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
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
Proof of work
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
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
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
Execution
Gossip-based protocol Current blockchain state 19
New transaction Let’s transfer 10BTC from my account to Alice’s account 20
23 Mining into a block
Mining into a block 24
25 Consensus
26 Consensus
Consensus 27
Consensus 28
Consensus 29
30 Consensus
Resolving a fork

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