Constructing Optimistic Multi-party Contract Signing Protocols

We give an explicit, general construction for optimistic multi-party contract signing protocols. Our construction converts a sequence over any finite set of signers into a protocol specification for the signers. The inevitable trusted third party's role specification and computations are independent of the signer's role specification. This permits a wide variety of protocols to be handled equally by the trusted third party. We give tight conditions under which the resulting protocols satisfy fairness and timeliness. We provide examples of several classes of protocols and we discuss lower bounds for the complexity of fair protocols, both in terms of bandwidth and minimum number of messages. Our results highlight the connection between optimistic fair contract signing protocols and the combinatorial problem of constructing sequences which contain all permutations of a set as subsequences. This connection is stronger than was previously realized.