Design and Implementation of a Certified Mail Exchange System Using Simultaneous Secret Exchange

In this paper we design a certified e-mail exchange system based on a simultaneous secret exchange protocol. Among others, we use the protocol proposed by Okamoto and Ohta since the number of rounds it requires is relatively small compared to other similar protocols. We first modify the method of Okamoto and Ohta to further reduce the number of rounds without breaking the fairness condition too much. Next, we propose three different communication models considering the existence of MTAs (mail transfer agents, e.g. SMTP server) and MUAs (mail user agents). Then we adopt two models, which have several advantages; for example, they enable send-and-forget for the sender, or enable to exchange messages between the sender and the receiver directly, depending on the environment of the users. Finally, we design details of the system and implement it as a prototype.