCT.At first，the watermark image is scrambled and encrypted.Then the vector image is divided into 8×8 blocks.Each block is transformed by DCT and watermark is embedded by quantization rules for the use of plus or minus.Watermark detection is based on positive and negative quantitative rules，and doesn’t need the original image.It’s a blind detection.At last，evaluating the performance of watermarking algorithm is done through the watermark correlation coefficient（NC） and Peak Signal to Noise Ratio（PSNR）.Experimental results show that the algorithm can effectively resist JPEG compression，noise，shearing，filtering attacks，and has a better robustness.

Key words: digital watermarking, Discrete Cosine Transform（DCT）, quantization of plus or minus, blind detection