Thunderella: Blockchains with Optimistic Instant Confirmation. Rafael Pass and Elaine Shi Cornell Tech & Cornell University

Transcription

1 Thunderella: Blockchains with Optimistic Instant Confirmation Rafael Pass and Elaine Shi Cornell Tech & Cornell University

2 State-machine replication (a.k.a. linearly ordered log, consensus, blockchain)

3 State-machine replication (a.k.a. linearly ordered log, consensus, blockchain) Consistency: honest nodes agree on log Liveness: TXs are incorporated soon

4 Classical (e.g. PBFT, Paxos) Blockchains

5 Classical (e.g. PBFT, Paxos) Fast (most of the time) Complex Asynch Blockchains

6 Classical (e.g. PBFT, Paxos) Fast (most of the time) Complex Asynch Simple Robust Slow Sync Blockchains (PoW and non-pow)

7 Thunderella As simple and robust as a blockchain Confirm in 2 actual network rounds in the optimistic case Fall back to blockchain when things go bad

8 Classical (e.g. PBFT, Paxos) Let s start with this Roadmap Blockchains (PoW and non-pow)

9 Classical (e.g. PBFT, Paxos) Roadmap Blockchains (PoW and non-pow)

10

11 Leader proposes transaction (Seq, )

12 Everyone ack s

13 Confirm on upon enough acks

14 Ex: Assume ⅔n+1 honest; wait for ⅔n+1 acks

15 Must intersect at an honest node ⅔n+1 ⅔n+1 Y Assume ⅔n+1 honest

16 Must intersect at an honest node ⅔n+1 ⅔n+1 Y Assume 1/3n malicious

17 Must intersect at an honest node ⅔n+1 ⅔n+1 Y Thus X = Y

18 Assume ⅔ honest and online

19 Assume ⅔ honest and online Consistency Liveness

20 Assume ⅔ honest and online Consistency Liveness Consistency No liveness

21 How do we achieve liveness?

22 How do we achieve liveness? You don t want to know [PBFT, Paxos...]

23 Anatomy of classical consensus Simple normal path Complicated recovery path

24 Classical (e.g. PBFT, Paxos) Roadmap Blockchains (PoW and non-pow)

25 blockchain Thunderella

26 Thunderella for permissionless Thunderella for permissioned

27 Thunderella for permissionless For concreteness, we ll focus on this

28 Leader/ accelerator blockchain miners Committee (recent miners/ stakeholders)

29 Optimistic mode: Instant confirmation honest and online majority honest 3/4 fraction honest and online

30 But, still SECURE as long as: Arbitrary deviation! majority honest majority honest (but need not be online)

31 Must intersect at an honest node ¾n+1 X ¾n+1 Y Assume ½n+1 honest

32 Propose (seq, ) Ack

33 Propose (seq, ) Ack ¾ acks: notarized

34 Confirm maximal lucky sequence ¾ voted: notarized

35 No liveness when ¾ voted: notarized Confirm maximal lucky sequence

36 blockchain collects evidence of

37 blockchain collects evidence of Now enter slow mode

38 What evidence do we collect? Need: faulty nodes cannot implicate honest leader

39 Miners tell blockchain everything they know

40 What evidence do we collect? k blocks

41 What evidence do we collect? k blocks has not appeared in a lucky sequence

42 blockchain collects evidence of Now enter slow mode

43 Nodes have different logs when entering slow mode Now enter slow mode

44 Need: agree on log before entering slow mode Now enter slow mode

45 Stop acking Stop new optimistic transactions output Tell others what Share you knowledge Miners tell blockchain All knowledge what they blockchain know Grace period: k blocks

46 Summary has not appeared in a lucky sequence k blocks Grace period Slow mode

47 Optimistic mode: Instant confirmation honest and online majority honest 3/4 fraction honest and online

48 But, still SECURE as long as: Arbitrary deviation! majority honest majority honest (but need not be online)

49 Thank you.