How to Solve the Byzantine Generals Problem

When people say Bitcoin will change the world they are not necessarily crazy. What they are actually referring to is the revolutionary potential of Bitcoin’s underlying invention: the blockchain.

The blockchain is the decentralised and distributed public ledger of all transactions. Every user on the network shares the ledger. It is revolutionary because it allows for agreement on a list of transactions without a centralised party to keep track of everything.

This decentralised ledger represents a critical advancement in technology. Specifically, it is the solution to a long standing problem in computer science: the Byzantine General’s problem. This was previously thought to be impossible to solve.

byzantine generals problem

An Old Computer Science Problem

The Byzantine generals problem is an allegory used to explain the fundamental problem. The concept can be a little difficult to grasp. We like a challenge. So, we will try and make it simple.

Some generals are outside a city with their respective armies. They all want to invade the city. If at least half of them attack at the same time they’ll win. They known this. However, they must attack at the same time otherwise they will lose not. The generals can only co-ordinate their attack via messages.


Treachery is of course possible and generals can send fake messages. When faced with the potential dishonesty there is know way know if a message is true. So how these armies, dispersed over the battle-field, co-ordinate the time to attack together?

The solution actually turns out to be quite simple. Attach a cost to sending a message and ensure, through the network, that only one general can send a message at a time. The cost is known as a proof-of-work scheme. A message can’t be faked as it will not contain the mathematical solution that is needed to authenticate it.

A dishonest general will only succeed if the majority of the other generals collude with him. But this is unlikely. This is because the system rewards generals for their work maintaining the system.
There it is; a time-stamped and distributed ledger over a peer-to-peer network: a technological tour de force, as Bill Gates put it.

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