Failure model Model n nodes in the system f are faulty Because blockchains

Failure model model n nodes in the system f are

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Failure model Model n nodes in the system f are faulty Because blockchains protect ownership, there is an incentive for an attacker to steal the goods of others The fault model is Byzantine (i.e., arbitrary) 12
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Solution for Byzantine consensus Limiting the number f of failures is key to solving consensus There are solutions when f < n/3 [CL02] [CL02] M. Castro and B. Liskov. Practical byzantine fault tolerance and proactive recovery. ACM Trans. Comput. Syst., 20(4):398{461, Nov. 2002. 13
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Proof of work
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Sybil attack A Sybil attack is an attack where a malicious user forges identities It is named after the subject of the book Sybil , a case study of a woman diagnosed with dissociative identity disorder. Some solutions [CL02] are prone to Sybil attacks where an adversary generates fake faulty nodes to have f n/3 consensus impossible. 15
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Miners Specialised peers, called miners , receive a reward for verifying transactions provably solving a cryptopuzzle [Bla02] to append a new transaction block to the blockchain. Cryptopuzzle: given a block and a threshold , a miner repeatedly: selects a nonce and applies a pseudo-random function to this block and the selected nonce …until it obtains a result lower than the threshold. The nonce is included in the block: getting the block takes time, but validating that the nonce is correct is easy [Bla02] A. Black, “Hashcash - a denial of service counter-measure”, Cypherspace, TR 2002. 16
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Proof-of-Work The nonce is included in the block, this is the proof-of-work [DN93] : finding the nonce takes time, but validating that the nonce is correct is easy. Everyone can verify that someone lied about having solved the puzzle [DN93] C. Dwork and M. Naor. Pricing via processing or combatting junk mail. In Proceedings of the 12th Annual International Cryptology Conference on Advances in Cryptology, CRYPTO '92, pages 139-147, 1993. 17
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Execution
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Gossip-based protocol Current blockchain state 19
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New transaction Let’s transfer 10BTC from my account to Alice’s account 20
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Broadcast 21
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Broadcast 22
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23 Mining into a block
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Mining into a block 24
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25 Consensus
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26 Consensus
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Consensus 27
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Consensus 28
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Consensus 29
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30 Consensus
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Resolving a fork
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