Hao Wang Yuyi Ou Jie Ling Lu Liang Xiang Xu Certified Email Delivery with Offline TTP. 15-20 2007 IAS http://doi.ieeecomputersociety.org/10.1109/IAS.2007.27 conf/IEEEias/2007 db/conf/IEEEias/IEEEias2007.html#WangOLLX07