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# System prepended metadata

title: <center><i class="fa fa-edit"></i> 1.5 A Simple Cryptocurrency </center>
tags: [Blockchain]

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# <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)

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### Goofycoin
![](https://i.imgur.com/ELGicP2.jpg)
:::danger
**Problem: Double-Spending Attack**

![](https://i.imgur.com/VwvxVkJ.jpg)
:::
:::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

![](https://i.imgur.com/ZJ83AWQ.png)

Valid because Scrooge said so

![](https://i.imgur.com/oxZF5oy.jpg)

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)