Baumwaidner Waidner

This paper is referenced by Asokan Schunter and Mukhamedov Ryan. It is an abuse-free asynchronous multi-party optimistic contract signing protocols. Say there are n parties:

• it requires $(n+1)n(n-1)$ messages which may be high;
• but only $n+2$ rounds which is optimal;
• in the optimistic case the end signed result is $(C,n+1)$, thus the contract $C$ has to specify $n$;
• but in the pessimistic case the end signed result is $(C,i)$ plus some countersigning by $T$.

It uses only standard asymmetric cryptography.

Outline of the protocol

• Send to $j=1…n$ : $(C,1)_{Pi}$.
• Await from $j=1…n$ : $(C,1)_{Pj}$. Else resolve.

For $t=2…n+1$ do:

• Send to $j=1…n$ : $((C,t-1)_{P1}…(C,t-1)_{Pn})_{Pi},(C,t)_{Pi}$.
• Await from $j=1…n$ : $((C,t-1)_{P1}…(C,t-1)_{Pn})_{Pj},(C,t)_{Pj}$. Else resolve.

Resolve is a subprotocol, where $T$ intervenes, it aims at completing the protocol by asking $T$ to take a decision whether the contract is valid and provide a certificate of that. If provided enough grounds for doing it, $T$ will do so.