关键词: 半脆弱水印, 图像认证, DCT变换, 密钥

Abstract: A digital image semi-fragile watermarking algorithm is presented based on the character of authentication and the elements of JPEG.The algorithm splits the original image to some blockes and do DCT transform，chooses three low-frequency coefficients in every block，quantizes them and chooses the least significance bits，then does the xor operations with the corresponding bits of water mark.，the result is the key.At the same time，we choose three high-frequency，and do the same operation and generate the other key.We use the two keys to extract the water mark to authenticate the image.The result of experiment show that this algorithm has good robust against JPEG，and frangibility against the familiar attacks，and beseem to selective authentication for image.Since the algorithm do not modify the original image，so it can be used in multimedia and authentication system that required high quality image.

Key words: semi-fragile watermarking, image authenticate, DCT transform, key