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[2a00:1450:4864:20::229]) by gmr-mx.google.com with ESMTPS id 5b1f17b1804b1-43d1fe6434fsi2234875e9.1.2025.03.16.07.15.55 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 16 Mar 2025 07:15:55 -0700 (PDT) Received-SPF: pass (google.com: domain of jameson.lopp@gmail.com designates 2a00:1450:4864:20::229 as permitted sender) client-ip=2a00:1450:4864:20::229; Received: by mail-lj1-x229.google.com with SMTP id 38308e7fff4ca-30c44a87b9cso28469371fa.3 for ; Sun, 16 Mar 2025 07:15:55 -0700 (PDT) X-Gm-Gg: ASbGncvFSgEHe9QWAcT08lYzxrZcjbDv32iR1hYlx84aejfJpxiYxmF4CopV6RrozOW dbHEfCXA5zBGAys9507MsBFVs658E25b1vaSG25rzrn2rfEqKB2m3us9MFDCZR9qBE2m8fnyNH3 PVGfgiywBv1aE3HOQ8J4HQa8ZP5Q== X-Received: by 2002:a05:6512:3da5:b0:545:1082:91a1 with SMTP id 2adb3069b0e04-549c38ef274mr3538418e87.7.1742134552488; Sun, 16 Mar 2025 07:15:52 -0700 (PDT) MIME-Version: 1.0 From: Jameson Lopp Date: Sun, 16 Mar 2025 10:15:40 -0400 X-Gm-Features: AQ5f1JrNq00tsq_U4JuK5RhBTaN8AFIC5QxY2bsCf8imzQofvy5gUyUEm7OxwXQ Message-ID: Subject: [bitcoindev] Against Allowing Quantum Recovery of Bitcoin To: Bitcoin Development Mailing List Content-Type: multipart/alternative; boundary="000000000000270e760630764fbf" X-Original-Sender: jameson.lopp@gmail.com X-Original-Authentication-Results: gmr-mx.google.com; dkim=pass header.i=@gmail.com header.s=20230601 header.b=B1TkAjcQ; spf=pass (google.com: domain of jameson.lopp@gmail.com designates 2a00:1450:4864:20::229 as permitted sender) smtp.mailfrom=jameson.lopp@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com; dara=pass header.i=@googlegroups.com Precedence: list Mailing-list: list bitcoindev@googlegroups.com; contact bitcoindev+owners@googlegroups.com List-ID: X-Google-Group-Id: 786775582512 List-Post: , List-Help: , List-Archive: , List-Unsubscribe: , X-Spam-Score: 0.0 (/) --000000000000270e760630764fbf Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable The quantum computing debate is heating up. There are many controversial aspects to this debate, including whether or not quantum computers will ever actually become a practical threat. I won't tread into the unanswerable question of how worried we should be about quantum computers. I think it's far from a crisis, but given the difficulty in changing Bitcoin it's worth starting to seriously discuss. Today I wish to focus on a philosophical quandary related to one of the decisions that would need to be made if and when we implement a quantum safe signature scheme. Several Scenarios Because this essay will reference game theory a fair amount, and there are many variables at play that could change the nature of the game, I think it's important to clarify the possible scenarios up front. 1. Quantum computing never materializes, never becomes a threat, and thus everything discussed in this essay is moot. 2. A quantum computing threat materializes suddenly and Bitcoin does not have quantum safe signatures as part of the protocol. In this scenario it would likely make the points below moot because Bitcoin would be fundamentally broken and it would take far too long to upgrade the protocol, wallet software, and migrate user funds in order to restore confidence in the network. 3. Quantum computing advances slowly enough that we come to consensus about how to upgrade Bitcoin and post quantum security has been minimally adopted by the time an attacker appears. 4. Quantum computing advances slowly enough that we come to consensus about how to upgrade Bitcoin and post quantum security has been highly adopted by the time an attacker appears. For the purposes of this post, I'm envisioning being in situation 3 or 4. To Freeze or not to Freeze? I've started seeing more people weighing in on what is likely the most contentious aspect of how a quantum resistance upgrade should be handled in terms of migrating user funds. Should quantum vulnerable funds be left open to be swept by anyone with a sufficiently powerful quantum computer OR should they be permanently locked? "I don't see why old coins should be confiscated. The better option is to > let those with quantum computers free up old coins. While this might have > an inflationary impact on bitcoin's price, to use a turn of phrase, the > inflation is transitory. Those with low time preference should support > returning lost coins to circulation." - Hunter Beast On the other hand: "Of course they have to be confiscated. If and when (and that's a big if) > the existence of a cryptography-breaking QC becomes a credible threat, th= e > Bitcoin ecosystem has no other option than softforking out the ability to > spend from signature schemes (including ECDSA and BIP340) that are > vulnerable to QCs. The alternative is that millions of BTC become > vulnerable to theft; I cannot see how the currency can maintain any value > at all in such a setting. And this affects everyone; even those which > diligently moved their coins to PQC-protected schemes." > - Pieter Wuille I don't think "confiscation" is the most precise term to use, as the funds are not being seized and reassigned. Rather, what we're really discussing would be better described as "burning" - placing the funds *out of reach of everyone*. Not freezing user funds is one of Bitcoin's inviolable properties. However, if quantum computing becomes a threat to Bitcoin's elliptic curve cryptography, *an inviolable property of Bitcoin will be violated one way or another*. Fundamental Properties at Risk 5 years ago I attempted to comprehensively categorize all of Bitcoin's fundamental properties that give it value. https://nakamoto.com/what-are-the-key-properties-of-bitcoin/ The particular properties in play with regard to this issue seem to be: *Censorship Resistance* - No one should have the power to prevent others from using their bitcoin or interacting with the network. *Forward Compatibility* - changing the rules such that certain valid transactions become invalid could undermine confidence in the protocol. *Conservatism* - Users should not be expected to be highly responsive to system issues. As a result of the above principles, we have developed a strong meme (kudos to Andreas Antonopoulos) that goes as follows: Not your keys, not your coins. I posit that the corollary to this principle is: Your keys, only your coins. A quantum capable entity breaks the corollary of this foundational principle. We secure our bitcoin with the mathematical probabilities related to extremely large random numbers. Your funds are only secure because truly random large numbers should not be guessable or discoverable by anyone else in the world. This is the principle behind the motto *vires in numeris* - strength in numbers. In a world with quantum enabled adversaries, this principle is null and void for many types of cryptography, including the elliptic curve digital signatures used in Bitcoin. Who is at Risk? There has long been a narrative that Satoshi's coins and others from the Satoshi era of P2PK locking scripts that exposed the public key directly on the blockchain will be those that get scooped up by a quantum "miner." But unfortunately it's not that simple. If I had a powerful quantum computer, which coins would I target? I'd go to the Bitcoin rich list and find the wallets that have exposed their public keys due to re-using addresses that have previously been spent from. You can easily find them at https://bitinfocharts.com/top-100-richest-bitcoin-addresses.html Note that a few of these wallets, like Bitfinex / Kraken / Tether, would be slightly harder to crack because they are multisig wallets. So a quantum attacker would need to reverse engineer 2 keys for Kraken or 3 for Bitfinex / Tether in order to spend funds. But many are single signature. Point being, it's not only the really old lost BTC that are at risk to a quantum enabled adversary, at least at time of writing. If we add a quantum safe signature scheme, we should expect those wallets to be some of the first to upgrade given their incentives. The Ethical Dilemma: Quantifying Harm Which decision results in the most harm? By making quantum vulnerable funds unspendable we potentially harm some Bitcoin users who were not paying attention and neglected to migrate their funds to a quantum safe locking script. This violates the "conservativism" principle stated earlier. On the flip side, we prevent those funds plus far more lost funds from falling into the hands of the few privileged folks who gain early access to quantum computers. By leaving quantum vulnerable funds available to spend, the same set of users who would otherwise have funds frozen are likely to see them stolen. And many early adopters who lost their keys will eventually see their unreachable funds scooped up by a quantum enabled adversary. Imagine, for example, being James Howells, who accidentally threw away a hard drive with 8,000 BTC on it, currently worth over $600M USD. He has spent a decade trying to retrieve it from the landfill where he knows it's buried, but can't get permission to excavate. I suspect that, given the choice, he'd prefer those funds be permanently frozen rather than fall into someone else's possession - I know I would. Allowing a quantum computer to access lost funds doesn't make those users any worse off than they were before, however it *would* have a negative impact upon everyone who is currently holding bitcoin. It's prudent to expect significant economic disruption if large amounts of coins fall into new hands. Since a quantum computer is going to have a massive up front cost, expect those behind it to desire to recoup their investment. We also know from experience that when someone suddenly finds themselves in possession of 9+ figures worth of highly liquid assets, they tend to diversify into other things by selling. Allowing quantum recovery of bitcoin is *tantamount to wealth redistribution*. What we'd be allowing is for bitcoin to be redistributed from those who are ignorant of quantum computers to those who have won the technological race to acquire quantum computers. It's hard to see a bright side to that scenario. Is Quantum Recovery Good for Anyone? Does quantum recovery HELP anyone? I've yet to come across an argument that it's a net positive in any way. It certainly doesn't add any security to the network. If anything, it greatly decreases the security of the network by allowing funds to be claimed by those who did not earn them. But wait, you may be thinking, wouldn't quantum "miners" have earned their coins by all the work and resources invested in building a quantum computer? I suppose, in the same sense that a burglar earns their spoils by the resources they invest into surveilling targets and learning the skills needed to break into buildings. What I say "earned" I mean through productive mutual trade. For example: * Investors earn BTC by trading for other currencies. * Merchants earn BTC by trading for goods and services. * Miners earn BTC by trading thermodynamic security. * Quantum miners don't trade anything, they are vampires feeding upon the system. There's no reason to believe that allowing quantum adversaries to recover vulnerable bitcoin will be of benefit to anyone other than the select few organizations that win the technological arms race to build the first such computers. Probably nation states and/or the top few largest tech companies= . One could certainly hope that an organization with quantum supremacy is benevolent and acts in a "white hat" manner to return lost coins to their owners, but that's incredibly optimistic and foolish to rely upon. Such a situation creates an insurmountable ethical dilemma of only recovering lost bitcoin rather than currently owned bitcoin. There's no way to precisely differentiate between the two; anyone can claim to have lost their bitcoin but if they have lost their keys then proving they ever had the keys becomes rather difficult. I imagine that any such white hat recovery efforts would have to rely upon attestations from trusted third parties like exchanges. Even if the first actor with quantum supremacy is benevolent, we must assume the technology could fall into adversarial hands and thus think adversarially about the potential worst case outcomes. Imagine, for example, that North Korea continues scooping up billions of dollars from hacking crypto exchanges and decides to invest some of those proceeds into building a quantum computer for the biggest payday ever... Downsides to Allowing Quantum Recovery Let's think through an exhaustive list of pros and cons for allowing or preventing the seizure of funds by a quantum adversary. Historical Precedent Previous protocol vulnerabilities weren=E2=80=99t celebrated as "fair game"= but rather were treated as failures to be remediated. Treating quantum theft differently risks rewriting Bitcoin=E2=80=99s history as a free-for-all rat= her than a system that seeks to protect its users. Violation of Property Rights Allowing a quantum adversary to take control of funds undermines the fundamental principle of cryptocurrency - if you keep your keys in your possession, only you should be able to access your money. Bitcoin is built on the idea that private keys secure an individual=E2=80=99s assets, and unauthorized access (even via advanced tech) is theft, not a legitimate transfer. Erosion of Trust in Bitcoin If quantum attackers can exploit vulnerable addresses, confidence in Bitcoin as a secure store of value would collapse. Users and investors rely on cryptographic integrity, and widespread theft could drive adoption away from Bitcoin, destabilizing its ecosystem. This is essentially the counterpoint to claiming the burning of vulnerable funds is a violation of property rights. While some will certainly see it as such, others will find the apathy toward stopping quantum theft to be similarly concerning. Unfair Advantage Quantum attackers, likely equipped with rare and expensive technology, would have an unjust edge over regular users who lack access to such tools. This creates an inequitable system where only the technologically elite can exploit others, contradicting Bitcoin=E2=80=99s ethos of decentralized powe= r. Bitcoin is designed to create an asymmetric advantage for DEFENDING one's wealth. It's supposed to be impractically expensive for attackers to crack the entropy and cryptography protecting one's coins. But now we find ourselves discussing a situation where this asymmetric advantage is compromised in favor of a specific class of attackers. Economic Disruption Large-scale theft from vulnerable addresses could crash Bitcoin=E2=80=99s p= rice as quantum recovered funds are dumped on exchanges. This would harm all holders, not just those directly targeted, leading to broader financial chaos in the markets. Moral Responsibility Permitting theft via quantum computing sets a precedent that technological superiority justifies unethical behavior. This is essentially taking a "code is law" stance in which we refuse to admit that both code and laws can be modified to adapt to previously unforeseen situations. Burning of coins can certainly be considered a form of theft, thus I think it's worth differentiating the two different thefts being discussed: 1. self-enriching & likely malicious 2. harm prevention & not necessarily malicious Both options lack the consent of the party whose coins are being burnt or transferred, thus I think the simple argument that theft is immoral becomes a wash and it's important to drill down into the details of each. Incentives Drive Security I can tell you from a decade of working in Bitcoin security - the average user is lazy and is a procrastinator. If Bitcoiners are given a "drop dead date" after which they know vulnerable funds will be burned, this pressure accelerates the adoption of post-quantum cryptography and strengthens Bitcoin long-term. Allowing vulnerable users to delay upgrading indefinitely will result in more laggards, leaving the network more exposed when quantum tech becomes available. Steel Manning Clearly this is a complex and controversial topic, thus it's worth thinking through the opposing arguments. Protecting Property Rights Allowing quantum computers to take vulnerable bitcoin could potentially be spun as a hard money narrative - we care so greatly about not violating someone's access to their coins that we allow them to be stolen! But I think the flip side to the property rights narrative is that burning vulnerable coins prevents said property from falling into undeserving hands. If the entire Bitcoin ecosystem just stands around and allows quantum adversaries to claim funds that rightfully belong to other users, is that really a "win" in the "protecting property rights" category? It feels more like apathy to me. As such, I think the "protecting property rights" argument is a wash. Quantum Computers Won't Attack Bitcoin There is a great deal of skepticism that sufficiently powerful quantum computers will ever exist, so we shouldn't bother preparing for a non-existent threat. Others have argued that even if such a computer was built, a quantum attacker would not go after bitcoin because they wouldn't want to reveal their hand by doing so, and would instead attack other infrastructure. It's quite difficult to quantify exactly how valuable attacking other infrastructure would be. It also really depends upon when an entity gains quantum supremacy and thus if by that time most of the world's systems have already been upgraded. While I think you could argue that certain entities gaining quantum capability might not attack Bitcoin, it would only delay the inevitable - eventually somebody will achieve the capability who decides to use it for such an attack. Quantum Attackers Would Only Steal Small Amounts Some have argued that even if a quantum attacker targeted bitcoin, they'd only go after old, likely lost P2PK outputs so as to not arouse suspicion and cause a market panic. I'm not so sure about that; why go after 50 BTC at a time when you could take 250,000 BTC with the same effort as 50 BTC? This is a classic "zero day exploit" game theory in which an attacker knows they have a limited amount of time before someone else discovers the exploit and either benefits from it or patches it. Take, for example, the recent ByBit attack - the highest value crypto hack of all time. Lazarus Group had compromised the Safe wallet front end JavaScript app and they could have simply had it reassign ownership of everyone's Safe wallets as they were interacting with their wallet. But instead they chose to only specifically target ByBit's wallet with $1.5 billion in it because they wanted to maximize their extractable value. If Lazarus had started stealing from every wallet, they would have been discovered quickly and the Safe web app would likely have been patched well before any billion dollar wallets executed the malicious code. I think the "only stealing small amounts" argument is strongest for Situation #2 described earlier, where a quantum attacker arrives before quantum safe cryptography has been deployed across the Bitcoin ecosystem. Because if it became clear that Bitcoin's cryptography was broken AND there was nowhere safe for vulnerable users to migrate, the only logical option would be for everyone to liquidate their bitcoin as quickly as possible. As such, I don't think it applies as strongly for situations in which we have a migration path available. The 21 Million Coin Supply Should be in Circulation Some folks are arguing that it's important for the "circulating / spendable" supply to be as close to 21M as possible and that having a significant portion of the supply out of circulation is somehow undesirable= . While the "21M BTC" attribute is a strong memetic narrative, I don't think anyone has ever expected that it would all be in circulation. It has always been understood that many coins will be lost, and that's actually part of the game theory of owning bitcoin! And remember, the 21M number in and of itself is not a particularly important detail - it's not even mentioned in the whitepaper. What's important is that the supply is well known and not subject to change. Self-Sovereignty and Personal Responsibility Bitcoin=E2=80=99s design empowers individuals to control their own wealth, = free from centralized intervention. This freedom comes with the burden of securing one's private keys. If quantum computing can break obsolete cryptography, the fault lies with users who didn't move their funds to quantum safe locking scripts. Expecting the network to shield users from their own negligence undermines the principle that you, and not a third party, are accountable for your assets. I think this is generally a fair point that "the community" doesn't owe you anything in terms of helping you. I think that we do, however, need to consider the incentives and game theory in play with regard to quantum safe Bitcoiners vs quantum vulnerable Bitcoiners. More on that later. Code is Law Bitcoin operates on transparent, immutable rules embedded in its protocol. If a quantum attacker uses superior technology to derive private keys from public keys, they=E2=80=99re not "hacking" the system - they're simply foll= owing what's mathematically permissible within the current code. Altering the protocol to stop this introduces subjective human intervention, which clashes with the objective, deterministic nature of blockchain. While I tend to agree that code is law, one of the entire points of laws is that they can be amended to improve their efficacy in reducing harm. Leaning on this point seems more like a pro-ossification stance that it's better to do nothing and allow harm to occur rather than take action to stop an attack that was foreseen far in advance. Technological Evolution as a Feature, Not a Bug It's well known that cryptography tends to weaken over time and eventually break. Quantum computing is just the next step in this progression. Users who fail to adapt (e.g., by adopting quantum-resistant wallets when available) are akin to those who ignored technological advancements like multisig or hardware wallets. Allowing quantum theft incentivizes innovation and keeps Bitcoin=E2=80=99s ecosystem dynamic, punishing complac= ency while rewarding vigilance. Market Signals Drive Security If quantum attackers start stealing funds, it sends a clear signal to the market: upgrade your security or lose everything. This pressure accelerates the adoption of post-quantum cryptography and strengthens Bitcoin long-term. Coddling vulnerable users delays this necessary evolution, potentially leaving the network more exposed when quantum tech becomes widely accessible. Theft is a brutal but effective teacher. Centralized Blacklisting Power Burning vulnerable funds requires centralized decision-making - a soft fork to invalidate certain transactions. This sets a dangerous precedent for future interventions, eroding Bitcoin=E2=80=99s decentralization. If quantu= m theft is blocked, what=E2=80=99s next - reversing exchange hacks? The system must= remain neutral, even if it means some lose out. I think this could be a potential slippery slope if the proposal was to only burn specific addresses. Rather, I'd expect a neutral proposal to burn all funds in locking script types that are known to be quantum vulnerable. Thus, we could eliminate any subjectivity from the code. Fairness in Competition Quantum attackers aren't cheating; they're using publicly available physics and math. Anyone with the resources and foresight can build or access quantum tech, just as anyone could mine Bitcoin in 2009 with a CPU. Early adopters took risks and reaped rewards; quantum innovators are doing the same. Calling it =E2=80=9Cunfair=E2=80=9D ignores that Bitcoin has never pr= omised equality of outcome - only equality of opportunity within its rules. I find this argument to be a mischaracterization because we're not talking about CPUs. This is more akin to talking about ASICs, except each ASIC costs millions if not billions of dollars. This is out of reach from all but the wealthiest organizations. Economic Resilience Bitcoin has weathered thefts before (MTGOX, Bitfinex, FTX, etc) and emerged stronger. The market can absorb quantum losses, with unaffected users continuing to hold and new entrants buying in at lower prices. Fear of economic collapse overestimates the impact - the network=E2=80=99s antifrag= ility thrives on such challenges. This is a big grey area because we don't know when a quantum computer will come online and we don't know how quickly said computers would be able to steal bitcoin. If, for example, the first generation of sufficiently powerful quantum computers were stealing less volume than the current block reward then of course it will have minimal economic impact. But if they're taking thousands of BTC per day and bringing them back into circulation, there will likely be a noticeable market impact as it absorbs the new supply. This is where the circumstances will really matter. If a quantum attacker appears AFTER the Bitcoin protocol has been upgraded to support quantum resistant cryptography then we should expect the most valuable active wallets will have upgraded and the juiciest target would be the 31,000 BTC in the address 12ib7dApVFvg82TXKycWBNpN8kFyiAN1dr which has been dormant since 2010. In general I'd expect that the amount of BTC re-entering the circulating supply would look somewhat similar to the mining emission curve: volume would start off very high as the most valuable addresses are drained and then it would fall off as quantum computers went down the list targeting addresses with less and less BTC. Why is economic impact a factor worth considering? Miners and businesses in general. More coins being liquidated will push down the price, which will negatively impact miner revenue. Similarly, I can attest from working in the industry for a decade, that lower prices result in less demand from businesses across the entire industry. As such, burning quantum vulnerable bitcoin is good for the entire industry. Practicality & Neutrality of Non-Intervention There=E2=80=99s no reliable way to distinguish =E2=80=9Ctheft=E2=80=9D from= legitimate "white hat" key recovery. If someone loses their private key and a quantum computer recovers it, is that stealing or reclaiming? Policing quantum actions requires invasive assumptions about intent, which Bitcoin=E2=80=99s trustle= ss design can=E2=80=99t accommodate. Letting the chips fall where they may avo= ids this mess. Philosophical Purity Bitcoin rejects bailouts. It=E2=80=99s a cold, hard system where outcomes r= eflect preparation and skill, not sentimentality. If quantum computing upends the game, that=E2=80=99s the point - Bitcoin isn=E2=80=99t meant to be safe or = fair in a nanny-state sense; it=E2=80=99s meant to be free. Users who lose funds to q= uantum attacks are casualties of liberty and their own ignorance, not victims of injustice. Bitcoin's DAO Moment This situation has some similarities to The DAO hack of an Ethereum smart contract in 2016, which resulted in a fork to stop the attacker and return funds to their original owners. The game theory is similar because it's a situation where a threat is known but there's some period of time before the attacker can actually execute the theft. As such, there's time to mitigate the attack by changing the protocol. It also created a schism in the community around the true meaning of "code is law," resulting in Ethereum Classic, which decided to allow the attacker to retain control of the stolen funds. A soft fork to burn vulnerable bitcoin could certainly result in a hard fork if there are enough miners who reject the soft fork and continue including transactions. Incentives Matter We can wax philosophical until the cows come home, but what are the actual incentives for existing Bitcoin holders regarding this decision? "Lost coins only make everyone else's coins worth slightly more. Think of > it as a donation to everyone." - Satoshi Nakamoto If true, the corollary is: "Quantum recovered coins only make everyone else's coins worth less. Think > of it as a theft from everyone." - Jameson Lopp Thus, assuming we get to a point where quantum resistant signatures are supported within the Bitcoin protocol, what's the incentive to let vulnerable coins remain spendable? * It's not good for the actual owners of those coins. It disincentivizes owners from upgrading until perhaps it's too late. * It's not good for the more attentive / responsible owners of coins who have quantum secured their stash. Allowing the circulating supply to balloon will assuredly reduce the purchasing power of all bitcoin holders. Forking Game Theory >From a game theory point of view, I see this as incentivizing users to upgrade their wallets. If you disagree with the burning of vulnerable coins, all you have to do is move your funds to a quantum safe signature scheme. Point being, I don't see there being an economic majority (or even more than a tiny minority) of users who would fight such a soft fork. Why expend significant resources fighting a fork when you can just move your coins to a new address? Remember that blocking spending of certain classes of locking scripts is a tightening of the rules - a soft fork. As such, it can be meaningfully enacted and enforced by a mere majority of hashpower. If miners generally agree that it's in their best interest to burn vulnerable coins, are other users going to care enough to put in the effort to run new node software that resists the soft fork? Seems unlikely to me. How to Execute Burning In order to be as objective as possible, the goal would be to announce to the world that after a specific block height / timestamp, Bitcoin nodes will no longer accept transactions (or blocks containing such transactions) that spend funds from any scripts other than the newly instituted quantum safe schemes. It could take a staggered approach to first freeze funds that are susceptible to long-range attacks such as those in P2PK scripts or those that exposed their public keys due to previously re-using addresses, but I expect the additional complexity would drive further controversy. How long should the grace period be in order to give the ecosystem time to upgrade? I'd say a minimum of 1 year for software wallets to upgrade. We can only hope that hardware wallet manufacturers are able to implement post quantum cryptography on their existing hardware with only a firmware update= . Beyond that, it will take at least 6 months worth of block space for all users to migrate their funds, even in a best case scenario. Though if you exclude dust UTXOs you could probably get 95% of BTC value migrated in 1 month. Of course this is a highly optimistic situation where everyone is completely focused on migrations - in reality it will take far longer. Regardless, I'd think that in order to reasonably uphold Bitcoin's conservatism it would be preferable to allow a 4 year migration window. In the meantime, mining pools could coordinate emergency soft forking logic such that if quantum attackers materialized, they could accelerate the countdown to the quantum vulnerable funds burn. Random Tangential Benefits On the plus side, burning all quantum vulnerable bitcoin would allow us to prune all of those UTXOs out of the UTXO set, which would also clean up a lot of dust. Dust UTXOs are a bit of an annoyance and there has even been a recent proposal for how to incentivize cleaning them up. We should also expect that incentivizing migration of the entire UTXO set will create substantial demand for block space that will sustain a fee market for a fairly lengthy amount of time. In Summary While the moral quandary of violating any of Bitcoin's inviolable properties can make this a very complex issue to discuss, the game theory and incentives between burning vulnerable coins versus allowing them to be claimed by entities with quantum supremacy appears to be a much simpler issue. I, for one, am not interested in rewarding quantum capable entities by inflating the circulating money supply just because some people lost their keys long ago and some laggards are not upgrading their bitcoin wallet's security. We can hope that this scenario never comes to pass, but hope is not a strategy. I welcome your feedback upon any of the above points, and contribution of any arguments I failed to consider. --=20 You received this message because you are subscribed to the Google Groups "= Bitcoin Development Mailing List" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to bitcoindev+unsubscribe@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/bitcoindev/= CADL_X_cF%3DUKVa7CitXReMq8nA_4RadCF%3D%3DkU4YG%2B0GYN97P6hQ%40mail.gmail.co= m. --000000000000270e760630764fbf Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
The quantum computing debate is heating up. There are many= controversial aspects to this debate, including whether or not quantum com= puters will ever actually become a practical threat.

I won't tr= ead into the unanswerable question of how worried we should be about quantu= m computers. I think it's far from a crisis, but given the difficulty i= n changing Bitcoin it's worth starting to seriously discuss. Today I wi= sh to focus on a philosophical quandary related to one of the decisions tha= t would need to be made if and when we implement a quantum safe signature s= cheme.

Several Scenarios
Because this ess= ay will reference game theory a fair amount, and there are many variables a= t play that could change the nature of the game, I think it's important= to clarify the possible scenarios up front.

1. Quantum computing ne= ver materializes, never becomes a threat, and thus everything discussed in = this essay is moot.
2. A quantum computing threat materializes suddenly = and Bitcoin does not have quantum safe signatures as part of the protocol. = In this scenario it would likely make the points below moot because Bitcoin= would be fundamentally broken and it would take far too long to upgrade th= e protocol, wallet software, and migrate user funds in order to restore con= fidence in the network.
3. Quantum computing advances slowly enough that= we come to consensus about how to upgrade Bitcoin and post quantum securit= y has been minimally adopted by the time an attacker appears.
4. Quantum= computing advances slowly enough that we come to consensus about how to up= grade Bitcoin and post quantum security has been highly adopted by the time= an attacker appears.

For the purposes of this post, I'm envisio= ning being in situation 3 or 4.

To Freeze or not to= Freeze?
I've started seeing more people weighing in on what = is likely the most contentious aspect of how a quantum resistance upgrade s= hould be handled in terms of migrating user funds. Should quantum vulnerabl= e funds be left open to be swept by anyone with a sufficiently powerful qua= ntum computer OR should they be permanently locked?

"I don't see why old coins shoul= d be confiscated. The better option is to let those with quantum computers = free up old coins. While this might have an inflationary impact on bitcoin&= #39;s price, to use a turn of phrase, the inflation is transitory. Those wi= th low time preference should support returning lost coins to circulation.&= quot;=C2=A0
-= Hunter Beast

On the other hand:

<= blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-l= eft:1px solid rgb(204,204,204);padding-left:1ex">"Of course they have = to be confiscated. If and when (and that's a big if) the existence of a= cryptography-breaking QC becomes a credible threat, the Bitcoin ecosystem = has no other option than softforking out the ability to spend from signatur= e schemes (including ECDSA and BIP340) that are vulnerable to QCs. The alte= rnative is that millions of BTC become vulnerable to theft; I cannot see ho= w the currency can maintain any value at all in such a setting. And this af= fects everyone; even those which diligently moved their coins to PQC-protec= ted schemes."
- Pieter Wuille
I don't think &qu= ot;confiscation" is the most precise term to use, as the funds are not= being seized and reassigned. Rather, what we're really discussing woul= d be better described as "burning" - placing the funds out of = reach of everyone.

Not freezing user funds is one of Bitcoin'= ;s inviolable properties. However, if quantum computing becomes a threat to= Bitcoin's elliptic curve cryptography, an inviolable property of Bi= tcoin will be violated one way or another.

Fund= amental Properties at Risk
5 years ago I attempted to comprehensi= vely categorize all of Bitcoin's fundamental properties that give it va= lue. https://nakamoto.com/what-are-the-key-properties-of-bitcoin/

= The particular properties in play with regard to this issue seem to be:
=
Censorship Resistance - No one should have the power to prevent = others from using their bitcoin or interacting with the network.

= Forward Compatibility - changing the rules such that certain valid tran= sactions become invalid could undermine confidence in the protocol.

= Conservatism - Users should not be expected to be highly responsive = to system issues.

As a result of the above principles, we have devel= oped a strong meme (kudos to Andreas Antonopoulos) that goes as follows:
Not your keys, not y= our coins.

I posit that the corollary to this principle is:=

Your keys, only y= our coins.

A quantum capable entity breaks the corollary of= this foundational principle. We secure our bitcoin with the mathematical p= robabilities related to extremely large random numbers. Your funds are only= secure because truly random large numbers should not be guessable or disco= verable by anyone else in the world.

This is the principle behind th= e motto vires in numeris - strength in numbers. In a world with quan= tum enabled adversaries, this principle is null and void for many types of = cryptography, including the elliptic curve digital signatures used in Bitco= in.

Who is at Risk?
There has long been a= narrative that Satoshi's coins and others from the Satoshi era of P2PK= locking scripts that exposed the public key directly on the blockchain wil= l be those that get scooped up by a quantum "miner." But unfortun= ately it's not that simple. If I had a powerful quantum computer, which= coins would I target? I'd go to the Bitcoin rich list and find the wal= lets that have exposed their public keys due to re-using addresses that hav= e previously been spent from. You can easily find them at https://bitinfo= charts.com/top-100-richest-bitcoin-addresses.html

Note that a fe= w of these wallets, like Bitfinex / Kraken / Tether, would be slightly hard= er to crack because they are multisig wallets. So a quantum attacker would = need to reverse engineer 2 keys for Kraken or 3 for Bitfinex / Tether in or= der to spend funds. But many are single signature.

Point being, it&#= 39;s not only the really old lost BTC that are at risk to a quantum enabled= adversary, at least at time of writing. If we add a quantum safe signature= scheme, we should expect those wallets to be some of the first to upgrade = given their incentives.

The Ethical Dilemma: Quanti= fying Harm
Which decision results in the most harm?

By mak= ing quantum vulnerable funds unspendable we potentially harm some Bitcoin u= sers who were not paying attention and neglected to migrate their funds to = a quantum safe locking script. This violates the "conservativism"= principle stated earlier. On the flip side, we prevent those funds plus fa= r more lost funds from falling into the hands of the few privileged folks w= ho gain early access to quantum computers.

By leaving quantum vulner= able funds available to spend, the same set of users who would otherwise ha= ve funds frozen are likely to see them stolen. And many early adopters who = lost their keys will eventually see their unreachable funds scooped up by a= quantum enabled adversary.

Imagine, for example, being James Howell= s, who accidentally threw away a hard drive with 8,000 BTC on it, currently= worth over $600M USD. He has spent a decade trying to retrieve it from the= landfill where he knows it's buried, but can't get permission to e= xcavate. I suspect that, given the choice, he'd prefer those funds be p= ermanently frozen rather than fall into someone else's possession - I k= now I would.

Allowing a quantum computer to access lost funds doesn&= #39;t make those users any worse off than they were before, however it w= ould have a negative impact upon everyone who is currently holding bitc= oin.

It's prudent to expect significant economic disruption if l= arge amounts of coins fall into new hands. Since a quantum computer is goin= g to have a massive up front cost, expect those behind it to desire to reco= up their investment. We also know from experience that when someone suddenl= y finds themselves in possession of 9+ figures worth of highly liquid asset= s, they tend to diversify into other things by selling.

Allowing qua= ntum recovery of bitcoin is tantamount to wealth redistribution. Wha= t we'd be allowing is for bitcoin to be redistributed from those who ar= e ignorant of quantum computers to those who have won the technological rac= e to acquire quantum computers. It's hard to see a bright side to that = scenario.

Is Quantum Recovery Good for Anyone?

Does quantum recovery HELP anyone? I've yet to come across an= argument that it's a net positive in any way. It certainly doesn't= add any security to the network. If anything, it greatly decreases the sec= urity of the network by allowing funds to be claimed by those who did not e= arn them.

But wait, you may be thinking, wouldn't quantum "= miners" have earned their coins by all the work and resources invested= in building a quantum computer? I suppose, in the same sense that a burgla= r earns their spoils by the resources they invest into surveilling targets = and learning the skills needed to break into buildings. What I say "ea= rned" I mean through productive mutual trade.

For example:
<= br>* Investors earn BTC by trading for other currencies.
* Merchants ear= n BTC by trading for goods and services.
* Miners earn BTC by trading th= ermodynamic security.
* Quantum miners don't trade anything, they ar= e vampires feeding upon the system.

There's no reason to believe= that allowing quantum adversaries to recover vulnerable bitcoin will be of= benefit to anyone other than the select few organizations that win the tec= hnological arms race to build the first such computers. Probably nation sta= tes and/or the top few largest tech companies.

One could certainly h= ope that an organization with quantum supremacy is benevolent and acts in a= "white hat" manner to return lost coins to their owners, but tha= t's incredibly optimistic and foolish to rely upon. Such a situation cr= eates an insurmountable ethical dilemma of only recovering lost bitcoin rat= her than currently owned bitcoin. There's no way to precisely different= iate between the two; anyone can claim to have lost their bitcoin but if th= ey have lost their keys then proving they ever had the keys becomes rather = difficult. I imagine that any such white hat recovery efforts would have to= rely upon attestations from trusted third parties like exchanges.

E= ven if the first actor with quantum supremacy is benevolent, we must assume= the technology could fall into adversarial hands and thus think adversaria= lly about the potential worst case outcomes. Imagine, for example, that Nor= th Korea continues scooping up billions of dollars from hacking crypto exch= anges and decides to invest some of those proceeds into building a quantum = computer for the biggest payday ever...

Downsides t= o Allowing Quantum Recovery
Let's think through an exhaustive= list of pros and cons for allowing or preventing the seizure of funds by a= quantum adversary.

Historical Precedent
= Previous protocol vulnerabilities weren=E2=80=99t celebrated as "fair = game" but rather were treated as failures to be remediated. Treating q= uantum theft differently risks rewriting Bitcoin=E2=80=99s history as a fre= e-for-all rather than a system that seeks to protect its users.

Violation of Property Rights
Allowing a quantum adve= rsary to take control of funds undermines the fundamental principle of cryp= tocurrency - if you keep your keys in your possession, only you should be a= ble to access your money. Bitcoin is built on the idea that private keys se= cure an individual=E2=80=99s assets, and unauthorized access (even via adva= nced tech) is theft, not a legitimate transfer.

Ero= sion of Trust in Bitcoin
If quantum attackers can exploit vulnera= ble addresses, confidence in Bitcoin as a secure store of value would colla= pse. Users and investors rely on cryptographic integrity, and widespread th= eft could drive adoption away from Bitcoin, destabilizing its ecosystem.
This is essentially the counterpoint to claiming the burning of vulner= able funds is a violation of property rights. While some will certainly see= it as such, others will find the apathy toward stopping quantum theft to b= e similarly concerning.

Unfair Advantage
= Quantum attackers, likely equipped with rare and expensive technology, woul= d have an unjust edge over regular users who lack access to such tools. Thi= s creates an inequitable system where only the technologically elite can ex= ploit others, contradicting Bitcoin=E2=80=99s ethos of decentralized power.=

Bitcoin is designed to create an asymmetric advantage for DEFENDING= one's wealth. It's supposed to be impractically expensive for atta= ckers to crack the entropy and cryptography protecting one's coins. But= now we find ourselves discussing a situation where this asymmetric advanta= ge is compromised in favor of a specific class of attackers.

Economic Disruption
Large-scale theft from vulnerable a= ddresses could crash Bitcoin=E2=80=99s price as quantum recovered funds are= dumped on exchanges. This would harm all holders, not just those directly = targeted, leading to broader financial chaos in the markets.

Moral Responsibility
Permitting theft via quantum compu= ting sets a precedent that technological superiority justifies unethical be= havior. This is essentially taking a "code is law" stance in whic= h we refuse to admit that both code and laws can be modified to adapt to pr= eviously unforeseen situations.

Burning of coins can certainly be co= nsidered a form of theft, thus I think it's worth differentiating the t= wo different thefts being discussed:

1. self-enriching & likely = malicious
2. harm prevention & not necessarily malicious

Both= options lack the consent of the party whose coins are being burnt or trans= ferred, thus I think the simple argument that theft is immoral becomes a wa= sh and it's important to drill down into the details of each.

Incentives Drive Security
I can tell you from a de= cade of working in Bitcoin security - the average user is lazy and is a pro= crastinator. If Bitcoiners are given a "drop dead date" after whi= ch they know vulnerable funds will be burned, this pressure accelerates the= adoption of post-quantum cryptography and strengthens Bitcoin long-term. A= llowing vulnerable users to delay upgrading indefinitely will result in mor= e laggards, leaving the network more exposed when quantum tech becomes avai= lable.

Steel Manning
Clearly this is a co= mplex and controversial topic, thus it's worth thinking through the opp= osing arguments.

Protecting Property Rights<= br>Allowing quantum computers to take vulnerable bitcoin could potentially = be spun as a hard money narrative - we care so greatly about not violating = someone's access to their coins that we allow them to be stolen!
But I think the flip side to the property rights narrative is that burning= vulnerable coins prevents said property from falling into undeserving hand= s. If the entire Bitcoin ecosystem just stands around and allows quantum ad= versaries to claim funds that rightfully belong to other users, is that rea= lly a "win" in the "protecting property rights" categor= y? It feels more like apathy to me.

As such, I think the "prote= cting property rights" argument is a wash.

Qua= ntum Computers Won't Attack Bitcoin
There is a great deal of = skepticism that sufficiently powerful quantum computers will ever exist, so= we shouldn't bother preparing for a non-existent threat. Others have a= rgued that even if such a computer was built, a quantum attacker would not = go after bitcoin because they wouldn't want to reveal their hand by doi= ng so, and would instead attack other infrastructure.

It's quite= difficult to quantify exactly how valuable attacking other infrastructure = would be. It also really depends upon when an entity gains quantum supremac= y and thus if by that time most of the world's systems have already bee= n upgraded. While I think you could argue that certain entities gaining qua= ntum capability might not attack Bitcoin, it would only delay the inevitabl= e - eventually somebody will achieve the capability who decides to use it f= or such an attack.

Quantum Attackers Would Only Ste= al Small Amounts
Some have argued that even if a quantum attacker= targeted bitcoin, they'd only go after old, likely lost P2PK outputs s= o as to not arouse suspicion and cause a market panic.

I'm not s= o sure about that; why go after 50 BTC at a time when you could take 250,00= 0 BTC with the same effort as 50 BTC? This is a classic "zero day expl= oit" game theory in which an attacker knows they have a limited amount= of time before someone else discovers the exploit and either benefits from= it or patches it. Take, for example, the recent ByBit attack - the highest= value crypto hack of all time. Lazarus Group had compromised the Safe wall= et front end JavaScript app and they could have simply had it reassign owne= rship of everyone's Safe wallets as they were interacting with their wa= llet. But instead they chose to only specifically target ByBit's wallet= with $1.5 billion in it because they wanted to maximize their extractable = value. If Lazarus had started stealing from every wallet, they would have b= een discovered quickly and the Safe web app would likely have been patched = well before any billion dollar wallets executed the malicious code.

= I think the "only stealing small amounts" argument is strongest f= or Situation #2 described earlier, where a quantum attacker arrives before = quantum safe cryptography has been deployed across the Bitcoin ecosystem. B= ecause if it became clear that Bitcoin's cryptography was broken AND th= ere was nowhere safe for vulnerable users to migrate, the only logical opti= on would be for everyone to liquidate their bitcoin as quickly as possible.= As such, I don't think it applies as strongly for situations in which = we have a migration path available.

The 21 Million = Coin Supply Should be in Circulation
Some folks are arguing that = it's important for the "circulating / spendable" supply to be= as close to 21M as possible and that having a significant portion of the s= upply out of circulation is somehow undesirable.

While the "21M= BTC" attribute is a strong memetic narrative, I don't think anyon= e has ever expected that it would all be in circulation. It has always been= understood that many coins will be lost, and that's actually part of t= he game theory of owning bitcoin!

And remember, the 21M number in an= d of itself is not a particularly important detail - it's not even ment= ioned in the whitepaper. What's important is that the supply is well kn= own and not subject to change.

Self-Sovereignty and= Personal Responsibility
Bitcoin=E2=80=99s design empowers indivi= duals to control their own wealth, free from centralized intervention. This= freedom comes with the burden of securing one's private keys. If quant= um computing can break obsolete cryptography, the fault lies with users who= didn't move their funds to quantum safe locking scripts. Expecting the= network to shield users from their own negligence undermines the principle= that you, and not a third party, are accountable for your assets.

I= think this is generally a fair point that "the community" doesn&= #39;t owe you anything in terms of helping you. I think that we do, however= , need to consider the incentives and game theory in play with regard to qu= antum safe Bitcoiners vs quantum vulnerable Bitcoiners. More on that later.=

Code is Law
Bitcoin operates on transpar= ent, immutable rules embedded in its protocol. If a quantum attacker uses s= uperior technology to derive private keys from public keys, they=E2=80=99re= not "hacking" the system - they're simply following what'= ;s mathematically permissible within the current code. Altering the protoco= l to stop this introduces subjective human intervention, which clashes with= the objective, deterministic nature of blockchain.

While I tend to = agree that code is law, one of the entire points of laws is that they can b= e amended to improve their efficacy in reducing harm. Leaning on this point= seems more like a pro-ossification stance that it's better to do nothi= ng and allow harm to occur rather than take action to stop an attack that w= as foreseen far in advance.

Technological Evolution= as a Feature, Not a Bug
It's well known that cryptography te= nds to weaken over time and eventually break. Quantum computing is just the= next step in this progression. Users who fail to adapt (e.g., by adopting = quantum-resistant wallets when available) are akin to those who ignored tec= hnological advancements like multisig or hardware wallets. Allowing quantum= theft incentivizes innovation and keeps Bitcoin=E2=80=99s ecosystem dynami= c, punishing complacency while rewarding vigilance.

Market Signals Drive Security
If quantum attackers start stealin= g funds, it sends a clear signal to the market: upgrade your security or lo= se everything. This pressure accelerates the adoption of post-quantum crypt= ography and strengthens Bitcoin long-term. Coddling vulnerable users delays= this necessary evolution, potentially leaving the network more exposed whe= n quantum tech becomes widely accessible. Theft is a brutal but effective t= eacher.

Centralized Blacklisting Power
Bu= rning vulnerable funds requires centralized decision-making - a soft fork t= o invalidate certain transactions. This sets a dangerous precedent for futu= re interventions, eroding Bitcoin=E2=80=99s decentralization. If quantum th= eft is blocked, what=E2=80=99s next - reversing exchange hacks? The system = must remain neutral, even if it means some lose out.

I think this co= uld be a potential slippery slope if the proposal was to only burn specific= addresses. Rather, I'd expect a neutral proposal to burn all funds in = locking script types that are known to be quantum vulnerable. Thus, we coul= d eliminate any subjectivity from the code.

Fairnes= s in Competition
Quantum attackers aren't cheating; they'= re using publicly available physics and math. Anyone with the resources and= foresight can build or access quantum tech, just as anyone could mine Bitc= oin in 2009 with a CPU. Early adopters took risks and reaped rewards; quant= um innovators are doing the same. Calling it =E2=80=9Cunfair=E2=80=9D ignor= es that Bitcoin has never promised equality of outcome - only equality of o= pportunity within its rules.

I find this argument to be a mischaract= erization because we're not talking about CPUs. This is more akin to ta= lking about ASICs, except each ASIC costs millions if not billions of dolla= rs. This is out of reach from all but the wealthiest organizations.

= Economic Resilience
Bitcoin has weathered thefts= before (MTGOX, Bitfinex, FTX, etc) and emerged stronger. The market can ab= sorb quantum losses, with unaffected users continuing to hold and new entra= nts buying in at lower prices. Fear of economic collapse overestimates the = impact - the network=E2=80=99s antifragility thrives on such challenges.
This is a big grey area because we don't know when a quantum compu= ter will come online and we don't know how quickly said computers would= be able to steal bitcoin. If, for example, the first generation of suffici= ently powerful quantum computers were stealing less volume than the current= block reward then of course it will have minimal economic impact. But if t= hey're taking thousands of BTC per day and bringing them back into circ= ulation, there will likely be a noticeable market impact as it absorbs the = new supply.

This is where the circumstances will really matter. If a= quantum attacker appears AFTER the Bitcoin protocol has been upgraded to s= upport quantum resistant cryptography then we should expect the most valuab= le active wallets will have upgraded and the juiciest target would be the 3= 1,000 BTC in the address 12ib7dApVFvg82TXKycWBNpN8kFyiAN1dr which has been = dormant since 2010. In general I'd expect that the amount of BTC re-ent= ering the circulating supply would look somewhat similar to the mining emis= sion curve: volume would start off very high as the most valuable addresses= are drained and then it would fall off as quantum computers went down the = list targeting addresses with less and less BTC.

Why is economic imp= act a factor worth considering? Miners and businesses in general. More coin= s being liquidated will push down the price, which will negatively impact m= iner revenue. Similarly, I can attest from working in the industry for a de= cade, that lower prices result in less demand from businesses across the en= tire industry. As such, burning quantum vulnerable bitcoin is good for the = entire industry.

Practicality & Neutrality of N= on-Intervention
There=E2=80=99s no reliable way to distinguish = =E2=80=9Ctheft=E2=80=9D from legitimate "white hat" key recovery.= If someone loses their private key and a quantum computer recovers it, is = that stealing or reclaiming? Policing quantum actions requires invasive ass= umptions about intent, which Bitcoin=E2=80=99s trustless design can=E2=80= =99t accommodate. Letting the chips fall where they may avoids this mess.
Philosophical Purity
Bitcoin rejects bailo= uts. It=E2=80=99s a cold, hard system where outcomes reflect preparation an= d skill, not sentimentality. If quantum computing upends the game, that=E2= =80=99s the point - Bitcoin isn=E2=80=99t meant to be safe or fair in a nan= ny-state sense; it=E2=80=99s meant to be free. Users who lose funds to quan= tum attacks are casualties of liberty and their own ignorance, not victims = of injustice.

Bitcoin's DAO Moment
Th= is situation has some similarities to The DAO hack of an Ethereum smart con= tract in 2016, which resulted in a fork to stop the attacker and return fun= ds to their original owners. The game theory is similar because it's a = situation where a threat is known but there's some period of time befor= e the attacker can actually execute the theft. As such, there's time to= mitigate the attack by changing the protocol.

It also created a sch= ism in the community around the true meaning of "code is law," re= sulting in Ethereum Classic, which decided to allow the attacker to retain = control of the stolen funds.

A soft fork to burn vulnerable bitcoin = could certainly result in a hard fork if there are enough miners who reject= the soft fork and continue including transactions.

Incentives Matter
We can wax philosophical until the cows come h= ome, but what are the actual incentives for existing Bitcoin holders regard= ing this decision?

"Lost coins only make everyone else's coins worth slightly more. = Think of it as a donation to everyone." - Satoshi Nakamoto
If true, the corollary is:

"Quantum recovered coins only make everyone else's c= oins worth less. Think of it as a theft from everyone." - Jameson Lopp=

Thus, assuming we get to a point where quantum resistant s= ignatures are supported within the Bitcoin protocol, what's the incenti= ve to let vulnerable coins remain spendable?

* It's not good for= the actual owners of those coins. It disincentivizes owners from upgrading= until perhaps it's too late.
* It's not good for the more atten= tive / responsible owners of coins who have quantum secured their stash. Al= lowing the circulating supply to balloon will assuredly reduce the purchasi= ng power of all bitcoin holders.

Forking Game Theor= y
From a game theory point of view, I see this as incentivizing u= sers to upgrade their wallets. If you disagree with the burning of vulnerab= le coins, all you have to do is move your funds to a quantum safe signature= scheme. Point being, I don't see there being an economic majority (or = even more than a tiny minority) of users who would fight such a soft fork. = Why expend significant resources fighting a fork when you can just move you= r coins to a new address?

Remember that blocking spending of certain= classes of locking scripts is a tightening of the rules - a soft fork. As = such, it can be meaningfully enacted and enforced by a mere majority of has= hpower. If miners generally agree that it's in their best interest to b= urn vulnerable coins, are other users going to care enough to put in the ef= fort to run new node software that resists the soft fork? Seems unlikely to= me.

How to Execute Burning
In order to b= e as objective as possible, the goal would be to announce to the world that= after a specific block height / timestamp, Bitcoin nodes will no longer ac= cept transactions (or blocks containing such transactions) that spend funds= from any scripts other than the newly instituted quantum safe schemes.
=
It could take a staggered approach to first freeze funds that are susce= ptible to long-range attacks such as those in P2PK scripts or those that ex= posed their public keys due to previously re-using addresses, but I expect = the additional complexity would drive further controversy.

How long = should the grace period be in order to give the ecosystem time to upgrade? = I'd say a minimum of 1 year for software wallets to upgrade. We can onl= y hope that hardware wallet manufacturers are able to implement post quantu= m cryptography on their existing hardware with only a firmware update.
<= br>Beyond that, it will take at least 6 months worth of block space for all= users to migrate their funds, even in a best case scenario. Though if you = exclude dust UTXOs you could probably get 95% of BTC value migrated in 1 mo= nth. Of course this is a highly optimistic situation where everyone is comp= letely focused on migrations - in reality it will take far longer.

R= egardless, I'd think that in order to reasonably uphold Bitcoin's c= onservatism it would be preferable to allow a 4 year migration window. In t= he meantime, mining pools could coordinate emergency soft forking logic suc= h that if quantum attackers materialized, they could accelerate the countdo= wn to the quantum vulnerable funds burn.

Random Tan= gential Benefits
On the plus side, burning all quantum vulnerable= bitcoin would allow us to prune all of those UTXOs out of the UTXO set, wh= ich would also clean up a lot of dust. Dust UTXOs are a bit of an annoyance= and there has even been a recent proposal for how to incentivize cleaning = them up.

We should also expect that incentivizing migration of the e= ntire UTXO set will create substantial demand for block space that will sus= tain a fee market for a fairly lengthy amount of time.

In Summary
While the moral quandary of violating any of Bitco= in's inviolable properties can make this a very complex issue to discus= s, the game theory and incentives between burning vulnerable coins versus a= llowing them to be claimed by entities with quantum supremacy appears to be= a much simpler issue.

I, for one, am not interested in rewarding qu= antum capable entities by inflating the circulating money supply just becau= se some people lost their keys long ago and some laggards are not upgrading= their bitcoin wallet's security.

We can hope that this scenario= never comes to pass, but hope is not a strategy.

I welcome your fee= dback upon any of the above points, and contribution of any arguments I fai= led to consider.

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