3.1 ‚Blockchain‘: A public book where all Bitcoin-Transactions are recorded

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Note
Before you continue with the story, I recommend to take a couple of minutes and read through the prior article again in order to refresh your present knowledge. The more often you read through, the more confident you will get with the technology behind cryptocurrencies. Take your time, to understand cryptocurrencies, as a Steemian it's worth it.
If something in the story is not clear to you, don’t hesitate to ask. There are several people who might have the same question. So shoot me a comment, if something is unclear. Me and other readers will appreciate it!

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In 2.4 Bitcoin Mining we saw that Alice (Miner) had to secure the Bitcoin-Network in order to finally get the reward in Bitcoins from the mathematical contest. Therefore she had to propose pending Bitcoin-Transactions (Envelopes) which should be executed.

After she chose some pending Bitcoin-Transactions (Envelopes), the visitors came on board and checked if the chosen envelopes are legitimate.

The visitors (Full-Nodes) look out for two things.

1: Only full-gapless envelopes are allowed to be transacted. Regarding our story in „1. Digital Signatures“ a full-gapless envelope is being transacted by putting it in an other right-half-envelope.

2: No two exemplars of the same envelope are on the boat (Double Spend).

In the last post we used an example where Bob sent Bitcoins to Charlie. That means Bob sent a full envelope with Bitcoins in it, to the right-half-envelope (Public Key) of Charlie.

In this article I want to point out a potential risk for the security of the Bitcoin-Network.

Imagine Bob makes a copy of his full envelope before he send it on the way to Charlie. But unlike in the prior post, he sends only the first envelope to Charlie with the Alice-Boat and the copy he sends with the next boat. Remember there is a mathematical contest every 10 minutes. Hence there is a boat every ten minutes, in which transactions can be transmitted. Each winner of that mathematical contest (Minning) has to secure the next boat which transmit Bitcoin-Transactions.

In the last post, the visitors checked all envelopes (including the envelope from Bob to Charlie) on the Alice-Boat for correctness. They came to the conclusion that all envelopes on the boat are legitimate. So the boat could take off and the transactions were executed.

Later on, Bob takes the copy of his envelope and this time he sends it to David with the next boat. That means, he wants to spend his Bitcoins twice - once to Charlie with Alice’s boat before, and now to David with the upcoming boat.

Technically this copy of the envelope would be legitimate as it fulfills the two conditions we mentioned so far. Again, these conditions are:

1. it is a full gapless envelope and
2. there is no other exemplar of that envelope on that boat.

The Bitcoin-Software took care of that Double-Spending problem. The visitors who are checking the boat for these two conditions, actually brought a record-notes with them. This record-book keeps track of all the Bitcoin-Transactions (Envelopes) which were ever transmitted. That means all envelopes which took off with any prior boat are recorded in that record-notes.

In that way the visitors can check if any of the envelopes on board has ever been on a prior boat and therefore was already transacted.

Lets update the conditions the visitors are looking out for:

1. All envelopes on board are full envelopes
2. There are no two exemplars of the same envelope on the boat
3. None of the envelopes or any copies of it, have ever been on a prior boat

If any of these three conditions is not fulfilled, the visitors will not let the boat take off. In this case all transactions on board will not be executed.

So if Bob sends the copy of his envelope to David, the visitors will see in their record-notes that Bobs envelope has already been on a boat before (Bob's transaction to CHARLIE). This violates the 3rd condition and therefore the visitors will not let the boat to take off. All transactions on that boat (including the copy of Bob’s envelope to DAVID) will not be executed.

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Summary
The visitors (Full Nodes) are looking out for three conditions which have to be fulfilled by the envelopes on board:

1. All envelopes on board are full envelopes(=Digital Signature).
2. There are no two exemplars of the same envelope on board(Double-Spend).
3. None of the envelopes or any copies of it, have ever been on a prior boat(Double-Spend).

For the third condition they have a record-note(Blockchain) with them, where they see all transactions, that have been executed before. This ‚record-note‘ gives you a first impression what the Bitcoin-Blockchain is. It is a public record-note which stores all Bitcoin-Transactions which have ever been transmitted. We will dive deeper into that in the next articles.

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