# <center><i class="fa fa-edit"></i> 1.5 A Simple Cryptocurrency </center>
###### tags: `Blockchain`
The following notes are taken from [Coursera](https://www.coursera.org/learn/cryptocurrency/home/week/2)

---

### Goofycoin
:::danger
**Problem: Double-Spending Attack**

:::
:::success
**Good cryptocurrency solves double-spending attack**
:::
### Scroogecoin
- Publish a history of all transactions (blockchain signed by Scrooge)
- Optimization: put multiple transactions in the same block


Valid because Scrooge said so


Valid if:
- Consumed coins valid
- Not already consumed
- Total value out = total value in
- Signed by owners of all consumed coins

Coins are immutable
- Immutable: can't be transferred, subdivided, or combined
Coins use transactions for immutable purposes
Problem: centralization (i.e. misbehaving Scrooge)