Practical Byzantine fault tolerance algorithm 

Practical Byzantine fault tolerance (PBFT) algorithm. Many algorithms are called Byzantine fault tolerant. The name comes from the allegory that presented the original problem.

Imagine an ancient Byzantine army moving to capture a city. The idea is to attack from all sides. Once the generals of the army reach the city, they must agree on when and how to attack. The difficulty is in how to agree. The generals can communicate only by messenger, but the messengers could be captured by the enemy, and there is the additional fear that one or more of the generals or their commanders are traitors.

The generals need a method to ensure that all the loyal generals agree on the same plan, and that a small number of possible traitors cannot cause the mission to fail.

The loyal generals will all do what the method says they will do, but the traitors might do anything. How can the generals create a method that ensures that, as long as most of them are loyal, their plans will succeed?

This allegory is also sometimes called the Chinese general's problem, as well as a few other names, but the issue remains the same: how can different parties securely communicate and reach an agreement about something when communication channels may not be secure, and when there may even be traitors in their midst.

In the case of blockchain, the generals in the story are the computers that are participating in the distributed network running the blockchain. The messengers represent the digital network that these machines are running on and the message protocols used by those machines. The goal is for the good computers or generals to decide which information on the network is valid while rooting out bad actors and preventing false information from being recorded on the blockchain.

The loyal generals in the story represent the operators of honest nodes that are interested in ensuring the integrity of the blockchain and the applications based on it, and that are therefore invested in making sure that only correct data is recorded. The traitors represent the many bad actors of the world who would love to falsify data (especially financial data) for personal gain or on behalf of some other antagonistic party. The motivations of the bad actors could be varied, from spending Bitcoin they do not truly possess or getting out of contractual obligations, or even trying to destroy the network as a form of currency control by a hostile government.