# <center><i class="fa fa-edit"></i> 2.3 Consensus Without Identity: The Blockchain </center>
###### tags: `Blockchain`
The following notes are taken from [Coursera](https://www.coursera.org/learn/cryptocurrency/home/week/2)

---
- Identiy is hard in a P2P system (Sybil Attack)
- Goal of Bitcoin: pseudonymity

:::warning
Weaker Assumption: select random node
:::

### Key Idea: Implicit Consensus
- In each round, a random node is picked
- Node proposes next block in the chain
- Other nodes implicitly accept or reject this block by:
1. Extending it *OR*
2. Ignoring it and extending chain from earlier block

**Every block contains hash of the block it extends**

### Consensus Algorithm (Simplified)
1. New transactions broadcasted to all nodes
2. Each node collects new transactions into a block
3. In each round a *random* node gets to broadcast its block
4. Other nodes accept the block only if all transactions in it are valid (unspent, valid signatures)
5. Nodes express their acceptance of a block by including its hash in the next block they create

Double-spend probability *decreases exponentially* with the number of confirmations
- Most common heuristic: 6 confirmations