What is the level of security of the data recorded on Bitcoin’s blockchain?
First of all, it must be said that, from a technical point of view, Bitcoin’s blockchain is nothing more than a public file, about 376 GB in size, of which there are tens of thousands of identical copies scattered around the world.
It is therefore not a single file held by a Bitcoin Fortune single manager who has the power to alter it or who can be hacked with a single attack, but it is tens of thousands of identical copies that are effectively unhackable.
In other words, it is impossible for someone to hack all the machines on which a copy is stored, in order to alter all of them.
To be honest, it might be enough to modify most of them to alter their content, but even that seems implausible.
Also because, if someone were able to alter many of them, there would still be a few unaltered ones left to use for comparisons, since some of the machines on which a copy is stored are practically unhackable.
This is very important because the protocol with which this file is built requires that anyone can check its correctness, given that the file is public and not encrypted.
If someone managed to alter a few copies of the Bitcoin blockchain, the alteration would immediately come to light, and the existence of a few unaltered copies would make it easy to compare them and verify without a shadow of a doubt which is the altered one and which is the original.
In reality, it would also be possible to make an altered copy of the blockchain without it violating the protocol, but it would be so expensive that very few could afford to do so.
Also, the older the blocks, the harder it would be to reconstruct them to conform to the protocol, so at most realistically one could create a blockchain where only the last few blocks are altered.
As can be seen from this reasoning, attempting to alter the Bitcoin blockchain is not worthwhile because it would be extremely difficult and expensive to do so, and it would be virtually impossible for anyone to notice.
A different matter, however, applies to new blocks, i.e. those that are added to the blockchain every 10 minutes or so. In this case, it would be possible to launch a 51% attack, to force the validator nodes to add new, altered blocks.
However, even in this case the cost would be considerable, and it would be practically impossible to do this without anyone noticing. So it is simply not worth doing.
In light of this, it is possible to say that it is virtually impossible to change the data once it has been recorded on the Bitcoin blockchain, where it therefore enjoys a very high degree of security.
It is still public data that can be seen in the clear by everyone, but if it is a cryptographic hash for example, even if it is read by anyone, it does not cause problems.