Security
In the context of distributed systems, Security is the science and art of building distributed systems that maintain their properties even when a subset of nodes are malicious or adversarial or Byzantine.
Fail-stop vs malicious behaviour: Malicious behaviour contrasts with fail-stop behaviour, where a node either follows its role, or just stops participating (it never does any of the above)
A node is malicious if we cannot assume anything about its behaviour. In particular, it may behave in the worst possible way needed to break the properties of our distributed system.
- Refusing to participate in the system
- Sending out messages to other nodes in contravention of the protocol
- Declaring a false identity
Origin of malicious behaviour: node has been under the control of a malicious entity.