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Why has nobody been able to hack bitcoin?
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In fact, somebody has!

Bitcoin Implementation Vulnerabilities

https://en.bitcoin.it/wiki/Value_overflow_incident relates an integer overflow attack on the Bitcoin implementations, not the protocol per se, which permitted the attacker to create 184 billion  BTC.

This change was quickly noticed, and the blockchain forked with new software that rejected this transaction (and any later blocks.)  While the hack was successful, the attacker did not try to sell their bitcoin, and the community was quickly able to repair the damage.

This was not the only software vulnerability discovered.   https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures has a long list of CVE's discovered in the bitcoind software which would permit denial-of-service attacks, double-spending, or even outright theft.

Bitcoin software has been thoroughly hacked.

Bitcoin Operational Vulnerabilities

In order to steal BTC, nobody needs to hack Bitcoin or the Bitcoin protocol.  Wallet implementations, currency exchanges, and people's desktops provide a much better cost/benefit tradeoff.  If you find a bug in the core Bitcoin protocol, it'll get patched and you probably won't benefit. If you steal Bitcoin from somebody's wallet, the community is likely to respond "SFYL."  Even large exchanges have suffered thefts due to computer intrusion.

Bitcoin has been stolen, many times, via hacking.

Bitcoin Design Vulnerabilities

So why is the Bitcoin protocol still considered secure? In part , it's just experience: a lot of people have tried to break it and failed. A key part of the engineering design was the use of existing cryptographic primitives instead of attempting to create new ones just for Bitcoin.  (Unlike some other blockchain projects I could mention.) These primitives have already been heavily analyzed and don't provide any known vulnerabilities.  The inventor of Bitcoin put them together in a way that is reasonable easy to understand and did not introduce any known vulnerabilities.  So the protocol has stood up to attack because it was built out of good parts.

However, even at this level there have been surprises. It was thought that miners would get rewards in proportion to their hashing power, but work on "selfish mining" showed that it was possible for a consortium of miners, even with less than 51% of the hashing power, to obtain disproportionate rewards.  (I will note that this result is controversial, but most of the criticism is about whether it is economically worthwhile to engage in this attack, rather than whether it is possible.)

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This is due to the blockchain technology on which the bitcoin is based. Let's analyze three of its main sources of security: the digital signature, the inputs, and the calculation of the hash of a block.

  Why technology is considered inviolable: cryptography at the service of the Blockchain 

Blockchain technology meets the four main criteria generally used in security: 

  •  Availability, provided by the distributed character of the Blockchain (all network participants have the same updated version) 
  •  Data integrity, provided by cryptographic mechanisms 
  • Traceability, ensured by the fact that the Blockchain is an open register and that anyone can verify that an action has taken place or that a data is present
  •  Confidentiality ensured by possible encryption of the data contained in the Blockchain 
  1.  The digital signature 

 Any  transaction (financial or otherwise) issued in the network of a  Blockchain is considered official only after passing a validation phase:  mining. This mining, carried out by a minor, ensures a high level of security thanks to three mechanisms:

  • The digital signature
  • Inputs
  • The calculation of the block hash

Let's start by approaching the notion of digital signature that is essential to fully understand how Blockchain works. It  allows the network to ensure that a transaction or information issued  by one of the participants is authentic, that is, the participant is the  one he claims to be and the content of his message. has  not been tampered with during transmission through the network  (remember that we are in a peer-to-peer network and information flows  from node to node).

Let's start by answering a simple question: what is a digital signature?

A  traditional handwritten signature would pose a major problem if it were  used in the digital world: it would be very simple to copy and reuse it  for fraudulent purposes. To avoid this problem, a digital signature is a sequence of numbers and letters. 

 A  digital signature depends mainly on two elements: the message on the one  hand (above the messages are financial transactions, but the message  could just as well be a text) and on the other hand a private key (that  one can see as a password) that owns the one who issues the transaction. The digital signature can be seen as the result of a function:

signature = f (message, private key)

The  private key is always the same but the message is always different:  that's why a digital signature is unique and corresponds to a given  transaction. Thus, it can not be copied and used to sign another transaction because the nodes would quickly realize that it does not match. 

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In the physical world it is the equivalent of asking you to take a one brick from the middle of a building that has been complete.   Could you do it? Nope . You would have to dismantle  the building piece by piece. in order to get that one brick. Someone would cop that there building is being dismantled because they are living there. and immediately have you arrested. Even with that you would need powerful tools. Anyway it is next to impossible.

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The way in which the bitcoin market works makes it "imposible", that is, it is possible but in a certain way it is absurd. The blockchain system works because of all the transactions in the world are recorded on all the computers of the world and for to be able to hack a transaction or make money appear from where it was not, you will have to hack every computer that has records, which in theory is possible, but that, being reasonable, is almost impossible and if we add the fact that the value of the bitcoin is given by the trust that people place in it, and if an exaggerated masive hack happen people would lose all the confidence in the bitcoin or any othe cryptocurrency and it wouldn't has value, so, whatever be the point of view where you focus it, that will no happen.

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