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Mon, 22 Jun 2026 04:49:56 -0700 (PDT) X-Received: by 2002:a05:6808:c2cb:b0:46e:c1cd:866a with SMTP id 5614622812f47-4896ac00afamr11454615b6e.27.1782128996883; Mon, 22 Jun 2026 04:49:56 -0700 (PDT) Received: by 2002:a05:690c:6602:b0:7fd:3980:ebd6 with SMTP id 00721157ae682-80101ab8203ms7b3; Mon, 22 Jun 2026 04:33:45 -0700 (PDT) X-Received: by 2002:a05:690c:968a:b0:7fd:e629:2a6b with SMTP id 00721157ae682-80133503bb0mr143592757b3.31.1782128023876; Mon, 22 Jun 2026 04:33:43 -0700 (PDT) Date: Mon, 22 Jun 2026 04:33:43 -0700 (PDT) From: waxwing/ AdamISZ To: Bitcoin Development Mailing List Message-Id: <2acb0488-6016-4ae3-b459-0bec5bee9d78n@googlegroups.com> In-Reply-To: References: <_z6_JUmphCkUYvI6gemSFMD9Sb_rDL03IQbtZQCNlk6pmioGEQBir_gMyZCfticFa8Ttfc0xoFHdxR07_MNuAfBu8do_h5IDf2apVk1w1BM=@wuille.net> <81QWqov2xqthGLjORSKtR2jDDosHG7gjK91LNQ61iIBNRQaJPsu6wTA63d3KjVdROO2VZy5zr3Buo5A8UXb3Ue5E4zc2qWYn65gRxmmFLKg=@wuille.net> Subject: Re: [bitcoindev] Aligning privacy incentives in P2MR MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_376588_877775687.1782128023399" X-Original-Sender: ekaggata@gmail.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: -0.5 (/) ------=_Part_376588_877775687.1782128023399 Content-Type: multipart/alternative; boundary="----=_Part_376589_62003666.1782128023399" ------=_Part_376589_62003666.1782128023399 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable > It is a common misconception that we can use toy curves as canaries.=20 Unfortunately we can't trust succinct proofs on small curves because they= =20 could be classically forgeable. Pollard-Rho can break ECDLP for points of= =20 order n=E2=80=8B with only sqrt(n)=E2=80=8B work today. For example, in 201= 6 these=20 researchers broke a 117-bit binary-field curve using FPGAs=20 , and this paper's=20 authors broke a 112-bit prime-field curve using a cluster of 200=20 playstations !=20 This hinges on what you consider 'toy'. Since there are good O(sqrt(n))=20 algos, a 128 bit curve is not really secure (you're in the range where=20 non-trivial endomorphisms and other technical tricks could make a=20 difference, but anyway it's pretty clearly within 'state actor' range). But= =20 that maybe is not a valid basis for dismissing even 160 bit curves (80 bits= =20 seems out of range?), let alone the 192 bit curves that have been=20 standardized in the past. The more interesting question for me is whether just looking for=20 lower-entropy secrets in secp256k1 (i.e. the canary explicitly tells the=20 attackers that the discrete log < t << n), or alternatively looking for=20 properly chosen secrets from a smaller curve, is equivalent, from the point= =20 of view of Shor. As per my "is it =E2=80=9Cthe same=E2=80=9D to solve a 192= bit secret on=20 secp, as to solve a same-size secret on secp192r1" question from [1]. (I=20 actually wrote "secp" not "secp256k1" or "r1", I forgot that we are talking= =20 about 2 different curve equations; that's another knot to untangle; is it= =20 only the size of the group that matters? seems like the curve eqn shouldn't= =20 matter here). These questions are easy to ask but maybe quite hard to=20 answer! I think canaries are unlikely to be useful, but only 'moderately unlikely',= =20 such that creating them may not be a total waste of time. Regards, AdamISZ/waxwing [1] https://delvingbitcoin.org/t/qcap-a-bitcoin-native-quantum-canary-alert= /2498/6 On Thursday, June 18, 2026 at 9:41:01=E2=80=AFPM UTC-3 conduition wrote: > Now I have some points I'd like to respond to about rescue protocols. Thi= s=20 > is largely tangential to the P2MR/P2TRv2 question. > > In either case, I consider anything that requires hardcoding specific=20 > wallet designs (BIP32 or otherwise) into Bitcoin's consensus rules (and= =20 > only allowing those coins to survive) to be squarely in disaster-recovery= =20 > land. It feels like embracing arbitrariness, and giving up on the=20 > permissionlessness that makes Bitcoin interesting > > It's fair to say such a solution is arbitrary, insofar as the knowledge= =20 > asymmetries like BIP32 CKD are arbitrary. But can you propose any=20 > alternatives which are not arbitrary? > > I would not count *hoping for majority migration* to be a valid solution.= =20 > We ought to consider and plan for the contingency that migration to the P= Q=20 > output type is underwhelming, even if that is not the *best* outcome,=20 > because such an outcome is still likely even if the new output type were= =20 > magically 10x cheaper and more private than P2TR. > > My impression is that your approach is to have an answer for many possibl= e=20 > futures, including ones where Q-day arrives within just a few years. But= =20 > optimizing for disaster-recovery also means reducing the chances of=20 > preserving Bitcoin as we know it in the scenarios where a successful=20 > migration is still possible. And if Q-day does arrive that soon, I don't= =20 > see what we can do today that would preserve Bitcoin in a form we care=20 > about anyway. By accepting that, we can focus on the futures where our=20 > choices today can still materially improve the outcome. > > Mostly yes. I gotta agree with AJ's quote here. In chess, one wins by=20 > forcing the worst possible outcome to be victory. In probabilistic games,= =20 > one wins by maximizing the chance of success and minimizing the cost of= =20 > failure. We should be doing the same thing here. > > A quantum "disaster recovery" scenario, as you put it, is absolutely wort= h=20 > optimizing for IMO, because with our current trajectory it seems very=20 > plausible, and yet its severe consequences can be mostly mitigated at=20 > little cost. I don't think we should abandon such survivable scenarios ju= st=20 > because they're less palatable than others. > > I dispute that rescue protocols would discourage PQ migration. Quite the= =20 > opposite: They *force* PQ migration even for inactive users, by turning= =20 > vulnerable UTXOs into PQ-safe ones retroactively. > > Even if rescue protocols do massively discourage migration to PQ outputs,= =20 > this would be of little consequence *exactly because rescue protocols=20 > would** exist* (which is their beauty). Even in the most extreme case=20 > where we deploy a PQ output type and exactly *nobody* migrates to it, we= =20 > can still use rescue protocols to achieve the same rate of=20 > ownership-preservation as if all quantum-recoverable coins (BIP32, hashed= =20 > addr, etc) had migrated to PQ outputs *immediately*. It'll be less=20 > efficient and more awkward, but far from hopeless. > > Again, the real target demographic that we need to focus on migrating is= =20 > the *quantum unrecoverable subset*, i.e. coins which have no quantum-hard= =20 > knowledge asymmetry, like satoshi's coins, P2PK wallets, and JBOK wallets= =20 > with exposed pubkeys. If we expect to have rescue protocols, then we don'= t=20 > really care what address format these coins move to, as long as it's to= =20 > *something* quantum-recoverable. But convincing them to move at all seems= =20 > hard, because their owners are probably dead or the keys are lost. How to= =20 > handle such coins after Q-day is an open question, and it seems to boil= =20 > down to the good old "freeze-or-steal" debate.=20 > > If Q-day comes before migration can reach meaningful levels (what those= =20 > are is probably a matter of perspective), then I think there isn't much o= f=20 > an interesting future for Bitcoin anyway. > > The future is only as interesting as we make it. Even if Q-Day comes=20 > before most coins migrate, we have an opportunity to make that future=20 > interesting, using rescue protocols. Maybe some folks don't want to work = on=20 > them or would rather sell their coins first, and that's totally valid, bu= t=20 > I implore you to at least keep your mind open to the possible futures tha= t=20 > rescue protocols open up, because it is unclear exactly which future we a= re=20 > bound for and I'd rather that neutral internet money still exists in each= =20 > of them. > > We have to play the cards we are dealt, and right now we've been dealt a= =20 > very fortunate hand because of BIP32 (thanks to you sipa!): Most users=20 > have at least *some* secret knowledge that a CRQC cannot forge, and=20 > that's a pretty awesome privilege, one which even extends to other=20 > blockchains whose wallets use BIP32, or its derivatives. You should be ve= ry=20 > proud of that IMO =F0=9F=99=82 > > > Disaster-recovery: neither the "predictable/planned consensus change" of = Later=20 > nor the "everyone takes responsiblity for their own funds" works, and the= =20 > only way to avoid a large percentage of bitcoin becoming a reward for=20 > quantum research is to replace EC spend paths with a ZK proof of knowledg= e=20 > of a BIP32 seed or similar, requiring a hard fork. > > @AJ rescue protocols can be deployed via soft fork, because we'd be=20 > constricting rules and not relaxing them. We'd require any EC spends to= =20 > *also* satisfy the rescue protocol in addition to the existing consensus= =20 > rules (signatures etc). Of course, there might be external reasons to=20 > deploy it as a hard fork, e.g. to roll back mass theft if we don't time= =20 > things right, or to disentangle from quantum-theft advocates, but I think= =20 > we should aim for soft fork compatibility if possible. > > > the "quantum-safe" hard-fork variant of bitcoin would be fairly described= =20 > as a utxo-bootstrapped altcoin, would compete in the market with the=20 > "quantum-unsafe" bitcoin thatexisting nodes would continue to follow, and= =20 > possibly there would be many slightly varying such altcoins competing wit= h=20 > each other, eg on exactly what height the utxo snapshot was taken or what= =20 > coins become spendable. It would not be a fun time for holders of affecte= d=20 > utxos. > > I really hope we do not end up in a situation with competing rescue=20 > protocol deployments. I don't think snapshots are necessary so that's not= =20 > really a factor. > As for the exact conditions which permit rescue... that could indeed get= =20 > dicey if we need to collate a bunch of different rescue protocols into on= e=20 > Frankensteinian mess. The most popular proposal will probably be one whic= h=20 > covers all the most common hard-relations, but this still needs more=20 > research. > If we think of rescue protocols more abstractly, the spender just needs t= o=20 > prove their script pubkey commited to a quantum-hard relation, and that t= he=20 > spender knows the witness to that relation. It doesn't actually matter=20 > which exact relation - BIP32, hashed address, musig, whatever - just that= =20 > it's quantum-hard. Maybe there is a way to formulate a two-step proof=20 > system general enough to rescue UTXOs with any such relation, by first=20 > proving the relation is quantum-hard, then proving you know the witness t= o=20 > the relation. That would be awesome but still an open problem. > > regards, > conduition > > > On Wednesday, June 17th, 2026 at 11:20 PM, Anthony Towns=20 > a...@erisian.com.au wrote: > > On Tue, Jun 16, 2026 at 08:09:08PM +0000, Pieter Wuille wrote: > > I want to first correct a potential misunderstanding here, because > I realize the terms "Later" and "Never" aren't very descriptive. They > are specifically about an expected and relied-upon expectation that an > EC-disabling fork will happen that at least applies to the output type > itself, in time. "Later" is the expectation that such a disabling will > happen after the output type is introduced, but still in time (so, before > Q-day). Outputs without a strong expectation that their EC paths/opcodes > will be disabled, or not in time, I classify under "Never". > > I believe here you're instead arguing for P2MR ("Merkle-Never") > over all "Later" options. That was my previous point: I think (solely) > having "Never" output types like P2MR is just utterly insufficient for > any worthwhile migration. It's so incompatible with today's workflows > that it either won't be adopted, or (possibly inadvertently) adopted > in an insecure fashion. Yes, it gives people the option to safeguard > their own coins, but to me that's disaster recovery territory - I think > we ought to prioritize maximizing the chances for saving the currency > as a whole in case Q-day comes, not a small subset of individual users' > coins. P2MR (alone) doesn't really achieve much in that regard in my view= , > thus we at least need something of the "Later" class in addition. > > I'm not sure I follow/agree with the logic here. I think the general idea > is: > > 1) we create some new address types that allow post-quantum spending, but > also allow efficient quantum-vulnerable spending that can be used prior > to Q-day > > 2) many people migrate to these new address types > > 3) Q-day arrives > > 4) all spending goes via the post-quantum paths, and everyone's funds are > safe > > There are three main variations to this, I think: > > Later-dominant: towards the end of (2) but prior to (3), the > quantum-vulnerable spend paths are disabled in a predictable, planned > manner, preventing coin theft, but not disrupting active usage > significantly (or not disrupting it any more than the proximity of > Q-day already is). > > Self-reliance: Q-day goes from maybe to definitely faster than consensus > changes can be coordinated, and the only reason people's funds remain > safe is that they can (and do) keep the quantum-vulnerable spend > paths secret. > > Disaster-recovery: neither the "predictable/planned consensus change" of > Later nor the "everyone takes responsiblity for their own funds" > works, and the only way to avoid a large percentage of bitcoin > becoming a reward for quantum research is to replace EC spend paths > with a ZK proof of knowledge of a BIP32 seed or similar, requiring > a hard fork. Such a hard fork would be certain to be controversial > ("why at this height: I received a payment five blocks after // > my funds were stolen by a quantum attacker five blocks earlier // > why are only BIP32 funds recoverable not scheme X"), but if no other > approach works, might still be the ultimately outcome. > > So the point here was just: if you already accept the need for a "Later" > output type (=3D one with builtin-in EC disabling expected from the start= ), > P2TRv2 is preferable over P2MR+ED, because: > > As far as I can see, only having P2TRv2 addresses would rule out the > "self-reliance" path here. > > Picking when Q-day will occur, either individually for determining > your own security posture, or collectively for organising a consensus > change seems very difficulty: it involves evaluating both complex state > of the art physics research, but also estimating the covert abilities > of national governments and the incentives to attack bitcoin prior to > revealing their capabilities. To me, that doesn't bode well for a smooth > and fast deployment of a consensus change in advance of problems occuring= . > > It's possible that general carelessness on behalf of users would also > rule out the effectiveness of a self-reliance approach: if only the most > conscientious 1% of users make use of P2MR securely, that might secure 10= % > of funds, but having 90% of the bitcoin supply crash probably wouldn't be > very valuable either. But even then, that may make the "disaster-recovery= " > approach less problematic, by ensuring the 1%/10% who were conscientious > can avoid being part of the "my funds were stolen by a quantum attacker" > contingent, eg. > > Theorycrafting for a second here. It's reasonable to conjecture fee > rates will be much higher post-Q-day, and thus P2MR's 32 byte advantage > over P2TRv2 will yield significant savings for end-users in absolute > terms. If fee rates inflate 10x higher after Q-day, then 8 vbytes becomes > significant (100 sats per spend or more). > > I don't think that is the right way to look at. 8vb/input is about > an additional 14% today (vs a taproot key-path spend), but with the > post-quantum signatures we have available now that's likely to reduce > to ~7% (SHRINCS) or ~1% (winternitz). So, post-Q-day, by dropping 32B, > you're only getting an expected savings in fees / increase in block > capacity on that order of magnitude, ie: 1%-7%. That's nice to have, > for sure, but doesn't compare to introducing a new address type that > puts the PQ sigs in an extension block, or one that uses ZK proofs to > do cross-input or cross-transaction signature aggregation, eg. So a 32B > witness overhead for an unused/unusable key-path post-Q-day doesn't seem > very relevant to me. > > FWIW, I don't think the P2TRv2 EC-disabling fork needs to be timed > perfectly. The expectation should be just that it happens before Q-day, > and when it looks inevitable or the fear about that is large enough. > > FWIW, I would define "timed perfectly" precisely as "EC disabling > fork happens before Q-day". Maybe allowing some additional months of > EC-efficiency would be a win while not taking out too much migration time= , > but for me "perfection" here means "no one who upgraded lost money via > quantum-related vulnerabilities". > > One reason I'm doubtful is that I think for some the "it looks inevitable= " > threshold has already been crossed, eg: > > So let me attempt to partially fill the silence, similarly to what > Scott Aaronson did in his April 29 post. Given everything I know, > including scary non-public information, I now put the odds of qday by > 2032 at 50%. 10% by 2030. > > IMO a good target date for migration is 2029, roughly 3.5 years > out. 2029 happens to be the date selected by Google, Cloudflare, and > the Ethereum Foundation. > > https://x.com/drakefjustin/status/2061793725299224676 > > So, here it is: if quantum computers start breaking cryptography a > few years from now, don=E2=80=99t you dare come to this blog and tell me = that > I failed to warn you. This post is your warning. Please start switching > to quantum-resistant encryption, and urge your company or organization > or blockchain or standards body to do the same. > > https://scottaaronson.blog/?p=3D9718 > > Personally, that leaves me at a minimum very skeptical of the utility > of introducing either P2MR or P2TRv2 (etc) approaches that don't also > introduce a quantum-safe spending path on day one. > > First a meta-point here: the reason I like separating the discussion into= =20 > different dimensions than just "P2TRv2 vs P2MR" is because there are more= =20 > options than those two, and not every argument applies to the same=20 > separation into two classes. Let me list them explicitly here, roughly in= =20 > decreasing order of how strongly I feel about them: > - We need at least a "Later" option for meaningful migration, i.e. P2TRv2= =20 > or P2MR+ED. > - Within the "Later" option, P2TRv2 is preferable. > - A "Later" option benefits from being the only PQC migration strategy,= =20 > making it a Schelling point. > > Correct me if I'm mistaken, but I think P2TRv2 is preferable only in the > "Later-dominant" scenario, and only to the degree that it's slightly > cheaper prior to Q-day. If it were the only available option, that would > increase the risk of loss involved with both the other approaches. (I > don't think P2TRv2 is meaningfully more private than P2MR in the way > taproot v1 is, as presumably you'd only adopt that address format if > you wanted to have a post-quantum script path) > > You'd have to convince everyone to deploy and then adopt P2TRv2 today=20 > under the public knowledge that it is insecure and their coins are more= =20 > likely to be stolen. That's a hard sell. > > How does one pitch P2TRv2 to users? "It will be quantum secure one day we= =20 > promise because everyone is incentivized to fix it later as Bitcoin will= =20 > die if we don't." > > How do you pitch P2MR? "It's quantum secure if you use it correctly." > To me, the pitch is "Bitcoin can only remain valuable if we mostly/all=20 > migrate." for both. P2TRv2 is just much easier to adopt, because P2MR (or= =20 > any "Never" output type) fundamentally requires many users to change thei= r=20 > workflows entirely. > > Let's call NoEC-day the earlier of activation of a soft-fork disabling > EC-spends on P2MR/P2TRv2 or Q-day. NoEC-day to some extent is presumably > equal to "the day the bitcoin ecosystem has finally agreed that CRQCs > are inevitable". > > My (cynical?) view is the only people who will adopt either P2TRv2 or > P2MR prior to NoEC-day being schedule will be people who are willign > to use those features in a quantum-safe way -- that is, keeping their > EC pubkeys secret, and only revealing those EC pubkeys to spend them > immediately, prior to NoEC-day. In that view, the EC-spend-paths of such > coins prior to NoEC-day are only an opportunistic "make spends cheaper" > shortcut, they don't allow the funds to be used in lightning channels > or to let your wallet be audited via sharing an xpub or anything similar. > > Perhaps I'm wrong: it's my opinion, not a technical fact; it's possible > that lightning software could get an upgrade to generate post-quantum > signatures for channel commitments or HTLC claims, I just think it's > pretty unlikely that that will happen at a meaningful scale when everyone > has much more immediate and less theoretical things to work on prior to > NoEC-day, especially when the efficiency/performance of such changes is > likely to be very low. > > This focus on allowing individual users the ability to safeguard their > coins just feels like a red herring: [...] > > While I appreciate your point, I also feel that "allowing individual > users the ability to safeguard their coins" is pretty close to the entire > point of Bitcoin. :) > > In either case, I consider anything that requires hardcoding > specific wallet designs (BIP32 or otherwise) into Bitcoin's consensus > rules (and only allowing those coins to survive) to be squarely in > disaster-recovery land. It feels like embracing arbitrariness, and > giving up on the permissionlessness that makes Bitcoin interesting - > if the community shows it can get consensus on effectively burning > coins except those that match a whitelist, it feels hard to maintain > censorship-freeness as a value, even if the whitelist includes most of > the (active) coins. That is of course not to say such techniques aren't > interesting to work on, but to me, their place is in disaster recovery > scenarios to save what's left, after Q-day, if migration attempts were > unsuccessful. In such a setting, I think we're already in effectively an > altcoin-with-UTXO-bootstrapped-from-Bitcoin territory, and a (possibly > growing) set of ways to recover burned coins can be hardforked in. > > Just for the record, I think the above is an accurate description of the > "disaster-recovery" scenario above: the "quantum-safe" hard-fork variant > of bitcoin would be fairly described as a utxo-bootstrapped altcoin, > would compete in the market with the "quantum-unsafe" bitcoin that > existing nodes would continue to follow, and possibly there would be > many slightly varying such altcoins competing with each other, eg on > exactly what height the utxo snapshot was taken or what coins become > spendable. It would not be a fun time for holders of affected utxos. > > My impression is that your approach is to have an answer for many > possible futures, including ones where Q-day arrives within just a few > years. > > "The key of strategy... is not to choose a path to victory, but to choose > so that all paths lead to a victory." > > -- https://tvtropes.org/pmwiki/pmwiki.php/Main/XanatosGambit > > But optimizing for disaster-recovery also means reducing the > chances of preserving Bitcoin as we know it in the scenarios where a > successful migration is still possible. And if Q-day does arrive that > soon, I don't see what we can do today that would preserve Bitcoin in > a form we care about anyway. By accepting that, we can focus on the > futures where our choices today can still materially improve the outcome. > > Preserving bitcoin as a personally-possessible inflation resistant > store of value seems both possible and worth caring about, even if other > constraints means that many people can't afford to personally hold it > (and have to go through ETFs/exchanges/banks) and that it can't be used > for day-to-day transactions. Would be very disappointing if true, and > even given Q-day in a few years I expect we could do better than just > that, but it doesn't feel like a throw-in-the-towel event to me. > > Essentially yes though, I expect the majority of holders will probably > migrate to PQ addresses via rescue protocols, either on Bitcoin or a fork > thereof. Even if we can't come to consensus and deploy a new output type, > we'll still be able to rescue most coins. It's just that we'd have nowher= e > to rescue them to, so we ought to make PQ wallets available soon, so we'r= e > not in a rush to build them later when we need them. If a PQ wallet can > use cheap EC signatures while they're still trustworthy, all the better > > FWIW, that's my guess on how things would play out if the near-term Q-day > timelines I've seen (ie, 2029 to 2035) match reality. I hope that's > pessimistic (either because the Q-day timelines are bad estimates, or > because migration happens in a more orderly fashion), but I guess we'll > see. I don't rate my ability to evaluate Q-day predictions very highly. > > - A (not-quite-CR)-QC breaks 128-bit ECDLP, say. > > I'm not in a position to judge, but the google paper claims: > > "Indeed, if a leading quantum architecture encounters and overcomes > all its scaling challenges before producing a device able to > solve (for example) 32-bit ECDLP, then there may be little time > between the breaking of 32-bit ECDLP and the breaking of 256-bit > ECDLP. Furthermore, the community should not expect to see published > demonstrations of the most advanced quantum error-correction > architectures and quantum algorithms deployed to cryptanalytic > problems. Thus, a successful public demonstration of Shor=E2=80=99s > algorithm on a 32-bit elliptic curve should not be seen as a wake-up > call to adopt PQC as much as a potential signal that PQC adoption > has already failed." > > and places the required tiffoli gates and logical qubits for a 32-bit > break at about (300k, 200) vs (10M, 600) for 128-bit or (80M, 1100) > for 256-bit. > > Of course, if you believe it's the only possible future, I understand > you'd come to a different conclusion. But is it really? Do you think > this isn't a plausible future: > > - A P2TRv2 type (let's leave aside whether P2MR or P2QR gets added too)= =20 > gets introduced in the next few years, with hash-based PQC opcodes. > - Over the course of the next decade or so, it gets adopted by practicall= y=20 > all active users. > > I think it might be better to look at that scenario in a more fine-graine= d > way? I think your "Late-ish" scenario is: > > 1) P2TRv2 (or whatever) is introduced > 2) Some PQ opcodes get enabled, allowing expensive but PQ-safe spend-path= s > via those outputs > 3) P2PK, P2PKH, P2WPKH, P2WSH, P2TR all become obsolete in favour of=20 > P2TRv2, > but EC spend paths continue to be what's used in practice. > 4) Some better PQ solutions become available, allowing cheap PQ-safe=20 > spend-paths > 5) Everyone switches from EC paths to the new PQ paths. > 6) NoEC-day happens, but no one is impacted. > > I think your "Soon-ish" scenario differs as of step (4): > > 4) NoEC-day happens. Massive disruption because the "what's used in=20 > practice" > path breaks, but everything is recoverable. > 5) Post-quantum approaches get even higher priority > > I'm skeptical of step 3 here; and would expect to see something more like= : > > 3') Only a small proportion of users (ie, the most conscientious/fearful) > switch to P2TRv2 with most preferring to stick with what works > > That has no real impact on the Late-ish scenario, but changes the Soon-is= h > scenario to either: > > 4'a) NoEC-day happens substantially before Q-day; people hurry to migrate > to P2TRv2, but it mostly works. > > or > > 4'b) NoEC-day happens essentially at the same time as Q-day; coins get > stolen and we hit the disaster-recovery scenario. > > Perhaps the difference between (3') and (3) playing out in reality > is just having nearly everyone agree that the upgrade is essential, > and rather than leaving it to self-interest/market-dynamics, having a > consistent push that every single wallet/service that doesn't deprecate > every current address type is a danger to the entire ecosystem, even > absent widespread agreement on when/if Q-day will happen. Arguably that > would be easier to agree on if the incremental cost of EC spend paths > in P2TRv2 prior to NoEC-day/Q-day versus current spend paths is near to > zero, rather than up to ~14% per input. > > Cheers, > aj > > -- > You received this message because you are subscribed to a topic in the=20 > Google Groups "Bitcoin Development Mailing List" group. > To unsubscribe from this topic, visit=20 > https://groups.google.com/d/topic/bitcoindev/p8AVEmAtWdA/unsubscribe. > To unsubscribe from this group and all its topics, send an email to=20 > bitcoindev+...@googlegroups.com. > To view this discussion visit=20 > https://groups.google.com/d/msgid/bitcoindev/ajN9be00Br-O3-9R%40erisian.c= om.au > . > > --=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/= 2acb0488-6016-4ae3-b459-0bec5bee9d78n%40googlegroups.com. ------=_Part_376589_62003666.1782128023399 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
> It is a common misconception that we can use toy curves as canari= es.=20 Unfortunately we can't trust succinct proofs on small curves because=20 they could be classically forgeable. Pollard-Rho can break ECDLP for=20 points of order n=E2=80=8B with only sqrt(n)=E2= =80=8B work today. For example,=C2=A0in 2016 these researchers broke a 117-bit binary-field curve using FPGAs<= /a>, and this paper's authors broke a 112-b= it prime-field curve using a cluster of 200 playstations!=C2=A0

This hinges on what you consider 'toy'. Since there are= good O(sqrt(n)) algos, a 128 bit curve is not really secure (you're in the= range where non-trivial endomorphisms and other technical tricks could mak= e a difference, but anyway it's pretty clearly within 'state actor' range).= But that maybe is not a valid basis for dismissing even 160 bit curves (80= bits seems out of range?), let alone the 192 bit curves that have been sta= ndardized in the past.

The more interesting ques= tion for me is whether just looking for lower-entropy secrets in secp256k1 = (i.e. the canary explicitly tells the attackers that the discrete log < = t << n), or alternatively looking for properly chosen secrets from a = smaller curve, is equivalent, from the point of view of Shor. As per my=C2= =A0"is it =E2=80=9Cthe same=E2=80=9D to solve a 192 bit secret on secp, as = to solve a same-size secret on secp192r1" question from [1]. (I actually wr= ote "secp" not "secp256k1" or "r1", I forgot that we are talking about 2 di= fferent curve equations; that's another knot to untangle; is it only the si= ze of the group that matters? seems like the curve eqn shouldn't matter her= e). These questions are easy to ask but maybe quite hard to answer!

I think canaries are unlikely to be useful, but only 'm= oderately unlikely', such that creating them may not be a total waste of ti= me.

Regards,
AdamISZ/waxwing

[1]=C2=A0https://delvingbitcoin.org/t/qcap-a-bitcoin-nati= ve-quantum-canary-alert/2498/6

On Thursday, June 18, 2026 at 9= :41:01=E2=80=AFPM UTC-3 conduition wrote:
<= span>Now I have some points I'd like to respond to about rescue protoco= ls. This is largely tangential to the P2MR/P2TRv2 question.

In eithe= r case, I consider anything that requires hardcoding specific wallet design= s (BIP32 or otherwise) into Bitcoin's consensus rules (and only allowin= g those coins to survive) to be squarely in disaster-recovery land. It feel= s like embracing arbitrariness, and giving up on the permissionlessness tha= t makes Bitcoin interesting

It's fair to say such a solution is arbitrary, insofar a= s the knowledge asymmetries like BIP32 CKD are arbitrary. But can you propo= se any alternatives which are not arbitrary?

I would not count=C2=A0hoping=C2=A0for majority migration=C2=A0to be a valid solution. We ought to consider and plan for = the contingency that migration to the PQ output type is=C2=A0underwhelming,= even if that is not the=C2=A0best=C2=A0ou= tcome, because such an outcome is still likely even if the new output type = were magically 10x cheaper and more private than P2TR.


My impression is t= hat your approach is to have an answer for many possible futures, including= ones where Q-day arrives within just a few years. But optimizing for disas= ter-recovery also means reducing the chances of preserving Bitcoin as we kn= ow it in the scenarios where a successful migration is still possible. And = if Q-day does arrive that soon, I don't see what we can do today that w= ould preserve Bitcoin in a form we care about anyway. By accepting that, we= can focus on the futures where our choices today can still materially impr= ove the outcome.

Mostly yes. I gotta agree with AJ's quote here. In chess, one w= ins by forcing the worst possible outcome to be victory. In probabilistic g= ames, one wins by maximizing the chance of success and minimizing the cost = of failure. We should be doing the same thing here.

A quantum "disaster recovery" scenario= , as you put it, is absolutely worth optimizing for IMO, because with our c= urrent trajectory it seems very plausible, and yet its severe consequences = can be mostly mitigated at little cost. I don't think we should abandon= such survivable scenarios just because they're less palatable than oth= ers.

I dispute that rescu= e protocols would discourage PQ migration. Quite the opposite: They= =C2=A0force=C2=A0PQ migration even for inactive = users, by turning vulnerable UTXOs into PQ-safe ones retroactively.

Even if rescue protocols do mass= ively discourage migration to PQ outputs, this would be of little consequen= ce exactly because rescue protocols would=C2=A0exist (which i= s their beauty). Even in the most extreme case where we deploy a PQ output = type and exactly=C2=A0nobody=C2=A0migrates= to it, we can still use rescue protocols to achieve the same rate of owner= ship-preservation as if all quantum-recoverable coins (BIP32, hashed addr, = etc) had migrated to PQ outputs=C2=A0immediately. It= 9;ll be less efficient and more awkward, but far from hopeless.

<= p style=3D"background-color:rgb(255,255,255)">Again, the real target demogr= aphic that we need to focus on migrating=C2=A0is the=C2=A0q= uantum unrecoverable subset, i.e. coins which have no quantum-hard know= ledge asymmetry, like satoshi's coins, P2PK wallets, and JBOK wallets w= ith exposed pubkeys. If we expect to have rescue protocols, then we don'= ;t really care what address format these coins move to, as long as it's= to=C2=A0something=C2=A0quantum-recoverabl= e. But convincing them to move at all seems hard, because their owners are = probably dead or the keys are lost. How to handle such coins after Q-day is= an open question, and it seems to boil down to the good old "freeze-o= r-steal" debate.


If Q-day comes before migration can reach meani= ngful levels (what those are is probably a matter of perspective), then I t= hink there isn't much of an interesting future for Bitcoin anyway.

The fut= ure is only as interesting as we make it. Even if Q-Day comes before most c= oins migrate, we have an opportunity to make that future interesting, using= rescue protocols. Maybe some folks don't want to work on them or would= rather sell their coins first, and that's totally valid, but I implore= you to at least keep your mind open to the possible futures that rescue pr= otocols open up, because it is unclear exactly which future we are bound fo= r and I'd rather that neutral internet money still exists in each of th= em.

We have to p= lay the cards we are dealt, and right now we've been dealt a very fortu= nate hand=C2=A0because of BIP32=C2=A0(thanks to you sip= a!): Most users have at least=C2=A0some=C2=A0secret kno= wledge that a CRQC cannot forge, and that's a pretty awesome privilege,= one which even extends to other blockchains whose wallets use BIP32, or it= s derivatives. You should be very proud of that IMO=C2=A0=F0=9F=99=82


Disaster-recovery: neither the &q= uot;predictable/planned consensus change" of=C2=A0Later nor the= "everyone takes responsiblity for their own funds" works, and th= e only way to avoid a large percentage of bitcoin becoming a reward for qua= ntum research is to replace EC spend paths with a ZK proof of knowledge of = a BIP32 seed or similar, requiring=C2=A0a hard fork.

@AJ rescue protocols can be deployed via soft fork, because w= e'd be constricting rules and not relaxing them. We'd require any E= C spends to also=C2=A0satisfy the rescue protocol in addition to the= existing consensus rules (signatures etc).=C2=A0Of course, there might be = external reasons to deploy it as a hard fork, e.g. to roll back mass theft = if we don't time things right, or to disentangle from quantum-theft adv= ocates,=C2=A0but I think we should aim for soft fork compatibility i= f possible.


the &q= uot;quantum-safe" hard-fork variant=C2=A0of bitcoin would be fa= irly described as a utxo-bootstrapped altcoin, would compete in the market = with the "quantum-unsafe" bitcoin thatexisting nodes would contin= ue to follow, and possibly there would be=C2=A0many slightly varying such a= ltcoins competing with each other, eg on exactly what height the utxo snaps= hot was taken or what coins become spendable. It would not be a fun time fo= r holders of affected utxos.
I really hope we do not end up in a situation with co= mpeting rescue protocol deployments. I don't think snapshots are necess= ary so that's not really a factor.
As for the exact conditions which permit rescue... that = could indeed get dicey if we need to collate a bunch of different rescue pr= otocols into one Frankensteinian mess. The most popular proposal will proba= bly be one which covers all the most common hard-relations, but this still = needs more research.
If we think of rescue protocols more abstractly, the spender just needs to= prove their script pubkey commited to a quantum-hard relation, and that th= e spender knows the witness to that relation. It doesn't actually matte= r which exact relation - BIP32, hashed address, musig, whatever - just that= it's quantum-hard. Maybe there is a way to formulate a two-step proof = system general enough to rescue UTXOs with any such relation, by first prov= ing the relation is quantum-hard, then proving you know the witness to the = relation. That would be awesome but still an open problem.

regards,
conduition



On Wednesday, June 17th, 2026 at 11:20 PM, Anthony Towns a...@erisian.com.au wrote:

On Tue, Jun 16, 2026 at 08:09:08PM +0000, Pieter Wuille wrote:

I want to first correct a potential misunderstanding here, because
I realize the terms "Later" and "Never" aren't very= descriptive. They
are specifically about an expected and relied-upon expectation that an
EC-disabling fork will happen that at least applies to the output type
itself, in time. "Later" is the expectation that such a disabling= will
happen after the output type is introduced, but still in time (so, before Q-day). Outputs without a strong expectation that their EC paths/opcodes will be disabled, or not in time, I classify under "Never".

I believe here you're instead arguing for P2MR ("Merkle-Never&q= uot;)
over all "Later" options. That was my previous point: I think (so= lely)
having "Never" output types like P2MR is just utterly insufficien= t for
any worthwhile migration. It's so incompatible with today's workflo= ws
that it either won't be adopted, or (possibly inadvertently) adopted in an insecure fashion. Yes, it gives people the option to safeguard
their own coins, but to me that's disaster recovery territory - I think=
we ought to prioritize maximizing the chances for saving the currency
as a whole in case Q-day comes, not a small subset of individual users'=
coins. P2MR (alone) doesn't really achieve much in that regard in my vi= ew,
thus we at least need something of the "Later" class in addition.=

I'm not sure I follow/agree with the logic here. I think the general= idea
is:

1) we create some new address types that allow post-quantum spending, bu= t
also allow efficient quantum-vulnerable spending that can be used prior
to Q-day

2) many people migrate to these new address types

3) Q-day arrives

4) all spending goes via the post-quantum paths, and everyone's fund= s are
safe

There are three main variations to this, I think:

Later-dominant: towards the end of (2) but prior to (3), the
quantum-vulnerable spend paths are disabled in a predictable, planned
manner, preventing coin theft, but not disrupting active usage
significantly (or not disrupting it any more than the proximity of
Q-day already is).

Self-reliance: Q-day goes from maybe to definitely faster than consensus=
changes can be coordinated, and the only reason people's funds remain safe is that they can (and do) keep the quantum-vulnerable spend
paths secret.

Disaster-recovery: neither the "predictable/planned consensus chang= e" of
Later nor the "everyone takes responsiblity for their own funds"<= br> works, and the only way to avoid a large percentage of bitcoin
becoming a reward for quantum research is to replace EC spend paths
with a ZK proof of knowledge of a BIP32 seed or similar, requiring
a hard fork. Such a hard fork would be certain to be controversial
("why at this height: I received a payment five blocks after //
my funds were stolen by a quantum attacker five blocks earlier //
why are only BIP32 funds recoverable not scheme X"), but if no other approach works, might still be the ultimately outcome.

So the point here was just: if you already accept the need for a "L= ater"
output type (=3D one with builtin-in EC disabling expected from the start),=
P2TRv2 is preferable over P2MR+ED, because:

As far as I can see, only having P2TRv2 addresses would rule out the
"self-reliance" path here.

Picking when Q-day will occur, either individually for determining
your own security posture, or collectively for organising a consensus
change seems very difficulty: it involves evaluating both complex state
of the art physics research, but also estimating the covert abilities
of national governments and the incentives to attack bitcoin prior to
revealing their capabilities. To me, that doesn't bode well for a smoot= h
and fast deployment of a consensus change in advance of problems occuring.<= /p>

It's possible that general carelessness on behalf of users would als= o
rule out the effectiveness of a self-reliance approach: if only the most conscientious 1% of users make use of P2MR securely, that might secure 10%<= br> of funds, but having 90% of the bitcoin supply crash probably wouldn't = be
very valuable either. But even then, that may make the "disaster-recov= ery"
approach less problematic, by ensuring the 1%/10% who were conscientious can avoid being part of the "my funds were stolen by a quantum attacke= r"
contingent, eg.

Theorycrafting for a second here. It's reasonable to conjecture fee<= br> rates will be much higher post-Q-day, and thus P2MR's 32 byte advantage=
over P2TRv2 will yield significant savings for end-users in absolute
terms. If fee rates inflate 10x higher after Q-day, then 8 vbytes becomes significant (100 sats per spend or more).

I don't think that is the right way to look at. 8vb/input is about an additional 14% today (vs a taproot key-path spend), but with the
post-quantum signatures we have available now that's likely to reduce to ~7% (SHRINCS) or ~1% (winternitz). So, post-Q-day, by dropping 32B,
you're only getting an expected savings in fees / increase in block
capacity on that order of magnitude, ie: 1%-7%. That's nice to have, for sure, but doesn't compare to introducing a new address type that puts the PQ sigs in an extension block, or one that uses ZK proofs to
do cross-input or cross-transaction signature aggregation, eg. So a 32B
witness overhead for an unused/unusable key-path post-Q-day doesn't see= m
very relevant to me.

FWIW, I don't think the P2TRv2 EC-disabling fork needs to be timed perfectly. The expectation should be just that it happens before Q-day,
and when it looks inevitable or the fear about that is large enough.

FWIW, I would define "timed perfectly" precisely as "EC d= isabling
fork happens before Q-day". Maybe allowing some additional months of EC-efficiency would be a win while not taking out too much migration time,<= br> but for me "perfection" here means "no one who upgraded lost= money via
quantum-related vulnerabilities".

One reason I'm doubtful is that I think for some the "it looks = inevitable"
threshold has already been crossed, eg:

So let me attempt to partially fill the silence, similarly to what
Scott Aaronson did in his April 29 post. Given everything I know,
including scary non-public information, I now put the odds of qday by
2032 at 50%. 10% by 2030.

IMO a good target date for migration is 2029, roughly 3.5 years
out. 2029 happens to be the date selected by Google, Cloudflare, and
the Ethereum Foundation.

https://x.com/drakefjustin/status/2061793725299224676

So, here it is: if quantum computers start breaking cryptography a
few years from now, don=E2=80=99t you dare come to this blog and tell me th= at
I failed to warn you. This post is your warning. Please start switching
to quantum-resistant encryption, and urge your company or organization
or blockchain or standards body to do the same.

https://scottaaronson= .blog/?p=3D9718

Personally, that leaves me at a minimum very skeptical of the utility of introducing either P2MR or P2TRv2 (etc) approaches that don't also introduce a quantum-safe spending path on day one.

First a meta-point here: the reason I like separating the discussion int= o different dimensions than just "P2TRv2 vs P2MR" is because ther= e are more options than those two, and not every argument applies to the sa= me separation into two classes. Let me list them explicitly here, roughly i= n decreasing order of how strongly I feel about them:
- We need at least a "Later" option for meaningful migration, i.e= . P2TRv2 or P2MR+ED.
- Within the "Later" option, P2TRv2 is preferable.
- A "Later" option benefits from being the only PQC migration str= ategy, making it a Schelling point.

Correct me if I'm mistaken, but I think P2TRv2 is preferable only in= the
"Later-dominant" scenario, and only to the degree that it's s= lightly
cheaper prior to Q-day. If it were the only available option, that would increase the risk of loss involved with both the other approaches. (I
don't think P2TRv2 is meaningfully more private than P2MR in the way taproot v1 is, as presumably you'd only adopt that address format if you wanted to have a post-quantum script path)

You'd have to convince everyone to deploy and then adopt P2TRv2 toda= y under the public knowledge that it is insecure and their coins are more l= ikely to be stolen. That's a hard sell.

How does one pitch P2TRv2 to users? "It will be quantum secure one = day we promise because everyone is incentivized to fix it later as Bitcoin = will die if we don't."

How do you pitch P2MR? "It's quantum secure if you use it corre= ctly."
To me, the pitch is "Bitcoin can only remain valuable if we mostly/all= migrate." for both. P2TRv2 is just much easier to adopt, because P2MR= (or any "Never" output type) fundamentally requires many users t= o change their workflows entirely.

Let's call NoEC-day the earlier of activation of a soft-fork disabli= ng
EC-spends on P2MR/P2TRv2 or Q-day. NoEC-day to some extent is presumably equal to "the day the bitcoin ecosystem has finally agreed that CRQCs<= br> are inevitable".

My (cynical?) view is the only people who will adopt either P2TRv2 or P2MR prior to NoEC-day being schedule will be people who are willign
to use those features in a quantum-safe way -- that is, keeping their
EC pubkeys secret, and only revealing those EC pubkeys to spend them
immediately, prior to NoEC-day. In that view, the EC-spend-paths of such coins prior to NoEC-day are only an opportunistic "make spends cheaper= "
shortcut, they don't allow the funds to be used in lightning channels or to let your wallet be audited via sharing an xpub or anything similar.

Perhaps I'm wrong: it's my opinion, not a technical fact; it'= ;s possible
that lightning software could get an upgrade to generate post-quantum
signatures for channel commitments or HTLC claims, I just think it's pretty unlikely that that will happen at a meaningful scale when everyone has much more immediate and less theoretical things to work on prior to
NoEC-day, especially when the efficiency/performance of such changes is
likely to be very low.

This focus on allowing individual users the ability to safeguard their coins just feels like a red herring: [...]

While I appreciate your point, I also feel that "allowing individua= l
users the ability to safeguard their coins" is pretty close to the ent= ire
point of Bitcoin. :)

In either case, I consider anything that requires hardcoding
specific wallet designs (BIP32 or otherwise) into Bitcoin's consensus rules (and only allowing those coins to survive) to be squarely in
disaster-recovery land. It feels like embracing arbitrariness, and
giving up on the permissionlessness that makes Bitcoin interesting -
if the community shows it can get consensus on effectively burning
coins except those that match a whitelist, it feels hard to maintain
censorship-freeness as a value, even if the whitelist includes most of
the (active) coins. That is of course not to say such techniques aren't=
interesting to work on, but to me, their place is in disaster recovery
scenarios to save what's left, after Q-day, if migration attempts were<= br> unsuccessful. In such a setting, I think we're already in effectively a= n
altcoin-with-UTXO-bootstrapped-from-Bitcoin territory, and a (possibly
growing) set of ways to recover burned coins can be hardforked in.

Just for the record, I think the above is an accurate description of the=
"disaster-recovery" scenario above: the "quantum-safe" = hard-fork variant
of bitcoin would be fairly described as a utxo-bootstrapped altcoin,
would compete in the market with the "quantum-unsafe" bitcoin tha= t
existing nodes would continue to follow, and possibly there would be
many slightly varying such altcoins competing with each other, eg on
exactly what height the utxo snapshot was taken or what coins become
spendable. It would not be a fun time for holders of affected utxos.

My impression is that your approach is to have an answer for many
possible futures, including ones where Q-day arrives within just a few
years.

"The key of strategy... is not to choose a path to victory, but to = choose
so that all paths lead to a victory."

-- https://tvtropes.org/pmwiki/pmwiki.php/Main/XanatosGa= mbit

But optimizing for disaster-recovery also means reducing the
chances of preserving Bitcoin as we know it in the scenarios where a
successful migration is still possible. And if Q-day does arrive that
soon, I don't see what we can do today that would preserve Bitcoin in a form we care about anyway. By accepting that, we can focus on the
futures where our choices today can still materially improve the outcome.

Preserving bitcoin as a personally-possessible inflation resistant
store of value seems both possible and worth caring about, even if other constraints means that many people can't afford to personally hold it (and have to go through ETFs/exchanges/banks) and that it can't be used=
for day-to-day transactions. Would be very disappointing if true, and
even given Q-day in a few years I expect we could do better than just
that, but it doesn't feel like a throw-in-the-towel event to me.

Essentially yes though, I expect the majority of holders will probably migrate to PQ addresses via rescue protocols, either on Bitcoin or a fork thereof. Even if we can't come to consensus and deploy a new output typ= e,
we'll still be able to rescue most coins. It's just that we'd h= ave nowhere
to rescue them to, so we ought to make PQ wallets available soon, so we'= ;re
not in a rush to build them later when we need them. If a PQ wallet can
use cheap EC signatures while they're still trustworthy, all the better=

FWIW, that's my guess on how things would play out if the near-term = Q-day
timelines I've seen (ie, 2029 to 2035) match reality. I hope that's=
pessimistic (either because the Q-day timelines are bad estimates, or
because migration happens in a more orderly fashion), but I guess we'll=
see. I don't rate my ability to evaluate Q-day predictions very highly.=

- A (not-quite-CR)-QC breaks 128-bit ECDLP, say.

I'm not in a position to judge, but the google paper claims:

"Indeed, if a leading quantum architecture encounters and overcomes=
all its scaling challenges before producing a device able to
solve (for example) 32-bit ECDLP, then there may be little time
between the breaking of 32-bit ECDLP and the breaking of 256-bit
ECDLP. Furthermore, the community should not expect to see published
demonstrations of the most advanced quantum error-correction
architectures and quantum algorithms deployed to cryptanalytic
problems. Thus, a successful public demonstration of Shor=E2=80=99s
algorithm on a 32-bit elliptic curve should not be seen as a wake-up
call to adopt PQC as much as a potential signal that PQC adoption
has already failed."

and places the required tiffoli gates and logical qubits for a 32-bit break at about (300k, 200) vs (10M, 600) for 128-bit or (80M, 1100)
for 256-bit.

Of course, if you believe it's the only possible future, I understan= d
you'd come to a different conclusion. But is it really? Do you think this isn't a plausible future:

- A P2TRv2 type (let's leave aside whether P2MR or P2QR gets added t= oo) gets introduced in the next few years, with hash-based PQC opcodes.
- Over the course of the next decade or so, it gets adopted by practically = all active users.

I think it might be better to look at that scenario in a more fine-grain= ed
way? I think your "Late-ish" scenario is:

1) P2TRv2 (or whatever) is introduced
2) Some PQ opcodes get enabled, allowing expensive but PQ-safe spend-paths<= br> via those outputs
3) P2PK, P2PKH, P2WPKH, P2WSH, P2TR all become obsolete in favour of P2TRv2= ,
but EC spend paths continue to be what's used in practice.
4) Some better PQ solutions become available, allowing cheap PQ-safe spend-= paths
5) Everyone switches from EC paths to the new PQ paths.
6) NoEC-day happens, but no one is impacted.

I think your "Soon-ish" scenario differs as of step (4):

4) NoEC-day happens. Massive disruption because the "what's use= d in practice"
path breaks, but everything is recoverable.
5) Post-quantum approaches get even higher priority

I'm skeptical of step 3 here; and would expect to see something more= like:

3') Only a small proportion of users (ie, the most conscientious/fea= rful)
switch to P2TRv2 with most preferring to stick with what works

That has no real impact on the Late-ish scenario, but changes the Soon-i= sh
scenario to either:

4'a) NoEC-day happens substantially before Q-day; people hurry to mi= grate
to P2TRv2, but it mostly works.

or

4'b) NoEC-day happens essentially at the same time as Q-day; coins g= et
stolen and we hit the disaster-recovery scenario.

Perhaps the difference between (3') and (3) playing out in reality is just having nearly everyone agree that the upgrade is essential,
and rather than leaving it to self-interest/market-dynamics, having a
consistent push that every single wallet/service that doesn't deprecate=
every current address type is a danger to the entire ecosystem, even
absent widespread agreement on when/if Q-day will happen. Arguably that
would be easier to agree on if the incremental cost of EC spend paths
in P2TRv2 prior to NoEC-day/Q-day versus current spend paths is near to
zero, rather than up to ~14% per input.

Cheers,
aj

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