Blockchain security relies on consensus mechanisms for transaction validity, as discussed in my article at https://vulehuan.com/en/blog/2023/5/exploring-blockchain-security-the-role-of-cryptography-and-consensus-algorithms-in-preventing-hacking-and-fraud-elY8lGXDJX1. The two most used are Proof of Work (PoW) and Proof of Stake (PoS). This post examines each mechanism's strengths and weaknesses and why Ethereum 2.0 chose PoS.

Proof of Work

Bitcoin introduced PoW; several other cryptocurrencies adopted it. Miners solve intricate math problems to validate transactions and append new blocks. The first solver earns newly generated coins and transaction fees.

Like gold miners extracting gold from ore through hard rock and refining, PoW miners face technical difficulty. The successful gold miner is paid by the gold obtained.

Advantages of PoW

Advantages of PoW:

  • High security: the most secure mechanism in use, with robustness and attack resilience from high computational power.
  • Decentralization: many miners, so no single entity easily controls or manipulates the network.
  • Fairness: many can join and get a fair chance to validate and earn. More computing power means a higher chance to solve and add a block.
  • Proven track record: widely used and time-tested, dependable and trustworthy.
  • Sybil attack resistance: fake identities need significant computational power to validate and add blocks.

Disadvantages of PoW

  • PoW has disadvantages. Bitcoin is only one of many PoW cryptocurrencies.
  • High energy consumption: large computational power means high energy use and environmental concerns. CoinShares estimates average Bitcoin mining electricity at $0.05 per kilowatt-hour https://www.nytimes.com/interactive/2021/09/03/climate/bitcoin-carbon-footprint-electricity.html
  • Centralization of mining: rising difficulty makes small miners struggle against large pools, centralizing mining power.
  • Vulnerability to 51% attacks: one entity or group with over 50% of computing power can manipulate transactions. Cointelegraph reported Bitcoin Gold suffered a 51% attack in May 2018, overspending around $18 million worth of BTG; Bittrex removed the coin from its listing.
  • Slow processing: high computational requirements slow transactions, especially at peak congestion.
  • Inefficient resource use: energy-intensive PoW wastes resources. A Cambridge University study estimates Bitcoin's annual energy consumption exceeds Argentina's https://www.bbc.com/news/technology-56012952
  • Environmental impact: mining energy contributes to greenhouse gases and climate change. Rising demand may strain non-renewable sources https://www.jbs.cam.ac.uk/insight/2022/a-deep-dive-into-bitcoins-environmental-impact/

Proof of Stake

PoS addresses PoW limitations. Validators hold a specific amount of cryptocurrency in a "staking" wallet. They are selected to validate transactions and add blocks based on coins held.

Real-life parallels:

  • Shareholder voting: voting power is proportional to shares. More shares means more stake and influence.
  • Rental deposits: a security deposit covers damage or unpaid rent. It is often proportional to monthly rent, like a validator's stake to coins held.
  • Online marketplace reputation: eBay or Amazon seller reputation is based on sales and reviews, acting as collateral for trust and quality.
  • Social media reputation: Twitter or Instagram reputation is based on posts and engagement, acting as collateral for trust and value.

Popular PoS cryptocurrencies and platforms:

  • Ethereum (ETH): second-largest by market capitalization; Ethereum 2.0 is transitioning from PoW to PoS.
  • Cardano (ADA): decentralized PoS platform, designed as a more secure and sustainable alternative to PoW.
  • Polkadot (DOT): multi-chain PoS platform enabling interoperability between blockchain networks.
  • Binance Coin (BNB): created by Binance; uses PoS for various functions on the platform.
  • Cosmos (ATOM): decentralized network of independent PoS blockchains allowing interoperability.
  • Tezos (XTZ): PoS platform aiming to be a more secure and efficient alternative to PoW.
  • Avalanche (AVAX): decentralized dApp platform using PoS, processing thousands of transactions per second.

Advantages of PoS

Advantages of PoS:

  • Energy efficiency: PoS uses far less energy than PoW, so it is greener and cheaper. Digiconomist's Bitcoin Energy Consumption Index puts Bitcoin's PoW network at around 129.09 TWh per year as of March 2023. Cardano's PoS network is estimated at around 0.6 TWh per year, far less. Ethereum's PoW-to-PoS transition is expected to cut its energy use significantly. PoS is much more efficient.
  • Reduced centralization: validators are chosen by coins held, not computing power.
  • Increased participation: users earn rewards by holding and staking cryptocurrency.
  • Stability: consensus rests on stakeholder agreement; miners need not compete, reducing forks.
  • Governance: validators influence decisions by coins held and staked, enabling decentralized governance.
  • Scalability: PoS processes more transactions per second. According to blocktivity.info, EOS consistently ranks among the top blockchains for volume and speed, often handling more transactions per second than Bitcoin and Ethereum, both PoW.

Disadvantages of PoS

Disadvantages of PoS:

  • Centralization risk: larger validators can dominate. A staking wallet requires enough funds, potentially excluding smaller participants.
  • Initial distribution: if few people hold a large share of total supply, the network is more vulnerable to attacks.
  • Nothing at stake: validators can validate multiple conflicting versions because they need no significant computational resources, unlike PoW miners doing complex calculations. This can hinder consensus and weaken security.
    • Analogy: an election where voters can vote for multiple candidates, the winner gets the most votes, there is no penalty for voting for more than one, and voters are incentivized to vote for all. Voters lose nothing.
    • If a validator sees two blockchain versions with different histories, normally it would validate only the version with the most work (longest chain). In PoS, it could validate both without extra computational resources, splitting the blockchain into branches with different histories.
  • Difficulty upgrading: PoW can change its mining algorithm; PoS changes need a hard fork, which can be contentious and split the community.

Long-range attacks

Both PoW and PoS can face long-range attacks, but PoS may be more susceptible. An attacker reconstructs the chain from an earlier point and creates a longer alternative chain.

PoW needs significant computational power to outpace the existing chain and create a longer one. In PoS, an attacker with a large stake could rebuild from a much earlier point, create a longer chain, and gain control of the network.

Examples:

  • Replay attacks: an attacker intercepts legitimate transactions and retransmits them on another network, so the recipient gets the same cryptocurrency twice. This can happen during a hard fork when two chains share transaction history, allowing a double-spend on both. Developers can add unique transaction identifiers or use replay protection. Example: Alice sends one bitcoin to Bob; an attacker replays it on another blockchain. If Bob accepts, he receives one bitcoin from both Alice and the attacker, and the attacker can withdraw the original bitcoin before the network detects the double-spend.
  • History revision: an attacker forks from an earlier point and builds a longer chain, rewriting history.
  • Timejacking: an attacker manipulates transaction timestamps to fork history.
  • Checkpoint attacks: a central authority creates checkpoints to stop long-range attacks, but an attacker can still rewrite history before the first checkpoint.

Prevention methods: checkpoints hinder attacks but add centralization, contradicting decentralization. Finality gadgets ensure block validity and finality, making history rewrites harder. DPoS lets coin holders delegate voting power to trusted validators to create blocks, reducing long-range attacks. Cryptographic techniques such as unique transaction identifiers or replay protection prevent transaction replay across blockchains or networks. Each method has trade-offs; developers must weigh them for their protocol.

Why Ethereum 2.0 chose PoS?

Ethereum, the second-largest cryptocurrency, is upgrading to Ethereum 2.0, transitioning from PoW to PoS. The switch was implemented in 2022, offering improved security, lower energy consumption, and better support for new scaling solutions than PoW. For more information, visit https://ethereum.org/en/developers/docs/consensus-mechanisms/pos.

PoW and PoS both ensure transaction validity and network security. PoW is considered the most secure due to attack resilience, but has high energy consumption and centralized mining power. PoS addresses these limits with a more energy-efficient, eco-friendly solution. Ethereum 2.0 chose PoS for improved scalability, decentralization, and lower energy consumption. PoS also faces staking power centralization and the need for high network participation for security. Choosing between PoW and PoS depends on a network's specific needs and priorities.