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      <h2>A digital currency</h2>

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<small><i>From Wikipedia, the free encyclopedia</i></small><br>

Bitcoin is a cryptocurrency and a digital payment system invented by an unknown programmer, or a group of programmers, under the name Satoshi Nakamoto.It was released as open-source software in 2009.<br>

The system is peer-to-peer, and transactions take place between users directly, without an intermediary.These transactions are verified by network nodes and recorded in a public distributed ledger called a blockchain. Since the system works without a central repository or single administrator, bitcoin is called the first decentralized digital currency.<br>

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      <h2>WALLET</h2>

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      You can install a software on your computer or your phone to manage your bitcoins.<br>

      As Bitcoin is open source, there is a lot of such software avalaible.<br>

      It's called a wallet.<br>

      Those softwares are all talking with the same protocol. They are connected to the same network.<br>

      But, in fact, your true wallet is your 2 crypt keys : the public and the private one.<br>

      Everybody who knows those 2 values can spend your bitcoins.<br>

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      <h2>Transaction</h2>

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When Bob is sending 42 <span class="glyphicon glyphicon-bitcoin"></span> to Alice, the bitcoin protocol is creating a transaction.<br> 

A transaction is the balance between his Inputs and his Outputs.

<ul>

  <li>First the network needs to be sure that Bob has got enough <span class="glyphicon glyphicon-bitcoin"></span> in his wallet.<br>So Bob will include a list of transactions that he has received before.<br>This is the inputs of the transactions.<br>For example, previous transaction's amount are 10, 25 and another 10, so total inputs are 45 <span class="glyphicon glyphicon-bitcoin"></span></li>

   <li>As the input's sum is superior to the amount of the transaction, an ouput is added.<br>This output is the change to Bob.<br>In the example, this 3 <span class="glyphicon glyphicon-bitcoin"></span></li>

  <li>Fees are took by the network to process the transaction.<br>Let say it will take 0.5<span class="glyphicon glyphicon-bitcoin"></span>.<br>This is another ouput</li>

  <li>So Alice will get only a part of the transaction's amount.<br>this the last output, 41.5<span class="glyphicon glyphicon-bitcoin"></span></li>

</ul>

So the transaction is a made with :

<ul>

  <li>Inputs : a list of previous transactions that bob has received</li>

  <li>Outputs : a list of amounts to send to Alice, Bob, and fees</li>

  <li>A timestamp</li>

</ul>

Then the wallet will broadcast the transaction on the network.<br>

It will be sent to the area called "mempool".<br>

Nodes of the network will validate the transaction by verfying the Inputs and the Ouputs.<br>

A hash of the transaction is computed.<br>

So the transaction is sealed.

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      <h2>Block</h2>

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Then the bitcoin protocol is grouping validated transactions in blocks.<br>

So a block is an array of transactions.<br>

A block is made with :

<ul>

  <li>An array of transactions.<br>

That's what my script is drawing.<br>

Running the transactions array, it is computing the sum of each outputs of each transaction.<br>

Then, it draws a rectangle for each sum.<br>

  </li>

  <li>A "proof of work".<br>The miner is computing a special hash that depends on the array of transaction and that will answer to a constraint.<br>This hash is very difficult to compute. In fact it is not really computed, the miner must test every value until he find the solution.<br>

That why the network will give him a reward.<br>This reward is a special transaction that has no inputs.<br>

As this reward is in bitcoin, this is also a mean to control the money stock.<br>

The more you use the money, the more there is blocks, the more there is money.<br>

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      <h2>Blockchain</h2>

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Each block has a hash and a reference to the ihash of the previous block.<br>

So blocks are chained together.<br>

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