The limitations of cryptographic agility in Bitcoin

[Pieter Wuille]

Hi all,

A thread was recently started on this list about cryptographic agility in Bitcoin. I believe there are important limitations around that idea, and wanted to offer my own perspective. It's more a philosophical consideration, and is not intended as an argument against (or for) any particular proposal today.

The idea is giving users/wallets the ability to choose the cryptographic primitives used to protect their own coins, from a set of available primitives that may change over time. I think this ignores how important it is what others do with their coins. If others' coins lose value, for example due to fears about them becoming vulnerable to theft with cryptographic breakthroughs, so do your own due to fungibility, regardless of how well protected they are.

As an extreme thought-experiment, imagine that tomorrow a new cryptographic signature scheme FancySig is published, with all the features that Bitcoin relies on today: small signatures, fast to verify, presumed post-quantum, BIP32-like derivation, taproot-like tweaking, multisignatures, thresholds, ... Further assume that within the next year or two, voices (A) start appearing arguing that such a scheme should be adopted, because it's the silver bullet the ecosystem has been waiting for. At the same time, another camp (B) may appear cautioning against this, because the scheme is new, hasn't stood the test of time, and isn't well-analyzed. These two camps may find themselves in a fundamental disagreement:

• Camp (A), fearing an EC-break (CRQC or otherwise), wouldn't just want FancySig as an option - they would want (feasible or not) that near-everyone migrates to it, because the prospect of millions of BTC in EC-vulnerable coins threatens their own hodling.
  

• Camp (B), fearing the possibility that FancySig gets broken soon, possibly even classically, don't want to just not use FancySig; they would want near-nobody to migrate to it, because the prospect of a potential break of FancySig threatens their own hodling.

This is of course an extreme scenario, and in reality the divergence between camps may be much less incompatible, but I still think it's a good demonstration of the fact that despite being a currency for enemies, Bitcoin essentially requires users to share trust assumptions in the cryptography, and its technology in general.

A small note aside: you can argue that it is possible for people to store coins insecurely, like in an OP_TRUE script output, or with private keys that are made public. I don't think this possibility affects the point above, because it is not about what possibilities exist, but which approaches people are likely to / asked to adopt. Nobody is incentivized or encouraged to store coins in an OP_TRUE.

It is also good to ask what amounts are relevant here, to which I do not know the answer. Possibly, the prospect of 100k BTC becoming vulnerable to theft won't threaten the value of the other coins, while the prospect of 10M BTC becoming vulnerable may. Depending on where your belief about this lies, you may end up with very different conclusions. Still, to me it is clear that some threshold exists above which I would be uncomfortable with seeing that many coins move into an untrustworthy scheme, even if they are not my own coins.

This brings us to the question then how at all Bitcoin users can migrate to new cryptography, because we cannot assume that secp256k1 will last forever. And I think the answer is essentially that it requires the entire ecosystem to change their assumptions. This does not mean that adding a new opt-in cryptographic primitive is infeasible or a bad idea; it just means that adding FancySig as an option is changing the collective security assumption from "secp256k1 is secure" to "secp256k1 AND FancySig are secure" once FancySig gets adopted at scale, and the discussion about adding new primitives should be treated with the gravity that entails. And it means that disabling secp256k1 EC operations (or near-everyone migrating to FancySig, but I think that is unlikely) is the only way to change the collective security assumption from "secp256k1 AND FancySig are secure" to "FancySig is secure".

Because of that, if CRQCs or other EC-breaks become reality, or just the fear about them becomes significant enough, I do believe that disabling EC opcodes will be necessary in any economically-relevant surviving chain, due to the millions of BTC that are unlikely to ever move. Note that I am not arguing that "Bitcoin" will, should, or even can do this, and I'm quite sure that the inherent confiscation required will make the result uninteresting to me personally. It may be that such disabling only happens in a fork that doesn't end up being called Bitcoin, or it may be that this happens only after a CRQC has been demonstrated to exist. Still, given a severe enough threat I think it is inevitable, and I think that this means we shouldn't worry too much about what happens in chains in which EC is not disabled while simultaneously EC is considered insecure: such chains will be worthless anyway.

-- 

Pieter

[Ethan Heilman]

I agree with your overall claim that we should be cautious about which digital signature algorithms are added as op codes. The danger of a break in ECC is real, but so is the danger of adopting a newer signature algorithm with less vetting. These risks must both be taken seriously.

>  The idea is giving users/wallets the ability to choose the cryptographic primitives used to protect their own coins, from a set of available primitives that may change over time.

Agreed that allowing users to choose their cryptographic primitives from a menu of many options would be problematic:

- Skub and fragmentation over which algorithm is best (Camp A vs Camp B)
- Greater resource costs on wallets if they have to support many algorithms,
- Security analysis is spread over many different schemes,
- More algorithms increase chance any one of the algorithms is broken,

Ideally Bitcoin would only have one signature algorithm at a time.

Algorithm agility aims to provide a standardized upgrade path from a weakened algorithm to a new algorithm that the community overwhelmingly wishes to use. It should not require a menu of different signature opcodes. Yes, the backup signature algorithm exists, but it exists only to enable this secure upgrade path. In this light the high transaction fees of SLH_DSA can be seen as a benefit since they incentivize using SLH_DSA signatures only when absolutely necessary.

> A small note aside: you can argue that it is possible for people to store coins insecurely, like in an OP_TRUE script output, or with private keys that are made public. I don't think this possibility affects the point above, because it is not about what possibilities exist, but which approaches people are likely to / asked to adopt. Nobody is incentivized or encouraged to store coins in an OP_TRUE.

As Bitcoin moves to PQ is I want to ensure the solution is standardized and that wallets use the standard approaches. I worry that some wallets might roll their own adhoc protections if we don't have a  clear and well supported standard way to support PQ signatures.

>  I do believe that disabling EC opcodes will be necessary in any economically-relevant surviving chain, due to the millions of BTC that are unlikely to ever move. 

I agree that the incentives make that an extremely likely confiscatory soft-fork. While it seems like the most likely outcome, I don't feel comfortable making PQ plans that depend on this happening. I also don't feel comfortable advocating for a confiscatory soft-fork. For these reasons, I have not baked the assumption that a confiscatory soft-fork happens in my algorithm agility proposal.

No one can say for certain, but it seems reasonable to assume long exposure attacks will be practical years before short exposure attacks. If true, then a soft-fork to freeze P2PK/P2TR will happen before EC opcodes are frozen as P2SH will still be safe to use (as long as the public key has not been exposed). By the time short exposure attacks are practical, will enough P2SH,P2WSH vulnerable coins remain to justify freezing EC opcodes? What's the breakdown on the number of coins in P2SH and P2WSH outputs that haven't moved in 5 years? As you ask, how much is enough?

This question of what to do with EC opcodes is worth more discussion. My hope is that once everyone agrees that they are insecure, no one uses them, like no one using OP_TRUE scripts or OP_SHA1, but that hope is not justified, what should we do? opcode anti-discount? Disabling them like you propose?

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[Light]

Hi Pieter,

> A small note aside: you can argue that it is possible for people to
> store coins insecurely, like in an OP_TRUE script output, or with
> private keys that are made public. I don't think this possibility
> affects the point above, because it is not about what possibilities
> exist, but which approaches people are likely to / asked to adopt.
> Nobody is incentivized or encouraged to store coins in an OP_TRUE.

While it is true that "nobody is incentivized or encouraged to store coins in an OP_TRUE", I believe your argument is incomplete because it is missing a more realistic mass-theft vector, which is that as of today approximately 2.5 million BTC is held by less than two dozen centralized custodians.[1] These coins are at a real risk of theft, due to both private threat actors e.g. thefts from MtGox, Bitfinex, and scores of others, as well as public threat actors e.g. EO 6102.[2][3]

Despite this very real and large-scale theft risk, nobody is proposing that we freeze coins held on exchanges. And yet some people find this (freezing coins) to be a reasonable response to the risk of cryptographic breaks (approximately 6.5 millions coins at risk).[4][5] Curious! But I digress.

> This brings us to the question then how at all Bitcoin users can
> migrate to new cryptography, because we cannot assume that secp256k1
> will last forever. And I think the answer is essentially that it
> requires the entire ecosystem to change their assumptions. This does
> not mean that adding a new opt-in cryptographic primitive is infeasible
> or a bad idea; it just means that adding FancySig as an option is
> changing the collective security assumption from "secp256k1 is secure"
> to "secp256k1 AND FancySig are secure" once FancySig gets adopted at
> scale, and the discussion about adding new primitives should be treated
> with the gravity that entails. And it means that disabling secp256k1 EC
> operations (or near-everyone migrating to FancySig, but I think that is
> unlikely) is the only way to change the collective security assumption
> from "secp256k1 AND FancySig are secure" to "FancySig is secure".
>
> Because of that, if CRQCs or other EC-breaks become reality, or just
> the fear about them becomes significant enough, I do believe that
> disabling EC opcodes will be necessary in any economically-relevant
> surviving chain, due to the millions of BTC that are unlikely to ever

> move. Note that I am *not* arguing that "Bitcoin" *will*, *should*, or
> even *can* do this, and I'm quite sure that the inherent confiscation

> required will make the result uninteresting to me personally. It may be
> that such disabling only happens in a fork that doesn't end up being
> called Bitcoin, or it may be that this happens only after a CRQC has
> been demonstrated to exist. Still, given a severe enough threat I think
> it is inevitable, and I think that this means we shouldn't worry too
> much about what happens in chains in which EC is not disabled while
> simultaneously EC is considered insecure: such chains will be worthless
> anyway.

I don't believe the predominant "collective security assumption" is that "secp256k1 is secure". Everyone using bitcoin, going all the way back to the genesis block, has known (or should know) that secp256k1 is vulnerable to a CRQC, and therefore secp256k1 is not unconditionally secure. So I would rephrase the assumption to be that "secp256k1 is currently secure, and if it is ever at risk of being broken, there is a large incentive for a viable alternative to be discovered, implemented, and adopted by anyone who is concerned about this risk". Note that this assumption does not, and cannot, preclude the possibility that other users store their coins in insecure ways, even en masse (as shown empirically by the above example of millions of coins held by centralized custodians).

An alternative where "secp256k1 OR FancySig is secure" seems strictly preferable to me (assuming we have some reasonable degree of confidence in the security of FancySig at the time of activation) than relying only on "secp256k1 is secure".

As an aside, I note that because it has been known since before bitcoin even launched that secp256k1 is vulnerable to a CRQC, we cannot rule out the possibility that the people who leave their coins in CRQC-vulnerable addresses -- even after QR addresses are made available -- aren't doing so intentionally, as a kind of bounty for developing a CRQC. Freezing these coins would violate their intentions. This is just one example of the ethical problems associated with such proposals.

A perhaps even more fundamental "collective assumption" of bitcoin users is that "a redeem script that was valid at the time that a UTXO was encumbered will always remain valid". This is fundamental to bitcoin's nature as p2p electronic cash. If users woke up one day no longer able to assume that their redeem scripts would remain valid, then they would not be able to rely on bitcoin to store value over time and the system would quickly collapse. Thus, any proposal to disable features in a way that freezes coins is a proposal to destroy bitcoin.

Bitcoin can survive a temporary price dump, if that's what ends up happening with vulnerable coins. Bitcoin cannot survive a violation of the fundamental assumption that redeem scripts for existing UTXOs will always remain valid, and certainly not a violation at the scale that you describe.

Regards,
Light

[1] https://www.coinglass.com/Balance
[2] https://bitcointalk.org/index.php?topic=5090869.0
[3] https://en.wikipedia.org/wiki/Executive_Order_6102
[4] https://github.com/bitcoin/bips/pull/1895
[5] https://youtu.be/a_B8BnwagEU&t=150

On Fri, Feb 13, 2026, at 11:20 AM, Pieter Wuille wrote:
> Hi all,
>

> A thread <https://groups.google.com/g/bitcoindev/c/7jkVS1K9WLo> was

> recently started on this list about cryptographic agility in Bitcoin. I
> believe there are important limitations around that idea, and wanted to
> offer my own perspective. It's more a philosophical consideration, and
> is not intended as an argument against (or for) any particular proposal
> today.
>
> The idea is giving users/wallets the ability to choose the
> cryptographic primitives used to protect their own coins, from a set of
> available primitives that may change over time. I think this ignores

> how important it is what *others do with their coins*. If others' coins

> lose value, for example due to fears about them becoming vulnerable to
> theft with cryptographic breakthroughs, so do your own due to
> fungibility, regardless of how well protected they are.
>
> As an extreme thought-experiment, imagine that tomorrow a new
> cryptographic signature scheme FancySig is published, with all the
> features that Bitcoin relies on today: small signatures, fast to
> verify, presumed post-quantum, BIP32-like derivation, taproot-like
> tweaking, multisignatures, thresholds, ... Further assume that within
> the next year or two, voices (A) start appearing arguing that such a
> scheme should be adopted, because it's the silver bullet the ecosystem
> has been waiting for. At the same time, another camp (B) may appear
> cautioning against this, because the scheme is new, hasn't stood the
> test of time, and isn't well-analyzed. These two camps may find
> themselves in a fundamental disagreement:
>
> • Camp (A), fearing an EC-break (CRQC or otherwise), wouldn't just
> want FancySig as an option - they would want (feasible or not) that

> *near-everyone migrates to it*, because the prospect of millions of BTC

> in EC-vulnerable coins threatens their own hodling.
> • Camp (B), fearing the possibility that FancySig gets broken soon,
> possibly even classically

> <https://www.quantamagazine.org/post-quantum-cryptography-scheme-is-cracked-on-a-laptop-20220824/>,
> don't want to just not use FancySig; they would want *near-nobody to
> migrate to it*, because the prospect of a potential break of FancySig

> threatens their own hodling.
> This is of course an extreme scenario, and in reality the divergence
> between camps may be much less incompatible, but I still think it's a

> good demonstration of the fact that *despite being a currency for

> enemies, Bitcoin essentially requires users to share trust assumptions

> in the cryptography, and its technology in general*.

>
> A small note aside: you can argue that it is possible for people to
> store coins insecurely, like in an OP_TRUE script output, or with
> private keys that are made public. I don't think this possibility
> affects the point above, because it is not about what possibilities
> exist, but which approaches people are likely to / asked to adopt.
> Nobody is incentivized or encouraged to store coins in an OP_TRUE.
>
> It is also good to ask what amounts are relevant here, to which I do
> not know the answer. Possibly, the prospect of 100k BTC becoming
> vulnerable to theft won't threaten the value of the other coins, while
> the prospect of 10M BTC becoming vulnerable may. Depending on where
> your belief about this lies, you may end up with very different

> conclusions. Still, to me it is clear that *some* threshold exists

> above which I would be uncomfortable with seeing that many coins move
> into an untrustworthy scheme, even if they are not my own coins.
>
> This brings us to the question then how at all Bitcoin users can
> migrate to new cryptography, because we cannot assume that secp256k1
> will last forever. And I think the answer is essentially that it
> requires the entire ecosystem to change their assumptions. This does
> not mean that adding a new opt-in cryptographic primitive is infeasible
> or a bad idea; it just means that adding FancySig as an option is
> changing the collective security assumption from "secp256k1 is secure"
> to "secp256k1 AND FancySig are secure" once FancySig gets adopted at
> scale, and the discussion about adding new primitives should be treated
> with the gravity that entails. And it means that disabling secp256k1 EC
> operations (or near-everyone migrating to FancySig, but I think that is
> unlikely) is the only way to change the collective security assumption
> from "secp256k1 AND FancySig are secure" to "FancySig is secure".
>
> Because of that, if CRQCs or other EC-breaks become reality, or just
> the fear about them becomes significant enough, I do believe that
> disabling EC opcodes will be necessary in any economically-relevant
> surviving chain, due to the millions of BTC that are unlikely to ever

> move. Note that I am *not* arguing that "Bitcoin" *will*, *should*, or
> even *can* do this, and I'm quite sure that the inherent confiscation

> required will make the result uninteresting to me personally. It may be
> that such disabling only happens in a fork that doesn't end up being
> called Bitcoin, or it may be that this happens only after a CRQC has
> been demonstrated to exist. Still, given a severe enough threat I think
> it is inevitable, and I think that this means we shouldn't worry too
> much about what happens in chains in which EC is not disabled while
> simultaneously EC is considered insecure: such chains will be worthless
> anyway.
>
> --
> Pieter
>
>
>

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[Erik Aronesty]

As a thought experiment, it's interesting, and I agree.  One of the main complaints about covenants, for example, isn't that they aren't opt-in - they are, and most people understand that they don't enable special concessions more than multisig.  Instead, it's because of the perception of Nth order effects around their usage in technologies like defi and contracts.   We have all heard the headlines about evm hacks and defi hacks, and, to some extent, these steer "hard money" proponents toward Bitcoin more than any arguments around decentralization.

However, "Q-day" may never arrive—and might just be a physics experiment that proves finite energy is real.  What's more, FancySig1 was already proven insecure/broken.  We are now on FancySig3.  

Cloudflare has decided to ship hybrid FancySig3/ECC, requiring both for all SSL.  This approach would mitigate your concerns fully.  You cannot use either option alone.   You have to use both.   Sure, it's annoying.  But if we choose a sufficiently efficient FancySigN, we can build a system guaranteed to satisfy the risks of both the new system and the old one.

In your extreme thought experiment, the only rational choice is "use both".

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[Antoine Poinsot]

Hi John,

Without commenting on the (un)desirability of freezing vulnerable coins (i largely share your
concerns, but haven't yet formed an opinion of my own), i would like to surface one issue in part of
your reasoning.

You state:

> A perhaps even more fundamental "collective assumption" of bitcoin users is that "a redeem script
> that was valid at the time that a UTXO was encumbered will always remain valid". This is
> fundamental to bitcoin's nature as p2p electronic cash. If users woke up one day no longer able to
> assume that their redeem scripts would remain valid, then they would not be able to rely on
> bitcoin to store value over time and the system would quickly collapse. Thus, any proposal to
> disable features in a way that freezes coins is a proposal to destroy bitcoin.

That's quite the sweeping claim, but it's trivially incorrect, or it would make any soft fork
impossible. This is both disproven historically, since Bitcoin made previously-valid redeem scripts
invalid on at least 5 occasions (post Satoshi), and incompatible with many soft fork proposal,
including your own.

As long as soft forks are tolerated, a degree of nuance needs to be introduced to take into account
reasonable expectations from users, and not disabling potentially-vulnerable EC opcodes as opposed
to upgrade hooks needs to be defended on its own merit rather than appealing to the destruction of
Bitcoin. (And to be clear i think a strong case can be made in this regard, this is not to be
interpreted as an argument in favour of The Big Freeze.)

Best,
Antoine

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>

[Light]

Hi Antoine,

> it would make any soft fork impossible.

This is probably tangential to the topic of the thread, but since you bring up the confiscatory potential of soft forks in general I will go ahead and plant my flag here by saying that if given the choice between doing a soft fork but with a high likelihood of freezing coins, or doing a hard fork to avoid freezing coins, I would pick the hard fork every time.

> As long as soft forks are tolerated, a degree of nuance needs to be
> introduced to take into account reasonable expectations from users

I believe what I wrote accounts for this nuance. As far as I'm aware, no soft fork ever intentionally froze coins; on the contrary, my observation has been that protocol developers are exceedingly cautious to avoid this side effect.[1] It appears their efforts have not been in vain, since broad confidence in the security of bitcoin property rights remains intact, as evidenced by the millions of people who continue to use bitcoin to store value.

> and incompatible with many soft fork proposal, including your own.

I'm not sure what you're referencing here; I am not an author of any soft fork proposals.

[1] Apparently the P2SH soft fork did freeze a total of 0.044 BTC: https://bitcoin.stackexchange.com/a/115804
This seems unintentional since the coins were created after the activation client was published and shortly before the fork activated, and I could not find any record of alarms being raised at the time about the prospect of freezing these coins.

[waxwing/ AdamISZ]

Hi Pieter, list,

>  If others' coins lose value, for example due to fears about them becoming vulnerable to theft with cryptographic breakthroughs, so do your own due to fungibility, regardless of how well protected they are.

> Camp (A), fearing an EC-break (CRQC or otherwise), wouldn't just want FancySig as an option - they would want (feasible or not) that near-everyone migrates to it, because the prospect of millions of BTC in EC-vulnerable coins threatens their own hodling.

> the fact that despite being a currency for enemies, Bitcoin essentially requires users to share trust assumptions in the cryptography, and its technology in general.

>  it just means that adding FancySig as an option is changing the collective security assumption from "secp256k1 is secure" to "secp256k1 AND FancySig are secure"

I fundamentally disagree with this thesis you outlined here. The error in the second of the three quotes is subtle: it's not that rational users will not *want* others to migrate (of course, they will), it's that they will not *want* violation of property rights. See [1]

The fact that other coins are held in an insecure way is in no way a threat to my security. As you point out with e.g. OP_TRUE there is zero ability to prevent people doing this.

Suppose X% of the supply is held by negligent holders. Over time that X% will move away from those negligent holders to "thieves" (scare quotes because if literally held in outputs for which the unlocking script is publically derivable, it's debatable whether it's theft, even). The thieves will either be negligent themselves or not; in any case, over time, the coins will move to holders who are not negligent.

The only thing that the system as a whole *has* to promise is that there exists a safe, practical way to keep possession (and also users should not have to just "guess" which methods are secure!). The system is not required, nor can it, to prevent people choosing insecure storage methods, against the technical advice.

If value is held in large quantities in outputs which phase from secure to insecure then for sure there are cases (even when said technical advice is amply provided!) where this leads to turbulence in value (mostly due to death or key loss), but value cannot be controlled or predicted anyway. As per my message here: [1] there are private property rights principles which cannot be violated, else the entire purpose of the project is lost. Good safety-engineering reasoning is unfortunately not enough to trump such principles.

(I think the title of your thread is interesting though: is *agility* desirable? We're borrowing the term from other areas of the IT industry where it makes more sense perhaps; perhaps here "flexibility" is the more desirable part, at least if you take *my* side of this specific argument, which is that no, others' security failings do not change the promise made to users to protect their property. Flexibility helps exactly where there's a lot of uncertainty about the security of different schemes. If it costs $100 to move in quantum-secure way and $1 to move in a non-*, then it is much better that flexibility exists.)

[1] https://groups.google.com/g/bitcoindev/c/7jkVS1K9WLo/m/btgqa9t7AwAJ

Cheers,
AdamISZ/waxwing

[sadiq Ismail]

Hi list, light

I want to clarify a point in your argument.

> I don't believe the predominant "collective security assumption" is that "secp256k1 is secure". Everyone using bitcoin, going all the way back to the genesis block, has known (or should know) that secp256k1 is vulnerable to a CRQC, and therefore secp256k1 is not unconditionally secure. So I would rephrase the assumption to be that "secp256k1 is currently secure, and if it is ever at risk of being broken, there is a large incentive for a viable alternative to be discovered, implemented, and adopted by anyone who is concerned about this risk". Note that this assumption does not, and cannot, preclude the possibility that other users store their coins in insecure ways, even en masse (as shown empirically by the above example of millions of coins held by centralized custodians).

When we say a cryptographic scheme is "secure", that has a precise meaning: there is no known "efficient" algorithm in the sense of probabilistic and polynomial time PPT that breaks it. By that standard, secp256k1 is still secure today.
 A CRQC would break it, but a CRQC is not an "efficient" algorithm in this sense, and the same premise underlies the security arguments for any post-quantum scheme as well. So until we see an efficient PPT algorithm that breaks secp256k1, saying it is "secure" is still technically correct, and the proposed rephrasing isn't necessary.

Best, 

Sadiq

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[conduition]

Hi Pieter, and thanks for bringing this up.

I think your concern is valid. I'd agree with your thought experiment in the sense that there would definitely be competing movements for and against two such well-matched algorithms. However i don't see this debate affecting much in the real world, outside of twitter meme showdowns and reddit debates.

Anyone who wants quantum security but is also afraid of fully trusting untested new algorithms like FancySig could easily use a hybrid locking script which requires both BIP340 AND FancySig signatures. Or more efficiently they could commit their BIP340/FancySig pubkeys in separate tapscript leaves and use only BIP340 until quantum security is needed. This is even more secure than a hybrid locking script if we assume no address reuse. Users don't have to pick between algorithms until spending time, and even then they could reveal only a hybrid leaf for extra safety if they are unsure.

So no, i don't think we need to change our assumptions, even if an ideal PQ signature candidate appears tomorrow. We just need to design smart and secure standards for wallets to implement so that users get a (speculatively) quantum secure fallback without exposing themselves to risk of novel attacks on young cryptosystems.

regards,
conduition

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