How to avoid double spending ?

Problem

Order problem

We cannot depend on time as it is easily

falsifiable.

Double spend fraud

Invalidated by some nodes.

How to agree on an order ?

Blockchain.

Transaction chain

Block

Answer to a specific problem

First person to find a solution will broadcast the block to the

network

Several possible branches

You need cpu power to have your block broadcasted

You always switched to the longuest branch.

Half of nodes must be honnest.

I have constructed this presentation based on :· Bitcoin under the hood

· Mastering Bitcoin· Original paper

· Coursera

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