On the Significance of Quiescent Protocols for Asynchronous Perfectly Secure Message Transmission

We consider the problem of perfect (information-theoretically) secure message transmission (PSMT) from a sender $S$ to a receiver $R$ in asynchronous directed networks tolerating dual adversary. The adversary can control at most $t_p$ nodes in a passive fashion and at most $t_f$ nodes in a fail-stop fashion. We achieve PSMT in this setting with a new class of message transmission protocols called the quiescent protocols. Quiescent protocols ensure that the players eventually stop sending messages but in contrast to the usual terminating protocols, these protocols do not require the players to go into the halt state, from which they cannot take any further action. To show the significance of quiescent protocols, we first identify conditions in which it is impossible for any terminating protocol to achieve PSMT. To circumvent this impossibility, we transform the existing synchronous PSMT protocol into a quiescent protocol which accomplishes PSMT. By focusing on designing quiescent protocols rather than terminating protocols, we show that asynchrony in the network does not demand any extra connectivity over its state-of-the-art synchronous counterpart. This shows that quiescent protocols are befitted to solve PSMT in asynchronous networks with fail-stop faults.