But bitcoin will fork
This doesn’t fix the main problem.
What type of hard fork are we talking about? There are a few options to go at it:
Hard fork with as a result “BTC” becomes --> “BTC with new sig scheme where the old sig scheme is rendered unusable” This would mean all old coins are stuck. They all are on old addresses which only work with the old key pairs and the old signature scheme. THIS IS IMPOSSIBLE.
Hard fork with as a result BTC = old chain, and QRBTC is new chain
2A Before the hard fork, you will need to register and you can claim your new QRBTC by proving you owned BTC before the time of the fork. Can be done, but what is the value of the QRBTC blockchain and its coins? Not automatically the same as the original BTC. (By the way. How, for example, was the value of BCH established right after the fork?)
- You will exclude all users of BTC who didn’t register in time.
- The value of QRBTC will not be the same as the value of the original BTC. While the value of BTC could decline fast due to the fact the QRBTC is the chain that is pursued as the official chain. So the ones that sell their BTC first, will win. They profit of the top price of BTC, and of the price of QRBTC which will be an extra. But the ones that are late selling their BTC, will sell for a lesser price and might end up netting a loss including the price of QRBTC, so $BTC + $QRBTC might < $BTC before fork.
- This option is not possible when Quantum Computers can already crack ECDSA. Because in that case they could hack the addresses on the original chain and forge registration.
After the hard fork, you can claim your QRBTC by proving you own BTC.
- This has the same downsides as a soft fork. Because eventually anyone with a quantum computer would be able to find the private keys of the old BTC by deriving it from the public keys. That way they could forge ownership and still get the QRBTC through hacking.
- Also this has the downside of a hard fork where the new chain has a different value then the old chain and the price of the original BTC might decline fast. Crypto is a volatile market with uncertainty, but this would expand that uncertainty hugely.
Post-quantum Signature Schemes aren't a drop-in Replacement
A change from ECDSA-based addresses to quantum-safe addresses would be no small fork, and would potentially require disabling active addresses for a period of time while a fork was implemented, regardless of the specific cryptocurrency. This could have significant deleterious effects on a cryptocurrency-powered blockchain network, and, as we have experienced in creating our own blockchain, could also require the changing of significant sections of the cryptocurrency's code to accommodate the new security features, drawing into question the feasibility of implementation.
Finally, even if the signature scheme of a particular blockchain platform can be updated to something with Post-Quantum security, there will never be a 100% adoption rate among the total set of wallets. There will always be old wallets which are no longer accessible (due to lost keys) and which cannot be updated, and going forward there will be an ever-larger set of non-technical users unable/unwilling to take necessary steps in a timely fashion should a sudden QC threat emerge. The coins in those old wallets could be vulnerable if the associated account ever broadcast a transaction, and an attacker with a sufficiently powerful quantum computer could accumulate a non-trivial percentage of a coin's circulating supply and use it to manipulate the price of that cryptocurrency. It is not guaranteed that an attack of this sort would be immediately noticeable.
See: "Getting Ready for Post-Quantum Cryptography", NIST, https://nvlpubs.nist.gov/nistpubs/CSWP/NIST.CSWP.04282021.pdf