Efficiency and Security for Committee-Based Consensus Protocols [May 2022 Scholar]

In committee-based consensus protocols like delegated proof-of-stake (DPoS), a small elected committee validates blocks. Can this help scale blockchains while keeping them secure and decentralised?

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Executive Summary

• We introduce the key points of “Scaling Blockchains: Can Elected Committees Help?” (referred to as the ‘paper’ in this report) by Alon Benhaim, Brett Hemenway Falk, and Gerry Tsoukalas.

• In ‘committee-based consensus’, voters delegate to a small committee the rights to produce and certify blocks.
  • Often associated with Delegated Proof of Stake (DPoS) systems, stake-weighted voters elect a small group of block producers as a committee responsible for producing and validating blocks.
  • Most DPoS systems adopt approval voting, where each stake-holding voter “approves” preferred candidates to join the committee. 

• The paper lays out a model where a blockchain’s token holders vote to elect a committee of block producers according to an up-to-t, k-winner approval voting system; t is the maximum number of candidates each voter can vote for, and k indicates the candidates with the highest scores.
  • The goal of the model is to seek an optimal strategy to maximise the probability of electing an honest committee.
  • Data collected from EOS is used as empirical observations to analyse the real-world voting strategies.
  • There are two particular voting strategies illustrated: threshold voting and cardinal voting. 

• In the model:
  • The authors derive the formula to calculate the objective function of the optimisation, which is the probability that an elected committee is honest.
  • The voters’ behaviour dramatically depends on the low vs. high number of voters.

• The probability of success rapidly and exponentially converges to 100% when the signals are deemed not entirely uninformative.

• The minimum committee size required to achieve a failure probability is much smaller for the DPoS-like system compared with the randomly chosen committee (as in Algorand).

• The paper also illustrates the problems of approval voting:
  • One drawback of electing committees, compared to selecting random committees, is that elections seem to lead to stagnation.
  • A small, static set of block producers reduces decentralisation — the core tenet of blockchains and cryptocurrencies.

Read the full scholar article here: A Discussion on Efficiency and Security for Committee-Based Consensus Protocols