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The authors of this paper didn't have "blockchain" in mind. Imagine rockets, or airplanes, with redundant systems on board. If, say, 3 of 4 sensors report "lower the landing gear" while 1 of 4 reports "don't lower the landing gear", then a hard requirement of the Byzantine Generals Problem is that "lower the landing gear" wins, assuming a majority of sensors function correctly (loyal generals). Nakamoto consensus addresses "double-spend". Say I have exactly one coin, and I broadcast a transaction that sends the coin to Alice. All miners on the network consider my transaction a candidate for the next block, and get busy hashing proof-of-work. Moments later, I compose a transaction that sends my one coin to Bob. And, I start mining a block with only that transaction in it. Now, its a race. If I win the race, the coin goes to Bob. If anyone else wins, the coin goes to Alice. Importantly, the coin will not go to both Alice and Bob. In Nakamoto consensus, exactly one miner wins each race. And that miner, "loyal" or not, determines whether the coin goes to Alice or to Bob. To satisfy Byzantine Generals, in this particular example, the coin should always go to Alice. But in Nakamoto consensus it might end up going to either Alice or Bob. Solving double-spend in a network of untrusted nodes is a significant feat. (And, according to the market, extremely valuable.) However, as I think your question is designed to point out, Nakamoto consensus does not solve the Byzantine Generals Problem. No offense to Nakamoto, it solves a different problem. It's worth mentioning the concept of "byzantine faults" - meaning a node on the network might not be simply buggy, it might be designed by a malicious adversary to do any behavior that could exploit vulnerability in other nodes. So, if you're reading the next revolutionary ICO whitepaper and see, "we use Nakamoto Consensus to solve double-spend, even in the presence of byzantine adversaries", maybe keep reading. But if you read, "we use proof-of-work to solve the Byzantine Generals Problem", maybe save your money for another investment. |