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# System prepended metadata

title: Ethereum Consensus Specification

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# Ethereum Consensus Layer

## The Beacon Chain
The **Beacon Chain** is the coordination layer of Ethereum’s **Proof of Stake (PoS)** system. It manages:
- **Validators**
- **Staking**
- **Finalization of blocks**

Unlike **Proof of Work (PoW)**, where miners compete to produce blocks, the Beacon Chain introduces **validators** who participate in block production and finalization.

Ethereum’s **Proof of Stake (PoS)** consensus is based on the **Ethereum Consensus Specification**, which defines the rules that validators follow to **propose, attest, and finalize blocks**.

The two key components of Ethereum’s consensus mechanism are:
- **Casper FFG** (Finality Gadget)
- **LMD-GHOST** (Fork-Choice Rule)

## LMD-GHOST: The Fork Choice Rule
- LMD-GHOST stands for **Latest Message Driven, Greedy Heaviest Observed Subtree**.
- It is the **fork-choice rule** used to determine the head of the chain.

### How LMD-GHOST Works:
- The block with the **most validator attestations** is selected as the next head of the chain.
- Instead of selecting the **longest chain** (as in PoW), nodes choose the chain with the **most recent votes** (attestations) from validators.
- Validators cast **votes (attestations)** for blocks, and the fork choice rule follows the branch with the **heaviest accumulated weight**.

## Casper FFG: The Finality Mechanism
Casper FFG ensures that **decisions made on the blockchain are final** and cannot be changed. It works by having **checkpoints** that are confirmed by validators. 

Once a **checkpoint is finalized**, it cannot be reverted without **slashing validators**.

## Epochs and Checkpoints
Ethereum operates in **epochs**, where:
- **1 epoch = 32 slots**
- **1 slot = 12 seconds**
- **1 epoch ≈ 6.4 minutes**

### Checkpoint Process:
1. The **first block** in an epoch is selected as a **checkpoint**.
2. Validators vote to **justify** a checkpoint.
3. If a checkpoint receives **2/3 of the total stake**, it becomes **justified**.
4. If a **justified checkpoint** is confirmed in the next epoch, it becomes **finalized**.

## Block Proposal
- A **validator** is randomly selected via **RANDAO and VRF** to propose a block **every 12 seconds**.
- **VRF (Verifiable Random Function)** ensures that the randomness is **cryptographically secure**.

## Attestation Process
Validators participate in **each slot** by attesting to blocks based on:
- **Source**: The last **justified checkpoint**.
- **Target**: The **current checkpoint**.
- **Head**: The **best block** according to LMD-GHOST.

## Summary Table

| **Component**   | **Role** |
|---------------|---------|
| **LMD-GHOST** | Determines the canonical chain by selecting the block with the most validator votes. |
| **Casper FFG** | Ensures economic finality by justifying and finalizing checkpoints. |
| **Validators** | Propose blocks, attest to checkpoints, and follow slashing rules. |
| **Epochs & Slots** | Blocks are produced every **12 seconds** and reach finality in **6 minutes**. |
| **Finality Rule** | A block is **finalized** when it is part of a **checkpoint** that is **justified** and confirmed in the next epoch. |


