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.

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Fair-exchange protocols e-commerce digital signatures security