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

摘要： 依据内容认证的特点和JPEG压缩原理提出一种半脆弱水印嵌入方法。该方法对原始图像进行分块DCT变换，每块取三个DCT低频系数，把这三个系数量化后取最低有效位，与水印的对应位进行异或运算得到密钥;再取三个高频系数进行异或等运算得到另一密钥。认证时使用这两个密钥来恢复水印信息。实验结果表明，该方法对JPEG压缩具有很好的鲁棒性，同时对滤波和加噪等处理具有很好的易碎性，适合于图像的内容认证。由于该水印方法不改变原图像，因而可应用于画面清晰度要求高的多媒体和认证系统中。

关键词: 半脆弱水印, 图像认证, DCT变换, 密钥