Byzantine Generals' Problem and BFT

Byzantine Fault Tolerance (BFT) is a property of distributed systems that allows them to function correctly even when some components fail or act maliciously. BFT is crucial in decentralized networks, where trust among nodes cannot be assumed. In other words, a system exhibiting BFT can tolerate Byzantine faults, which are arbitrary failures that include malicious behavior. For a system to be Byzantine fault-tolerant, it must reach consensus despite these faults.

Practical Byzantine Fault Tolerance (pBFT)

pBFT is an algorithm designed to improve the efficiency of achieving BFT in practical applications. It works by having nodes communicate with each other to agree on the system's state. The algorithm operates in several rounds of voting, where nodes exchange messages to confirm the validity of transactions. pBFT ensures that as long as less than one-third of the nodes are faulty, consensus can be achieved.

Byzantine Generals' Problem

The Byzantine Generals’ Problem was conceived in 1982 as a logical dilemma that illustrates how a group of Byzantine generals may have communication problems when trying to agree on their next move. It illustrates the difficulty of achieving consensus in a distributed system with potentially faulty nodes. It is framed as a scenario where several Byzantine generals must agree on a coordinated attack plan, but some of them may be traitors trying to prevent consensus.

The dilemma assumes that each general has its own army and that each group is situated in different locations around the city they intend to attack. The generals can communicate with one another only by messenger. After observing the enemy they must decide on a common plan of action. It does not matter whether they attack or retreat, as long as all generals reach consensus, i.e., agree on a common decision in order to execute it in coordination.

The central challenge of the Byzantine Generals’ Problem is that the messages can get somehow delayed, destroyed or lost. In addition, even if a message is successfully delivered, one or more generals may choose (for whatever reason) to act maliciously and send a fraudulent message to confuse the other generals, leading to a total failure. If we apply the dilemma to the context of blockchains, each general represents a network node, and the nodes need to reach consensus on the current state of the system. Putting in another way, the majority of participants within a distributed network have to agree and execute the same action in order to avoid complete failure.

Therefore, the only way to achieve consensus in these types of distributed system is by having at least ⅔ or more reliable and honest network nodes. This means that if the majority of the network decides to act maliciously, the system is susceptible to failures and attacks (such as the 51% attack).

In blockchain and distributed ledger systems, the Byzantine Generals' Problem is analogous to nodes (validators or miners) needing to agree on the state of the ledger despite some nodes potentially acting maliciously.