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ETH Zurich – Distributed Computing Group
Roger Wattenhofer
Does a Blockchain Need Altruism? Roger Wattenhofer ETH Zurich - - PowerPoint PPT Presentation
Does a Blockchain Need Altruism? Roger Wattenhofer ETH Zurich - - PowerPoint PPT Presentation
Does a Blockchain Need Altruism? Roger Wattenhofer ETH Zurich Distributed Computing Group Do You Trust the Miners? Modeling Distributed Systems A ltruistic R ational C rash B yzantine Modeling Distributed Systems C rash R ational A ltruistic
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Byzantine Altruistic Rational Crash
Modeling Distributed Systems
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Byzantine Altruistic Rational Crash
Who are the Miners?
Modeling Distributed Systems
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“The system is secure as long as honest nodes collectively control more CPU power than any cooperating group of attacker nodes.”
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Mining is a Rational Business
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Mining is a Rational Business
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2009 2013 2018
Selfish Mining Timeline
2010
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Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 …
What is Selfish Mining
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Analysis
Simpler
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Original Algorithm
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Somewhat Simpler Algorithm
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Tx_0 Tx_1 … Tx_0 Tx_1 …
𝑒𝑞 > 𝑒𝑡
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Somewhat Simpler Algorithm
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Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 …
𝑒𝑞 = 𝑒𝑡 − 1
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Somewhat Simpler Algorithm
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Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 … Tx_0 Tx_1 …
𝑒𝑞 = 𝑒𝑡
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State Machine (Original & Simpler)
= 1 − 𝛽 𝛽: probability that selfish miner finds a block
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Stationary Distribution
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Computation…
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All 𝛾 Transitions
0 → 0: Block for honest miners 𝑗 + 1 → 𝑗 : Block for selfish miner (for 𝑗 > 2) 2 → 0: Two blocks for selfish miner 1 → 0: Race who wins next block with probability 𝛽 two blocks for selfish miner with probability 𝛾(1 − 𝛿) two blocks for honest miners with probability 𝛾𝛿 one block each
𝛿: probability that honest miners append block to selfish miner’s block (in race)
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Ratio of Selfish Blocks in Chain
𝛿: probability that honest miners append block to selfish miner’s block (in race)
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Selfish Miner Share
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Selfish Miner Share
𝛿 = 0: break even at 𝛽 = 1/3 𝛿 = 0.5: break even at 𝛽 = 1/4 𝛿 = 1: break even at 𝛽 > 0
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A Blockchain Without Altruism?
[Joint Work with Jakub Sliwinski]
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Simple Chains Are Too Simple
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Better: Expose Competition
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Our Rational Blockchain
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Always Refer to All Childless Blocks
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Only One Type of Reference
(Heaviest Reference is Your “Parent”)
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2 3 4 5 6
Block Ordering is Recursive
1
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Incentives
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Why Miners Should Always Refer to All Childless Blocks?
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Because of our Block Rewards!
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It’s Somewhat Complicated…
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Reward = 0.71 Reward = 0.71 Reward = 1
Motivating Block Rewards I
Reward = 1 Reward = 0.34 Reward = 1
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Reward = 0.72 Reward = 0.45 Reward = 0.91 Reward = 0.69
Motivating Block Rewards II
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Our Solution
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1 1
Block Penalty Example
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1 0.5 0.5
Block Penalty Example
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1 1 0.66 0.66 0.66 1 0.5 0.5
Block Penalty Example
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The Penalty Algorithm
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The Penalty Algorithm
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The Penalty Algorithm
1 1 1 1 1 1 1 1 0.66 0.66 0.66 0.66 0.66 0.33 0.33
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Transaction Fees
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A B fee = 10 A B fee = 10
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A B fee/2 = 5 A B fee/2 = 5
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“The system is secure as long as honest nodes collectively control more CPU power than any cooperating group of attacker nodes.”
rational
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Thank You!
Questions & Comments?
www.disco.ethz.ch Thanks to Jakub Sliwinski