Beyond One Third Byzantine Failures

The Byzantine agreement problem requires a set of $n$ processes to agree on a value sent by a transmitter, despite a subset of $b$ processes behaving in an arbitrary, i.e. Byzantine, manner and sending corrupted messages to all processes in the system. It is well known that the problem has a solution in a (an eventually) synchronous message passing distributed system iff the number of processes in the Byzantine subset is less than one third of the total number of processes, i.e. iff $n > 3b+1$. The rest of the processes are expected to be correct: they should never deviate from the algorithm assigned to them and send corrupted messages. But what if they still do? We show in this paper that it is possible to solve Byzantine agreement even if, beyond the $b$ ($< n/3$) Byzantine processes, some of the other processes also send corrupted messages, as long as they do not send them to all. More specifically, we generalize the classical Byzantine model and consider that Byzantine failures might be partial. In each communication step, some of the processes might send corrupted messages to a subset of the processes. This subset of processes - to which corrupted messages might be sent - could change over time. We compute the exact number of processes that can commit such faults, besides those that commit classical Byzantine failures, while still solving Byzantine agreement. We present a corresponding Byzantine agreement algorithm and prove its optimality by giving resilience and complexity bounds.