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I was thinking more of spending ALL of everybody's bitcoin in seconds because Sha256d would be cracked in nano seconds.
To be honest that would be the least of our concerns because a functioning quantum computer could brute force every banking system and missile system on earth in nano seconds too :D
Stonehedge-
I don't think there's a suitable quantum algorithm for immediate cracking of hashes like SHA-256 at silly speeds.
Things that rely on factorisation or discrete logarithms (e.g. RSA, etc) are definitely susceptible via Shor's algorithm in a quantam computer.
The best known quantum algorithm for cryptographic hashes is due to Grover et al and, at best, reduces the effectiveness of a hash to 2^(n/3) where n is the hash size. That's a significant reduction but far from crackable instantly.
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One can only hope
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It can't brute force a physical device.
There are already digital solutions for avoiding a 'quantum hack'.
https://theqrl.org/
Greenbank
Drano
hippy
mine ALL the remaining bitcoins in seconds