Consensus: how thousands of strangers agree
If anyone can add to the ledger, who decides which version is real? That's the consensus problem - and there's more than one answer.
What's going on
When you send a transaction, you sign it with your private key and it gets broadcast to the whole network. Every node checks it: do you have the balance, is the signature valid, is the address real? Only then can a new block be formed and added. But which node gets to add it, and how does everyone agree? That's consensus.
Proof of Work, used by Bitcoin, makes nodes (miners) race to solve a hard math puzzle. The winner adds the block and earns a reward. It's secure but, as Neetu noted, it burns enormous energy because everyone competes at once. Proof of Stake replaced that race with a deposit: validators are chosen based on how much they've staked, which uses a fraction of the energy. Proof of Authority goes further - validators are known, reputable entities, and if they approve a bad block, their reputation is on the line.
The point isn't that one is "best." Each trades off energy, speed, and how decentralized it is. The right choice depends on what the network is for.
A public coin wants maximum decentralization and might pick Proof of Stake. A consortium of known banks tracking trade documents might pick Proof of Authority - they already know and can hold each other accountable, so they don't need to burn energy proving it.
Key terms
- Proof of Work (PoW)
- Miners compete to solve a puzzle; winner adds the block. Secure, energy-hungry. (Bitcoin)
- Proof of Stake (PoS)
- Validators are chosen by how much they've staked. Far less energy.
- Proof of Authority (PoA)
- Known, reputable validators; bad behavior costs them their reputation.