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40 lines
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40 lines
2 KiB
Markdown
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title: Proof of Work in Cryptocurrencies | Monero Documentation
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---
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# Proof of Work in Cryptocurrencies
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> Proof of work is a Sybil protection mechanism
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## PoW protects against Sybil attack
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In decentralized cryptocurrencies **untrusted** actors confirm (blocks of) transactions.
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If threshold voting was employed then the scheme would break immediately.
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This is because nothing prevents a single actor from creating arbitrary number of pseudonyms and take over the voting.
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In distributed systems this is known as Sybil attack.
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Instead, cryptocurrencies employ proof of work. In the proof of work scheme,
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it is not the number of actors that counts. It is the amount of committed
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computational resources.
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This, of course, is much harder to game.
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To endanger the scheme, an attacker would have to actually control majority (>50%) of computational resources.
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In practice, attacker would need this control over significant period of time.
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## PoW is a leader election mechanism
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In distributed systems "leader election" is a process of establishing which node is responsible for (temporarily) coordinating the system.
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In cryptocurrencies PoW is used to elect the node that "wins" the next block.
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Using PoW for leader election was one of the key inventions introduced by Bitcoin.
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Competing nodes (known as "miners") work on a solution to artificial problem.
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Every now and then, someone randomly finds the solution.
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Chances are linearly proportional to committed computing power.
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The winner uses its solution to "underwrite" the block it assembled. Only blocks with valid solutions are accepted by the network.
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The winner also gets a reward for its work. The reward is a specific amount of cryptocurrency created "out of thin air" and assigned to self. The winner also gets all fees coming from transactions included in the block.
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The difficulty of the PoW problem is dynamically adjusted by the network, with the goal of finding blocks with a roughly constant rate (typically, every couple of minutes).
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