Improved Multi-party Contract Signing

A multi-party contract signing protocol allows a set of participants to exchange messages with each other with a view to arriving in a state in which each of them has a pre-agreed contract text signed by all the others. “Optimistic” such protocols allow parties to sign a contract initially without involving a trusted third party T . If all signers are honest and messages are not arbitrarily delayed, the protocol can conclude successfully without T ’s involvement. Signers can ask T to intervene if something goes amiss, for example, if an expected message is not received. Two multi-party contract signing protocols have been proposed so far. One solution to this problem was proposed by Garay and MacKenzie (DISC’99) based on private contract signatures, but it was subsequently shown to be fundamentally flawed (it fails the fairness property). Another more efficient protocol was proposed by Baum-Waidner and Waidner (ICALP’00). It has not been compromised, but it is based on a nonstandard notion of a signed contract. In this paper, we propose a new optimistic multi-party contract signing protocol based on private contract signatures. It does not use a nonstandard notion of a signed contract and has half the message complexity of the previous solution.