Quote from: gargavaar on December 24, 2017, 03:30:50 PM

So the general consensus is somewhere along the lines of "if quantum computing cracks Bitcoin, there will be bigger and more serious problems to worry about"?

Pretty close. Here are the facts:

1) Quantum computing (QC) is really hard. It's not just easy-in-theory-but-hard-in-practice, it's theoretically and practically hard. This is why there has been, to date, no definite demonstration of quantum speedup and this is why the quantum-skeptics in the thread are saying, "Quantum is a conspiracy, it's science made up by the government."

2) QC, when we do get it working, will not provide exponential speedup.

3) For certain kinds of problems, QC can provide quadratic speedup, which is a massive speedup. For symmetric ciphers, this probably just means you double your key size - where 128 bits of security used to be sufficient, now you need 256. No big deal. The real problem is with public-key encryption. But lay-people often forget that the quantum speedup blade cuts both ways. We can build encryption systems which take advantage of quantum speedup and make quantum cryptanalysis of PKE quadratically more difficult, mooting the theoretical advantage that cryptanalysts get from quantum speedup. In fact, this is why Bitcoin uses the public-key hash instead of the public-key itself and recommends against address-reuse; in the event of working, at-scale QC, your coins are still secured behind 128-bit-equivalent security as long as you don't reuse addresses or publish the public-keys for your addresses.

4) The most valuable uses of QC will not be for breaking encryption unless you're the military, in which case, you more or less don't care about civilian encrypted traffic. Even in the worst-case-conspiracy-scenario where the government has had quantum computers for decades, or whatever, it is highly unlikely that this immensely valuable equipment would be used to steal your $3,786 worth of Bitcoin. A civilian breakthrough in QC will result in a flurry of cryptographic updates to bring popular public-key encryption systems up-to-date. But even if a working QC with, say, 128 qubits were announced tomorrow, the initial applications of this QC would go to sciences like aerodynamic modeling (auto + aircraft fuel efficiency), traffic modeling (metropolitan commute + traffic efficiency), financial modeling (stock price predictions), medical research (drug development + protein-folding + cell modeling), and so on. Breaking HTTPS would be very far down on the list of priorities of anyone with enough disposable cash on hand to actually purchase and operate QC hardware. And we know that QC will be expensive because of point (1).