Optimistic Fair Exchange of Digital Signatures

We present a new protocol that allows two players to exchange digital signatures over the Internet in a fair way, so that either each player gets the other's signature, or neither player does. The obvious application is where the signatures represent items of value, for example, an electronic check or airline ticket. The protocol can also be adapted to exchange encrypted data. The protocol relies on a trusted third party, but is "optimistic", in that the third party is only needed in cases where one player attempts to cheat or simply crashes. A key feature of our protocol is that a player can always force a timely in fair termination, without the cooperation of the other player. A specialization of our protocol can be used for contract signing; this specialization is not only more efficient, but also has the important property that the third party can be held accountable for its actions: if it ever cheats, this can be detected and proven.