Abstract: We survey the state of non-repudiation cryptographic protocols, almost all of which have weaknesses. The paper proposes a perfect protocol which can be used to realize the strong fairness and non-repudiation of both parties even if the channels are not reliable. Usually the protocol only needs three interactions of participants and the transferring data remain confidential for others but the sender and the receiver, who are unknown to others. The Trusted Third Party(TTP) acts as a notary rather than a delivery and its bottleneck problem is solved completely. Finally the security of the protocol is analysed in detail.

Key words: cheating, undeniable, trusted third party, certificate authority