The Third Web #13 – Consensus Primer with Aparna Krishnan

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Aparna Krishnan is head of education at Blockchain at Berkeley and co-founder of Mechanism Labs, an open source blockchain research lab. Earlier this year, Aparna was awarded a scholarship by the DFINITY Foundation for Mechanism Labs’ research into consensus mechanisms. This episode is essentially a primer for advanced discussion of consensus in decentralized networks.

https://mechanismlabs.io/
https://github.com/aparnakr

https://medium.com/@aparnalocked

Aparna Krishnan
Co-founder mechanism labs open source research lab
All work and research is on Github
Telegram: @mechanism_labs
Co-founder of the education team at Blockchain Berkeley
Consensus researcher
Teaches executive education courses

Consensus
Proof of Work
Proof of Stake
Old Field

History
Cynthia Dwork developed stronger adversarial models
Did not have many applications
Blockchain has brought cryptography and consensus into the mainstream eye

Protocols
DFINITY
Tendermint
Bitcoin
Focus has been on proof of stake protocols
Mining may not be sustainable
Long term sustainabilities and lack of externalities is important
Proof of stake offers efficiently
As does proof of elapsed time and proof of space and time

Consensus
Sybil control
Coming to agreement relies on traditional consensus
PoS, PoW refer to the sybil control mechanism
PoS – Putting down capital
Financial penalty for misbehavior
Token holders are participants
PoW – burning energy
One cpu, one vote
No connection between token custody and rewards
P o Elapsed Time
Proof of Authority placing reputation
A cost of playing ball
Traditional Consensus (PBFT)
No concept of probabilistic finality
All honest nodes come to final agreement
Closed, permissioned
Blockchain
All nodes may with a high degree of probability agree
A probabilistic guarantee

Longest Chain Rule
Longest sequence of blocks is the
Ethereum
Ghost
Can be attacked by a stealthy entity
Open, permissionless, decentralized
Inefficient
Node churn
Better liveness properties

Early PoS 2013
PeerCoin
NXT
Bitshares
Primitive
State grinding attack vulnerable
Randomness derived from blockhashes

New Generation
DFINITY
Uses threshold relay decentralized randomness
Tendermint
Round robin
Has social layer fallback
Ethereum Casper
Thunderella
Algorand
Hides the block creator until the block has been created

In Tendermint a minority below the assumption cannot break finality
In Bitcoin >10% can change the finality through threatening to censor

Hard to get both economic and cryptographic security. Not clear that bribing and collusion attacks have been solved. No protocols have shown sophisticated economic models.

Mechanism Labs
Focussing on incentive schemes
What does it mean to have a stable, scalable protocol
Scalability

Proof of Replication
Recent work with Verifiable delay functions
In commit/reveal schemes there is an opportunity to manipulate by giving only one person the ability to reveal the randomness
Verifiable delay functions allow anyone to reveal the randomness
ASIC resistance because not parallelizable

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