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Bitcoin’s Transaction Processing

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  1. Bitcoin’s Transaction Processing Yonatan Sompolinsky joint work with Aviv Zohar

  2. Bitcoin decentralized cryptocurrency • hard to get • easy to pass • no authority, no money printing • pseudonymous • relatively secure • high transaction volume current protocol: conflictghost protocol: in accord

  3. Transactions in Bitcoin tx tx tx’ tx tx tx tx tx tx tx’ tx tx tx’ tx tx tx’ tx tx tx tx’ tx’ • irreversible Bob Calvin

  4. The Blockchain txs: txs: txs: ~ 10 min block #83 block #81 block #82 • consistent • grows in time • kept locally at each node Time

  5. Conflicting blockchains maintain one main chain only B1 A the ”longest chain” rule B2 delays => forks blocks discarded & txs cancelled Time

  6. Block tree vs Block chain 3 6 4 λ:= block creation rate = 6 per hourβ:= growth rate of main chain = 4 per hour efficiency:=β/λ=4/6<1 2 1 5 delays => forks => inefficiency Time

  7. Block size, Delay & Efficiency Block Propagation Time 0.95 Efficiency = 0 λ = block creation rateβ= growth rate of main chain • analysis of data provided to us by Decker and Wattenhofer

  8. Throughput (TPS) • Transactions Processed per Second = • (block size) X (growth rate of the main chain) • efficiency 2000 1/600 • TPS < 3.3 why? • TPS ~ 1736

  9. Security - double spend attack tx tx tx main chain Bob Calvin tx’ publish ! secret

  10. Success of attack • computational power needed to create blocks • share of blocks proportional to fraction of computational power • an attacker with computational power will eventually fail • conversely, an attacker with will always succeed • security threshold is thus

  11. Effect of increasing throughput • ↑ • efficiency↓ delay ↑ • TPS ↑ • security ↓

  12. Security limitation on TPS high TPS: more bandwidth needed Bitcoin today: secure butlow throughput TPS vulnerable TPS secure TPS attacker computational power (fraction)

  13. Greedy Heaviest Observed Sub-Tree longest chain • security ↓ efficiency ↓

  14. Greedy Heaviest Observed Sub-Tree main chain according to “longest” rule C1 4 10 D1 B tx B A 5 1 C2 6 1 1 D2 2 main chain according to “ghost” E1 D3 C3 main chain according to “longest” rule tx’ Time

  15. GHOST throughput • no security constraint • hardware constraints, e.g., network congestion, limit the feasible TPS • 8.5-fold increase of current bandwidth can withstandTPS = 2000 • in pipeline: improvement of ghost, same bandwidth more TPS • … • security • high transaction volume

  16. https://eprint.iacr.org/2013/881.pdf

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