Discussion

Neha's Writings

EthanHeilman: "A CRQC is an existential threat to Bitcoin (you might believe this is very low-likehood). Your measurement of this threat should literally be:(A) How likely you think it is a CRQC appears by a given time, multiplied by (B) How likely it is you think Bitcoin will not successfully upgrade by that time."It would interesting to survey people about their answers.My off the cuff answer is:2030: A=0.05, B=0.012035: A=0.50, B=0.0012045: A=~1.0, B=~0.0I reserve the right to change my mind on these answers at any point. This is not a serious prediction.

hackernudes: CRQC = cryptographically relevant quantum computer

sayYayToLife: Karl Popper calls this a psychological probability(% chance I go to the gym today). This is different from objective probability (% chance a dice lands on 5).

tromp: Good article with some dumb remarks like> Q: Stealing is illegal, so why would anyone use a CRQC to steal Bitcoin?> A: If you truly believe this, you really should value Bitcoin at 0 – it has many unnecessary components with a lot of overhead, like proof-of-work and digital signatures.Proof of work is necessary for two reasons:1) to fairly distribute all coins (it's not sufficient though, e.g. Bitcoin's halvings still concentrate wealth on early miners/adopters)2) to provide objective proof for the true transaction history, anchored in energy expenditure.A related article on Bitcoin Core resistance to upgrading: https://murmurationstwo.substack.com/p/bitcoin-developers-ar...

EthanHeilman: > 2) to provide objective proof for the true transaction history, anchored in energy expenditure.Why do you need this if you are willing to trust other people not to steal coins or lie?> 1) to fairly distribute all coinsSame question as above. If you don't care about perfidy, simply use the honor system for coin distribution.If you do care about perfidy, then you should probably care about people breaking the law to steal your coins.

lxgr: I guess the argument goes more like: If nobody were to attempt to steal anything, you don’t need security for your ledger anyway.

schoen: As was alluded to in the comments, my colleagues at Blockstream Research are doing some work on this with mechanisms called SHRINCS and SHRIMPS.Of course, inventing and demonstrating a quantum-resistant signature mechanism isn't the same thing as deploying it in consensus or upgrading everyone's UTXOs to it, and it's fair to say that there are many steps in between!

jaspanglia: I think we still have a 3-4 years of escape window to reach the necessary qubit range of breaking the encryption. But China is unstoppable and advancing rapidly, So crypto community needs to upgrade to Post-Quantum Cryptography before the threshold breaks.

glerk: One thing that is not addressed: say this quantum attack happens tomorrow and everyone agrees it was an attack, what would prevent the community (miners, node operators, and users) to hard fork the chain at a snapshot before the attack, patch the protocol, and call that Bitcoin? There would be loss of value of course, but it is not unrecoverable.It’s worth remembering that Ethereum forked for much less (not even a bug in the protocol, but a bug in a private application running on the protocol) and nobody seems too upset about it a decade later.

wmf: In theory nothing prevents that but it would be so contentious that the backlash (e.g. 90% drawdown) may be even worse than just letting the hacks stand.

Retr0id: A hard fork implies a difference in consensus rules, and what do you propose that difference be?Existing wallets need to actively commit to some PQ signature mechanism, prior to Q-day.

nehan: This work is important, and I'm looking forward to forming an opinion on it. Maybe a future post! For those who are interested, this is what I'm aware of:- Tim Ruffing proved that [Taproot's commitment scheme was quantum-resilient](https://eprint.iacr.org/2025/1307)- Jonas Nick and Mikhail Kudinov have proposed [SHRINCS](https://delvingbitcoin.org/t/shrincs-324-byte-stateful-post-...) and [SHRIMPS](https://x.com/n1ckler/status/2038695067754328095).

glerk: Even if Q-day means there is a way to deterministically retrieve any private key from a public key (is that what it means? or is the blast radius of q-day contained? This is a bit above my level of cryptography), I’m sure we could come up with something to minimize the damage. In the worst case, it might involve a claim process with an authority or consensus mechanism to prove who the rightful owner of the funds is and revert the unauthorized transactions on the new chain.Yes, this is not ideal! But if the wallet conversion requires active participation, preemptive measures are also not ideal.

memnips: Somewhat ironic question, but as ETFs holdings of BTC continue to grow, is there a possibility that the custodians of those ETFs start to have a backup plan for ETF holders or create an alliance to push a fork forward? The management fee those companies generate is non-trivial, so they're incentivized to stay ahead of this.Now, of course, the irony here would be traditional finance infrastructure winning out over decentralized, which could definitely deal a psychological blow to BTC's perceived value... but it's something I've been thinking about lately as this existential threat rises on the horizon.

pants2: Microstrategy is already pushing/funding quantum resilience for Bitcoin, so yes!

glerk: Letting the hack stand means the chain comes to a halt and all value is destroyed? Even if you’re a staunch bitcoin purist, I don’t think that’s the path you want to go on.

wmf: The chain wouldn't halt because mining won't be affected by quantum. If you see hacks happening you could race to move your coins into a PQ wallet before the hackers do. I'm assuming that PQ software will be available before the hacks. I agree that this is a very bad scenario.

nehan: Also, LetsEncrypt is very cool! Thanks for working on it.

EGreg: Apparently bitcoin foundation is already working on SHRINCS and SHRIMPS. But whether they will forcibly revoke keys of satoshi and all early bitcoin whales or not is another question!

wmf: Yes, if you read the fine print on the ETFs they tell you what they will do in case of a fork. Usually their custodian picks the "winning" chain at their discretion. There's a similar (although reversed) situation with stablecoins.

glerk: > How do you prove anything, after the key material is compromised?It’s a blockchain, so the simplest would be chain of custody until the chain points undeniably at you. This is not a pure cryptographic device, some social intervention might be needed here.

Q: Stealing is illegal, so why would anyone use a CRQC to steal Bitcoin?

xoa: >Q: Stealing is illegal, so why would anyone use a CRQC to steal Bitcoin?I've had this thought for awhile actually: how would reproducing some random number be legally "stealing" under any legal system in the world? Putting aside that cryptocurrencies have always been about "code decides" etc, that they're outside of the legal system entirely, but I'm struggling to see where there's any actual property interest here. Randomly generated numbers are not protected by IP in any way. There's no computer fraud act angle or the like here, nobody would be having so much as the slightest interaction with anyone else's private system. They'd merely be taking publicly available unprotected numbers and doing some math on them with their own quantum computer. Somebody else who has something related to those numbers is never deprived of them or interacted with in the slightest. There is nothing resembling "hacking", no flaws in the software exploited, all just math there from the start.I can understand how suddenly a lot of proponents might wish to cling to and push the idea that it's "illegal" or "stealing", but doesn't appear to be any meat on dem bones. Maybe they hope to generate support to get laws passed banning it, though hard to see that working out either. As a practical matter seems like they're just going to have to agree on a transition to new version using PQE algorithms and try to convert over before it's too late?

jfengel: I can't imagine that getting laws passed is going to help. The government can't just order a bank to restore funds, the way they can with regular currency. They could try forcing the culprit to return them, but it seems unlikely for the culprit to be in your jurisdiction.I suppose we could pass laws to prevent them from ever spending the money in a country that they can control. Even then, they'd have to find ways around the funds being "laundered" through mixers.

schoen: Isn't your bank balance in a bank database also "just a number"? That number still exists if it goes up or down.I understand that the bank's ownership of its computer means that hacking into it could be seen as (for example) a trespass. However, what if you somehow persuaded a bank employee to change someone's balance? The bank employee has some kind of authority to do this and the result is once again "just a number".OK, what if you display some fraudulent information somewhere that leads a bank employee to decide to update a balance?I don't want to entirely dismiss your intuition because after all there is lots of interest in not relying on legal systems to adjudicate issues related to cryptocurrency transactions. However, changing numbers and causing people or devices to change numbers is not inherently categorically exempt from being considered fraudulent. For that matter, computer fraud laws are often explicitly written to apply to unauthorized alteration of data, not just to unauthorized access to a specific device.You might try to defend this by saying* the ownership of cryptocurrency assets is defined as the ability to transfer them, and should not be further or separately interpreted apart from that ability, or* deceiving a protocol is less obviously wrongful (or at least harder to define) than deceiving a person, or* computer crime should require undermining someone's intent about the use of devices or data and that intent should be clearly manifested and meaningful, which it arguably isn't in a cryptocurrency system, or* offline institutions create some kind of intelligible notion of ownership that's related to the non-digital world and this kind of ownership is what laws about theft or fraud aim to protect rather than any other kind of ownership without that non-digital nexus. (although this doesn't seem to be empirically true as ownership of, for example, domain names has been recognized as a form of property by courts since at least Kremen v. Cohen in 2003, even though it is just a matter of a database entry and has no offline existence)These are interesting conceptual possibilities, but not necessarily persuasive for courts, law enforcement, or cryptocurrency end users.

QuantumNomad_: Cryptocurrency gains are taxable in many (most?) countries. Clearly the governments see cryptocurrency as something more than just random numbers without meaning.Likewise, when government agencies shut down dark net markets (DNMs), they will seize the cryptocurrency funds that the DNM had (from market fees etc., or even funds that belonged to customers and were in escrow etc. by the DNM) if they can (i.e. if they get access to the private keys of DNM owned wallets either by technical means or by convincing the operators of the DNM to hand over the keys). Again because the governments view cryptocurrencies as something more than just random numbers without meaning.