Are quantum computers a danger to the Bitcoin? What is the possibility of an attack on the network?
Can quantum computers destroy the Bitcoin?
If we take a look at the developments of high technologies, some people might have the legitimate idea whether a DTL, in this case Bitcoin, is safe from quantum computer (QC) attacks. The answer is yes! If we understand how QC works and how exactly BC works!
The performance of quantum computers is usually given by the number of qubits. So far there are quantum processors with 2 up to about 52 Qubits (as of 9.2019). More would result in problems, since an error occurs on average every 100 microseconds. It takes 100 nanoseconds to perform an operation. This allows a qubit to perform between 100 and 1000 operations before making an error. Accordingly, the error susceptibility of quantum computers with many qubits is higher than that of devices with few qubits. The error rate therefore increases with the number of error sources.
A hash function is a one-way function, QC could theoretically guess this hash value with correspondingly many qbits (256 qbits da 256 bit SHA) to recalculate the block history, but it cannot manipulate all about 3 million nodes, most of which run over the Tor network.
The next consideration is to calculate back a publickey using a QC. Which is mathematically impossible, because Satoshi Nakamoto took precautions especially in waiser foresprosichts, by the timelock of the PKey. A publickey is defined as a doublehash function with a timelock that would force a QC to work at the speed of a binary system, destroying the supposed advantages of QC, the unimaginable speed and processing power.
The quantum computer, if it had 256qbits, could easily find the hash value of the new block, the only question is what would happen to the difficulty, it would rise to infinity, because in seconds it would have found more than 2016 blocks!
The next conceivable scenario would be a brute force attack in which all private keys would be calculated. This would require progress in QC and its storage technology, which is not even imaginable from today's point of view, not to mention the energy that would be needed, we are talking about energies beyond our galaxy!
There are no less than 2^256 ~ 1.1579*10^77 keys that would have to be stored to bit accuracy, but a QC cannot achieve such accuracy in storage! So what happens when the quantum computer receives the command to calculate all possible seeds 24!*2048 ~ 1.127*10^27 (Seedwords: 24 ; 2048 words from BIP39)?
A similar problem as before with the private keys will occur. The generated combinations of words would have to be read by a supercomputer to derive all possible publickeys and check for entries in the whole ledger (directory, blockchain, value memory)! How high the computational effort for this is exactly, with the most modern computers I still have to calculate.
If there are logical errors in this text or all others, I am looking forward to your feedback.
Which attacks are there nevertheless how secure is the system really?
Possible danger for the network is, for example, a worldwide power failure due to a strong solar flare. In this scenario, however, humanity has completely different problems than the need for a currency that can be put back into operation once the infrastructure has been restored (whereby the hardware in the affected area is destroyed). There are some underground mining facilities that are hedged against such a scenario for a limited time. If a miner has more than 50% of the hash power, it can determine what is written in the block and, for example, execute double entries or determine which transactions are approved and which are prevented.
Now there is also the difficulty that does not immediately adapt to the computing power in the peer to peer network, which will result in much more time to find the next block. Since the miner has no interest in losing public confidence in the BTC, he will have no malicious intentions except to mine profitably in the interest! Possible manipulations could be analyzed later, since the Bitcoin blockchain is public in particular.
Another possible attack would be the Erebus attack, in which the nodes are provided with false information over a period of up to 30 to 45 days, with the aim of partitioning the network. The Erebus attack can easily be prevented by the node communicating a .onion address over the Tor network. The Block Explorer can be used to check how many nodes (nodes / peers) are active in the network and whether they communicate via Tor.