Just a small overview over what can and cannot be done (personal view only, open for debate)

1. If one has a working fault tolerant quantum computer RSA and ECC are done. ECC prior to RSA because of the key length. I could imagine that the first target would not be personalized attacks on your bank account but crypto currencies.
2. If someone claims to have a quantum secure crypto currency look at the underlying algorithm.
3. Real proof of concept is difficult because one needs access to a quantum computer and that access is not given to everyone. So if I would find some problems in some claimed quantum secure crypto currency I could write up a proof on paper why the lib sucks/the algo sucks but I could not prove it because I don’t have a quantum computer at hand. Any idea who to address in that case? Give a talk at a hacker con? Put it on Twitter? E-Mail the responsible person?
4. Quantum effects can generate nice random numbers. That is so far the main application I know of.
5. No, RSA is not broken (yet).
6. AES remains secure under current assumptions. Just double the key length and you are fine 🙂
7. Classical crypto thought to be quantum safe is in the making (NIST standardization with public mailing list)
8. Some of these algos are already dead or vulnerable to mathematical backdoors. The latter ones should be rather difficult to spot.
9. Using entanglement for key exchange gives a protocol that is theoretical secure. Has physical backdoors in some cases (laser leacking photons etc)
10. Multi-party protocols for quantum networks are in the making. Theoretically proven secure but hard to put into practize. Generating the entangled states for that is a real engineering problem.
11. Quantum protocols are mathematically proven secure as long as the foundations of quantum physics hold. Again these are an experimental theory – but the best tested physical theory to this date. Normal crypto does not come with mathematical provable security unless we can prove factoring not to be in P. The argument rather goes like “in the last 30 years no one discovered an efficient algo to break this” which is okayish but rather lame given that some mathematical problems are open for centuries.
12. If you want to exchange quantum information (qubits) between quantum computers you need quantum networks.
13. If you are a local SME or mom’s and pop’s shop quantum attacks should be the least of your worries. Better make sure you update your OS, don’t use 123456 as password etc.
14. If you are the next Snowden you might want to consider a PQC algo together with AES. Or just the plain old One Time Pad.
15. Many documentaries about quantum computers and even articles in respected media outlets overexagerate the capabilities of QC. In the end it all boils down to some arrows and how to turn them in space. The notation is a bit weird but in the end superposition is nothing else than the linear combination of vectors you know from high school.