Blockchain Protocol Solida: Reconfigurable Byzantine Consensus
Solida is a blockchain protocol based on reconfigurable Byzantine consensus, developed by Ling Ren in collaboration with Ittai Abraham, Dahlia Malkhi, Kartik Nayak, and Alexander Spiegelman at VMware Research. The protocol provides insights into Bitcoin, mining, double spending, permissionless consensus, Bitcoin consensus speed, PBFT for blockchains, and reconfiguration dynamics. It explores concepts like PoW, longest chain wins, PoW puzzles, PBFT consensus, and more.
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Presentation Transcript
Solida: A Blockchain Protocol Based on Reconfigurable Byzantine Consensus Ling Ren Joint work with Ittai Abraham, Dahlia Malkhi, Kartik Nayak and Alexander Spiegelman *Work done at VMware Research
Bitcoin Ordered blocks (of transactions) A block must be hard to generate -- mining Carol pays Bob $50 Bob pays Alice $10 Alice pays Bob $1 & Dan $2 Dan pays Carol $50 Double spending Carol pays Carol $50 2
Bitcoin Mining Mining: solve hard puzzles Puzzle Random function ffffffffca780f89 > Threshold ? nonce Proof-of-Work (PoW) Carol pays Bob $50 Bob pays Alice $10 Alice pays Bob $1 Alice pays Dan $2 Dan pays Carol $50 PoW PoW PoW 3
Permissionless Consensus Bitcoin: PoW + longest chain wins Bob Charlie Dave Alice Emily Newly minted coins & tx fees 4
Permissionless Consensus Bitcoin: PoW + longest chain wins Bob Charlie Dave Alice Emily 5
Bitcoin Consensus is Slow 10min / block, > 60min latency Byzantine consensus (e.g., PBFT) is instant Member 1 (Leader) Member 2 Member 3 Member 4 n = 3f+1 Pre-prepare Prepare PBFT Commit Courtesy of J. Li @ NYU 6
PBFT for Blockchains? [Peercensus, Byzcoin, Hybrid Consensus] Numerous challenges: Reconfiguration PBFT is permissioned Wedging TXs concurrent to reconfiguration altruistic vs. rational participants 7
Reconfiguration puzzle puzzle F PoW A B C D E PoW G PoW Committee 8
Reconfigure PBFT The newcomer acts as an external leader Leader Member 1 Member 2 Member 3 Member 4 Propose Accept Commit Propagate 9
Reconfigure PBFT - Contention Paxos-style leader election with ranks Leader B Leader A Member 1 Member 2 Member 3 Member 4 Propose Accept Commit 10
Reconfigure PBFT - Contention PoW as rank Leader B Leader A Member 1 Member 2 Member 3 Member 4 Status Propose Accept Commit 11
Reconfigure PBFT - Contention PoW as rank Leader B Leader A Member 1 Member 2 Member 3 Member 4 Status Propose Accept Commit 12
Problem with PoW Ranks A Byzantine leader can stall Leader A Member 1 Member 2 Member 3 Member 4 Elect Propose Accept Commit 13
A Fix with Epochs rank = (lifespan, PoW) Lifespan = # of PoWs seen so far Leader A Member 1 Member 2 Member 3 Member 4 Propose Accept Commit Elect 14
A Fix with Epochs Safety follows from PBFT rank = (epoch, PoW) Each member maintains a local epoch Proof-of-Work gives sybil-proof rank.epoch = median among 2f+1 replies Leader A and eventually unique leader Member 1 Member 2 Member 3 Lifespan prevents stalling Member 4 Propose Accept Commit Get-epoch Elect 15
Proof Sketch Goal: Adversary cannot get 1/3 seats Sketch: Message delay for honest users extra time to work on PoW for adversaries Synchrony? Seriously? Necessary for Bitcoin/PoW [PSS 16] Why PBFT if the model is synchronous? Actual speed vs. fault tolerance 16
Summary Bitcoin: PoW a PoW chain permissionless consensus Solida: PoW permissionless consensus
