An Optimistic Non-repudiation Protocol with Transparent Trusted Third Party

In this paper we consider a new and efficient optimistic nonrepudiation protocol. In a non-repudiation protocol, during which Alice wants to transmit a message to Bob, Alice has to send a non-repudiation of origin evidence to Bob (attesting that Alice is at the origin of the transmitted message), and Bob has to send a non-repudiation of receipt evidence to Alice (attesting Bob’s receipt of the message). Classical solutions propose to use a trusted third party to help realizing the exchange without giving any significant advantage to one of the two parties. In an optimistic protocol, the trusted third party intervenes only in case of problems during the communication between Alice and Bob. Classically, in a situation where an error occurs, evidences that have been digitally signed by the TTP are issued. Although these evidences are distinct from those produced by Alice and Bob in a faultless case, they have the same value in case of a dispute. In this paper we propose a protocol where the TTP produces the same evidences that Alice and Bob should have produced in a faultless protocol execution (this prevents, after a succesful protocol execution, to determine whether the TTP was involved or not).