Advances in Cryptology — EUROCRYPT'98

Volume 1403 of the series Lecture Notes in Computer Science pp 591-606


Optimistic fair exchange of digital signatures

Extended abstract
  • N. AsokanAffiliated withIBM Zürich Research Laboratory
  • , Victor ShoupAffiliated withIBM Zürich Research Laboratory
  • , Michael WaidnerAffiliated withIBM Zürich Research Laboratory


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 and fair termination, without the cooperation of the other player.