We present new protocols for fair exchange of electronic data (digital signatures, payment and confidential data) between two parties A and B. Novel properties of the proposed protocols include: 1) off-line trusted third party (TTP), i.e., TTP does not take part in the exchange unless one of the parties behaves improperly; 2) only three message exchanges are required in the normal situation; 3) true fair exchange, i.e., either A and B obtain each other's data or no party receives anything useful; no loss can be incurred to a party no matter how maliciously the other party behaves during the exchange. This last property is in contrast to previously proposed protocols with off-line TTP ([1] and [21]), where a misbehaving party may get other party's data while reuse to send his document to the other party, and the TTP can provide affidavits attesting to what happened during the exchange. To our knowledge, the protocols presented here are the first exchange protocols which use off-line TTP and at the same time guarantee true fair-exchange of digital messages. We introduce...

We study the benefits of revocable items (like electronic payments which can be undone by the bank) for the design of efficient fair exchange protocols. We exploit revocability to construct a new optimistic fair exchange protocol that even works with asynchronous communication channels. All previous protocols with comparable properties follow the idea of Asokan's exchange protocol for two generatable items [Aso98, ASW98]. But compared to that, our protocol is more efficient: We need less messages in the faultless case and our conflict resolution is less complicated. Furthermore, we show that the generatability, which is required by [Aso98, ASW98], is difficult to implement in the context of some electronic payments. Instead, revocability of payments may be much easier to realize. Thus, our new protocol is very well suited for the fair exchange of revocable payments for digital goods.

Abstract. In this paper we consider the problem of increasing the number of possible challenge values from 2 to s in various zero-knowledge cut and choose protocols. First we discuss doing this for graph isomorphism protocol. Then we show how increasing this number improves efficiency of protocols for double discrete logarithm and e-th root of discrete logarithm which are potentially very useful tools for constructing complex cryptographic protocols. The practical improvement given by our paper is 2-4 times in terms of both time complexity and transcript size. 1