Section: Scientific Foundations

Malicious process behaviors

When assuming that processes fail by simply crashing, bounds on resiliency (maximum number of processes that may crash), number of exchanged messages, number of communication steps, etc. either in synchronous and augmented asynchronous systems (recall that in purely asynchronous systems some problems are impossible to solve) are known. If processes can exhibit malicious behaviors, these bounds are seldom the same. Sometimes, it is even necessary to change the specification of the problem. For example, the consensus problem does not make sense if some processes can exhibit a Byzantine behavior and thus propose arbitrary value. The validity property of the consensus is changed to "if all correct processes propose the same value then only this value can be decided" instead of "a decided value is a proposed value". Moreover, the resilience bound of less than half of faulty processes is at least lowered to "less then a third of Byzantine processes". These are some of the aspects we propose to study in the context of the classical model of distributed systems, in peer-to-peer systems and in sensor networks.