Efficient Fair Exchange with Verifiable Confirmation of Signatures

We propose a new efficient protocol, which allows a pair of potentially mistrusting parties to exchange digital signatures over the Internet in a fair way, such that after the protocol is running, either each party obtains the other’s signature, or neither of them does. The protocol relies on an off-line Trusted Third Party (TTP), which does not take part in the exchange unless any of the parties behaves improperly or other faults occur. Efficiency of the protocol is achieved by using a cryptographic primitive, called confirmable signatures (or designated confirmer signatures in its original proposal [9]). We recommend using a new efficient confirmable signature scheme in the proposed fair exchange protocol. This scheme combines the family of discrete logarithm (DL) based signature algorithms and a zero-knowledge (ZK) proof on the equality of two DLs. The protocol has a practical level of performance: only a moderate number of communication rounds and ordinary signatures are required. The security of the protocol can be established from that of the underlying signature algorithms and that of the ZK proof used.