# Popular Blockchains Consensus Mechanism

## Proof of Work (PoW)

Used in: Bitcoin, Ethereum (before the Merge), Litecoin

### How it works:
Miners compete to solve a difficult cryptographic puzzle (finding a nonce). The first to solve it earns the right to propose the next block and receives a block reward. Other nodes verify the block and append it to the chain if valid.

### Pros:
- Battle-tested (used by Bitcoin since 2009)
- Very secure and decentralized

### Cons:
- High energy consumption
- Low throughput (slow block times)

## Proof of Stake (PoS)

Used in: Ethereum 2.0, Cardano, Solana (variant), Polkadot (variant)

### How it works:

Validators are chosen to propose and attest to blocks based on the amount of stake (cryptocurrency) they lock up.
Misbehaving validators can be slashed (lose stake).
Blocks are finalized through votes (attestations).

### Pros:
- Energy efficient (no mining)
- Economically secure via slashing
- Fast finality with designs like Ethereum’s Casper FFG

### Cons:
- "Rich get richer" dynamic
- Complex validator management

## Delegated Proof of Stake (DPoS)

Used in: EOS, TRON, Steem

### How it works:

Token holders vote for a small set of delegates (block producers).
These delegates are responsible for validating and producing blocks.
Delegates can be voted out if they misbehave.

### Pros:
- High throughput (fast block times)
- Governance baked in (voting for reps)
- Lower resource requirements for participation

### Cons:
- More centralized (small number of block producers)
- Prone to collusion and vote-buying

## Proof of Authority (PoA)
Used in: VeChain, private Ethereum chains (e.g., Microsoft Azure), BNB Smart Chain (variant)

### How it works:

A set of pre-approved authorities (validators) are trusted to produce blocks.
Their identities are known and tied to real-world organizations or individuals.

### Pros:
- Extremely fast and efficient
- Good for private/enterprise chains
- No mining or staking required

### Cons:
- Highly centralized — trust required in validators
- Not censorship-resistant

## Practical Byzantine Fault Tolerance (PBFT)
Used in: Hyperledger Fabric, Zilliqa (shard layer), Cosmos (Tendermint)

### How it works:
Validators go through multiple rounds of voting to reach consensus (pre-prepare, prepare, commit).
Can tolerate up to 1/3 of malicious nodes (Byzantine faults).
Deterministic finality — once a block is agreed upon, it’s final.

### Pros:
- Finality is instant (no need to wait for confirmations)
- Strong security against malicious actors

### Cons:
- Communication overhead grows with more validators
- Less scalable for large public networks

## Casper FFG (Finality Gadget)

Used in: Ethereum 2.0 (Beacon Chain)

### How it works:
Casper FFG is a hybrid consensus protocol built on top of a block proposal mechanism (originally PoW, now PoS).
It introduces finality through validator votes (attestations).

### Process:
- Validators propose blocks in each slot.
- Other validators attest to a block as justified and prepare it for finalization.
- If a block and its direct child are both justified and receive 2/3 votes, the parent block is finalized.
- Slashing penalties are applied if validators vote inconsistently (equivocation).

### Fault Tolerance:
- Tolerates up to 1/3 of validators being Byzantine.
- If 2/3 majority agrees, the block is finalized and cannot be reverted.

### Pros:
- Fast and deterministic finality — blocks are irreversibly finalized with 2/3 consensus.
- Energy efficient — operates on PoS.
- Slashing ensures accountability, deterring malicious validators.
- Enables a clear separation between block proposal and finality voting.

### Cons:
- Requires consistent participation from 2/3 of validators to maintain liveness.
- Finality can be delayed during validator inactivity or network partitions.
- Relies on accurate time synchronization and validator responsiveness.
- Additional complexity added to block validation and fork choice rules.