70 Int. J. of Applied Cryptography, Vol. 1, No. 1, 2008 An optimistic fair exchange protocol and its security in the universal composability framework Yusuke Okada*

Security services become crucial to many applications such as e-commerce payment protocols, electronic contract signing, and certified e-mail delivery with the phenomenal growth of the Internet. For these applications fair exchange must be assured. A fair protocol allows two members participating in a contract to exchange digital signatures over the Internet in a fair way, so that either each person gets the other’s signature, or neither person does. As more business is conducted over the Internet, the fair-exchange problem is gaining greater importance. The property abuse-freeness is necessary for contract signing. Abuse free means, if the protocol is not executed successfully, none of the two members involved in contract signing can show the validity of intermediate results to others. Here a contract-signing protocol in a multiple TTP scenario is described. This digital signature exchange protocol is optimistic, means the third trusted party (TTP) is involved only in the situations where one person is cheating or the communication channel is interrupted, i.e., TTP is off-line.

The wide qualities of peer-to-peer (P2P) network have given us many advantages and threats for enhancement of distributed computing. The best way to reduce threats is adding a reputation-based globally trusted model. Many present trust models are failing to restrain effectively some behaviors like collusive attacks, but pay no heed towards the security of this mechanism.