Dipartimento di Ingegneria informatica, automatica e gestionale

Schemes for the identification and replacement of two-faced Byzantine processes are presented. The detection is based on the comparison of the (blackbox) decision result of a Byzantine consensus on input consisting of the inputs of each of the processes, in a system containing n processes p1, …, pn. Process pi that received a gossiped message from pj with the input of another process pk, that differs from pk ’s input value as received from pk by pi, reports on pk and pj being two-faced. If enough processes (where enough means at least t+1, t < n, is a threshold on the number of Byzantine participants) report on the same participant pj to be two-faced, participant pj is replaced. If less than the required t+1 processes threshold report on a participant pj, both the reporting processes and the reported process are replaced. If one of them is not Byzantine, its replacement is the price to pay to cope with the uncertainty created by Byzantine processes. The scheme ensures that any two-faced Byzantine participant that prevents fast termination is eliminated and replaced. Such replacement may serve as a preparation for the next invocations of Byzantine agreement possibly used to implement a replicated state machine.