What will Happen to Bitcoin when SHA256 Needs to be Replaced?
All in theory: Moore’s Law will compromise all proof of work algorithms as newer mining equipment becomes affordable to fewer and fewer miners. Eventually one miner will have 51% control and it doesn’t need to be quantum.
*Edit: This already happened once back in July 2014 when the mining pool GHash.io passed a 51% hashrate.
*EndEdit
However quantum would be a definitive answer to this question because Moore’s Law is steadily approaching its limit for conventional CPU chips.
A functional 10000 qubit quantum computer can run Shor’s and Grover’s algorithm: Applying different approaches with Shor’s and Grover’s to associate private and public keys could break ECDSA 256.
The first 10000 qubit quantum computer could assert 51% control over any blockchain it targets.
If Bitcoin is still around like it is, a fork would happen to bb84, but before someone would almost certainly attack the blockchain at will if they could.
Quantum computers in 2019 by top qubit count:
- Google’s Bristlecone: 72-qubit gate-based
- IBM’s 50-qubit
- Intel: 49-qubit-Test-Chip
How long will it take to get to 10000 qubits? No one knows exactly. Quantum computers are still messy and unpredictable.
*Edit: Here’s a good point I didn’t mention:
Check out this simple JavaScript blockchain on my website. Adjust the mining difficulty 0s @duckblo mentions. Watch your browser crash for fun!!
*EndEdit
Shoutout to da homie Kyle Gibson.
