Bitcoins consensus chooses the deepest branch State BiPi the local blockchain

Bitcoins consensus chooses the deepest branch state

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Bitcoin’s consensus chooses the deepest branch State Bi,Pi the local blockchain view Each peer of the blockchain executes: Receive blocks Bj, Pj from j Bi = Bi Bj Pi = Pi Pj depth (b): if children(b) = then return 1 else return 1 + max c children(b) depth(c) Prune shortest branches at i b = genesis-block(Bi) while b.next ≠ block = argmax c children(b) { depth(c) } B = B {block} P = P { block,b } b = block Bi,Pi = B,P [Nak08] S. Nakamoto, “Bitcoin: a peer-to-peer electronic cash system,” 2008, . 33
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When is a transaction committed?
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Committed transaction Given a blockchain with parameter k , a block at index i is decided when the chain depth reaches i+k A transaction is committed if it belongs to a decided block 35
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Quiz
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Question 1 What assumption is needed here? 37
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Question 1 What assumption is needed here? Synchrony Without synchrony, no one can distinguish a committed transaction from a transaction that is not yet committed and that can possibly be rolled back later on 38
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The 51% attack
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Impact of mining power 10MH/s 3MH/s 2MH/s 1MH/s 3MH/s 40
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b1 b1 10MH/s 3MH/s 2MH/s 1MH/s 3MH/s Impact of mining power 41
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b2 b2 b1 b1 10MH/s 3MH/s 2MH/s 1MH/s 3MH/s Impact of mining power 42
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b2 b1 b3 b3 10MH/s 3MH/s 2MH/s 1MH/s 3MH/s Impact of mining power 43
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b4 b2 b1 b4 b3 10MH/s 3MH/s 2MH/s 1MH/s 3MH/s Impact of mining power 44
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b5 b2 b1 b4 b3 b5 10MH/s 3MH/s 2MH/s 1MH/s 3MH/s Impact of mining power 45
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b5 b2 b1 b4 b3 b5 10MH/s 3MH/s 2MH/s 1MH/s 3MH/s [Ros12] M. Rosenfeld, “Analysis of hashrate- based double-spending,” 2012. Impact of mining power 46
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Impact of network delay
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Impact of communication delay 48
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Blocks propagate fast b5 b1 b1 49
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Impact of communication delay b1 b2 b3 b4 b5 b5 50
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Impact of communication delay b1 b2 b3 b4 b5 b5 b4 b3 b2 b1 51
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Quiz
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