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.