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Hybrid Consensus: Efficient Consensus in the Permissionless Model - - PowerPoint PPT Presentation
Hybrid Consensus: Efficient Consensus in the Permissionless Model - - PowerPoint PPT Presentation
Hybrid Consensus: Efficient Consensus in the Permissionless Model Rafael Pass and Elaine Shi IC3 1 Blockchains Satoshi Nakamoto, 2009 Public database Shared & maintained between network participants Blockchain agreement protocol
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The Blockchain Agreement Problem
Set of N computational nodes
No inherent identity No PKI
Agree on a block
Includes a set of new transactions (or data)
Two properties
Agreement Validity: all transactions satisfy some validity conditions
Blo ck 3 Blo ck 2 Blo ck 1
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Scalability Issue
1 MB block per 10 mins 3-7 TXs per second
Support limited computations Several DoS attacks recently
Demand from Practice: 1,200 - 50,000 TXs/s
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Byzantine Agreement
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p
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p
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p
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p
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p
- Complete graph
- permissioned
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Agreement:
Every correct node chooses the same value If all the correct nodes have the same input, that input must be the value chosen
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Existing protocols are not scalable
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Number of nodes Transaction rate
Low High Small Large
xKnown static identity set xO(n2) messages x Constant TX rate x Re-parameterization is not a long term solution BFT Protocols Nakamoto Consensus
?
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Hybrid Consensus Idea
- Use PoW chain to elect a static committee
- honest nodes run the blockchain for csize + λ
blocks where csize = Θ(λ) denotes the targeted committee size.
- honest nodes would remove the trailing,
unstablized λ blocks from its local chain, and call the miners of the first csize blocks the BFT committee.
- Run BFT to agree on a new block
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Blocks
PoW block BFT block Σ: a set of signatures | Σ | ≥ csize/3 R-th committee
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h,nonce,TXs h’,Σ,TXs
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Step 1: Identity establishment Solve PoW
ID = H ( h, Nonce, txs) < D
The last csize confirmed blocks define members in a committee
Random seed for the PoW Difficulty
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Step 2: Propose BFT blocks
Run a classical Byzantine agreement protocol
- Members agree & sign on one valid BFT block
- Each committee serves for a period of time to
generate csize PoW blocks, e.g. 1 day.
- Broadcast the block to the network
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When each committee has at least C members? Each committee has O(λ)
O(λ 2) messages =>scalable
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Security guarantees
Due to the consistency property of PoW chain, all honest nodes agree on the same BFT committee. Due to the chain quality property of PoW chain, with appropriate overall parameters, we can ensure that more than 2/3 of the committee members are honest which is sufficient to ensure the security of the permissioned BFT protocol.
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Security guarantees
Due to the chain growth property of the pow chain, it will not take too long for the BFT committee to form
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T HANK YOU
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