OK. I'll explain. SHA-256 is used for hashing. Of coz it's used in a variety of applications. But if someone get a quantum computer and manage to falsify a digitally signed contract then only authentic owner of the contract will be harmed. If someone manage to falsify an SSL certificate then only visitors of the site will be harmed. But if someone manage to find block nonces every second, then everyone who uses bitcoins will be in troubles.
Quantum computers aren't a magic bullet. Yes using Shor's algorithm the search speed can be increased exponentially however at what cost? For example say once ASICs become mainstream the cost to attack/defend the network using ASICs is $20,000 per TH. Now say a quantum computer which could implement shor's algorithm on 256bit numbers could be built for $50,000 per TH equivelent. Who cares? An attacker is going to take the more economical option.
So quantum computer is only a threat if all 5 elements are true
a) it is possible to build a quantum computer which can implement shor's algorithm on 256bit numbers
b) it is possible to build a quantum computer large enough to 51% attack the network
c) it is possible to build a quantum computer that makes such attack more economical than ASIC based brute force
d) quantum technology can be restricted so that a computer meeting requirements a,b, c isn't available to "defenders"
e) Bitcoin protocol isn't changed to implement quantum resistant block hashing algorithm
The idea that a,b,c,d & e will all remain true at the same time is implausible. a & b are technical limitations and currently impossible although they MAY be possible in the future. c is likely only true if quantum computers are being mass produced. If c is true then it is very likely d isn't true. a,b,c &d aren't going to happen overnight so as implausible as that set on conditions is some years or decades before it becomes true Bitcoin could adopt a quantum reistant hashing algorithm making conditon e false.
This guy makes Bitcoin seem immortal.
