What is Byzantine Generals’ Problem?
Crypto Glossary Definition
A thought experiment that is intended to illustrate the difficulty of reaching consensus in a distributed system. In the problem, a group of generals who each command a portion of the army, surround a city. These generals must develop a plan to either attack or to retreat. Every general must reach a collective decision (if a unified decision is not reached, and some generals decide to attack, while others retreat, then the uncoordinated attack or retreat, the army will fail). Generals must coordinate via messengers (whom may lie) and generals themselves may be traitorous.
Why Byzantine Generals’ Problem Matters
This classic 1982 computer science thought experiment is the foundational problem blockchain consensus mechanisms exist to solve: how do you get a large group of mutually distrusting parties (some of whom might be actively lying) to agree on a single version of the truth without a trusted coordinator? Proof of Work and Proof of Stake are both, at their core, practical solutions to this exact problem.
Byzantine Generals’ Problem in Practice
Imagine explaining consensus mechanisms to someone new to crypto by walking through the original 1982 thought experiment it comes from: several generals, each commanding a separate division, have surrounded an enemy city, and their only hope of victory is attacking simultaneously, since a partial, uncoordinated attack would be worse than either a full attack or a full retreat. The generals can only communicate by sending messengers between camps, any of whom might be captured and have their message altered, and worse, some of the generals themselves might be traitors deliberately sending contradictory orders to sabotage the plan. The puzzle is figuring out how the loyal generals can still reach a reliable, coordinated decision despite unreliable communication and the presence of active liars in their own ranks. This maps directly onto the challenge facing a decentralized blockchain network: thousands of computers, run by strangers with no reason to trust one another, some of whom may be running modified or malicious software, still need to agree on a single, accurate version of the transaction ledger. Proof of Work solves it by making it prohibitively expensive to lie convincingly, and Proof of Stake solves it by making dishonesty financially costly through slashing. Both are, at their heart, modern engineering answers to that same forty-year-old military riddle.
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