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Quantum computing is not "growing rapidly", it is at best "fast approaching" (and even that is doubtful). There is no quantum computer that does any kind of use

Quantum computing is not "growing rapidly", it is at best "fast approaching" (and even that is doubtful). There is no quantum computer that does any kind of useful work today at all * - the biggest accomplishment of any quantum computer to date has been to run a simplified version of quantum factoring that calculated the prime factors of 15 are 3 and 5, if I remember correctly (maybe it was 21 = 3 × 7 actually?). Also, the blockchain is not encrypted, it is cryptographically signed. Any clients "decrypt" it all day every day, that is how you verify it. The security comes from making it hard to find a hash of a block that has the right structure. I don't think it is known whether any quantum algorithm could improve significantly on this problem (there might be a small improvement from using the quantum O(sqrt(n)) Grover's algorithm instead of the classical O(n) searches, but that's not hard to work around). Note that quantum computers are only known to be exponentially faster at a handful of very specific problems, not in the general case. While this is not established fully, it is considered highly likely by those in the field to be the case. * there are some computers that implement specific quantum algorithms that are sometimes called quantum computers, but those can't do the more interesting algorithms that could break encryption.

nadahalli 3y ago
The hashing part is kind of orthogonal. The real negative outcome of quantum computers getting reliable and powerful would be that ECDSA would be broken (using Shor's algorithm variants). This would make every coin stealable by anyone with access to such a computer. Some coins would be easier to steal than others (P2PK would be the easiest - and all of Satoshi's coins are P2PK). The Bitcoin network would keep working, but coin-ownership would be meaningless.
rcxdude 3y ago
You're forgetting the other very significant part of cryptography on the blockchain: the signing of transactions with wallet keys. If you can crack the public/private key system (and what bitcoin uses is not quantum-resistant), then you can sign transactions on other people's addresses. Now, this is not 100% catastrophic in principle: a break requires an address to have signed a transaction before, so in principle if you are cycling addresses like you're supposed to, you are safe, except you can't publish your transaction to the mempool anymore without risking being sniped by someone else, you need to send it to a miner you trust, which is somewhat damaging to the whole 'trustless' part of the idea. This is one reason amongst a few to be bearish on bitcoin over longer timescales, even as a 'store of value'.

nadahalli 3y ago

The hashing part is kind of orthogonal. The real negative outcome of quantum computers getting reliable and powerful would be that ECDSA would be broken (using Shor's algorithm variants). This would make every coin stealable by anyone with access to such a computer. Some coins would be easier to steal than others (P2PK would be the easiest - and all of Satoshi's coins are P2PK). The Bitcoin network would keep working, but coin-ownership would be meaningless.

rcxdude 3y ago

You're forgetting the other very significant part of cryptography on the blockchain: the signing of transactions with wallet keys. If you can crack the public/private key system (and what bitcoin uses is not quantum-resistant), then you can sign transactions on other people's addresses. Now, this is not 100% catastrophic in principle: a break requires an address to have signed a transaction before, so in principle if you are cycling addresses like you're supposed to, you are safe, except you can't publish your transaction to the mempool anymore without risking being sniped by someone else, you need to send it to a miner you trust, which is somewhat damaging to the whole 'trustless' part of the idea. This is one reason amongst a few to be bearish on bitcoin over longer timescales, even as a 'store of value'.