中文摘要：

针对电子商务中三方交易存在的问题，提出基于安全水印的三方交易系统．在买卖方水印协议中加入第3方，卖方将所拥有的图像委托给服务器，让服务器与买方进行交易．首先依靠同态加密和扩频，在卖方的原始图像中嵌入服务器的水印，并将含水印的图像发送给服务器．服务器接收到图像后，先通过扩频抖动调制嵌入用于追踪非法传播时锁定可疑买方的定位水印，然后与买方执行基于同态加密的买卖方水印协议．在电子商务平台三方交易中，基于安全水印的三方交易系统不仅可保证服务器和买方不被陷害，而且可追踪非法传播的买方或服务器，为大用户群提供安全交易方案．

英文摘要：

To ensure security of e-commerce, we propose a tripartite trade system based on secure watermarking. We add a third party （server） in the buyer-seller watermarking protocol. The seller sends his/her own image to the server, and let the server trade with the buyer. The seller first uses homomorphic encryption and spreading to embed the server＇s watermark in the original image and send the marked image to the server. Having received the marked image, the server uses spread-spectrum dither modulation to embed another watermark to be used for locating suspicious buyers who illegally distribute the image. The server then executes the buyer-seller watermarking protocol based on homomorphic encryption. In the tripartite trade of the e-commerce platform, the proposed system not only prevents both the server and buyer from being framed, but also can trace illegal distribution of any buyer or the server. The system also provides a solution to the secure online transactions for a large number of users.