Hashed keys are actually fully quantum secure

[Martin Habovštiak]

Hello list,

this is somewhat related to Jameson's recent post but different enough to warrant a separate topic.

As you have probably heard many times and even think yourself, "hashed keys are not actually secure, because a quantum attacker can just snatch them from mempool". However this is not strictly true.

It is possible to implement fully secure recovery if we forbid spending of hashed keys unless done through the following scheme:

0. we assume we have *some* QR signing deployed, it can be done even after QC becomes viable (though not without economic cost)

1. the user obtains a small amount of bitcoin sufficient to pay for fees via external means, held on a QR script

2. the user creates a transaction that, aside from having a usual spendable output also commits to a signature of QR public key. This proves that the user knew the private key even though the public key wasn't revealed yet.

3. after sufficient number of blocks, the user spends both the old and QR output in a single transaction. Spending requires revealing the previously-committed sigature. Spending the old output alone is invalid.

This way, the attacker would have to revert the chain to steal which is assumed impossible.

The only weakness I see is that (x)pubs would effectively become private keys. However they already kinda are - one needs to protect xpubs for privacy and to avoid the risk of getting marked as "dirty" by some agencies, which can theoretically render them unspendable. And non-x-pubs generally do not leak alone (no reason to reveal them without spending).

I think that the mere possibility of this scheme has two important implications:

* the need to have "a QR scheme" ready now in case of a QC coming tomorrow is much smaller than previously thought. Yes, doing it too late has the effect of temporarily freezing coins which is costly and we don't want that but it's not nearly as bad as theft

* freezing of *these* coins would be both immoral and extremely dangerous for reputation of Bitcoin (no comments on freezing coins with revealed pubkeys, I haven't made my mind yet)

If the time comes I'd be happy to run a soft fork that implements this sanely.

Cheers

Martin

[Lloyd Fournier]

This seems like a very clever idea. It allows us to mostly ignore the QC question until a threat actually materializes and then soft fork to disallow bare public key spending with minimal actions needed to be taken by users. Nice work!

A couple of important points:
- Taproot keys are also "hashed keys" since the internal key is technically hashed to produce the external. If you disallow key path spend you can apply the same rule by using the internal key to produce the commitment signature.
- Taproot keys are actually better hashed keys since you don't have to worry about whether you've revealed your public key on-chain in the past e.g. via address re-use if you use external key spends (since this doesn't reveal your internal key).

If this approach gains acceptance I think the main immediate action users can take is to move to a taproot wallet. I predict trying to advise people to move to p2pkh addresses or that p2pkh addresses are "fine" will create confusion since there are huge numbers of coins in p2pkh addresses whose public key has already been revealed and people may do address reuse without knowing it.
Also an attractive approach is to embed the QR signature scheme in a tapleaf before activating it so that most coins already have a QR spending path ready to go. This is more straightforward if taproot is normalized first.
I understand that people might feel "less protected" on a taproot address because they might get sniped by the QC attacker before the freezing fork has been activated but I don't think this is a serious concern relative to the millions of coins available with known public keys. We have to freeze it before they can be taken.

So outside of cryptography, the difficult task is to come to a social consensus mechanism about when to trigger the freezing soft fork. It should be done *before* a secp256k1 DLOG QC can be built but *after* we know that one can be built. Right now it is certainly not clear that one *can* be built ever and we won't have any indication this decade and maybe the next. It may be a matter of debate whether we've reached that point in 10 years (it certainly isn't now) and you can imagine malicious actors trying to subvert the process either to hold it back or to push it forward.

LL

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[Martin Habovštiak]

Antoine, "in addition to making spending old outputs invalid on their own, a rule which dictates they may only be spent along with a QR output at least X blocks old."

yes, this is what I meant but also the QR output must contain the commitment. This rule makes it not "a race". The attacker cannot make the commitment before knowing the private key and cannot reverse deep chain.

Augustin, you understand it correctly. Sadly, the dilemma is only mitigated for hashed keys, not revealed ones.

1. we would presumably bump segwit version, so we can do whatever we like. I assume it'd be something similar to today's Annex but there are likely more ways to do it with their pros and cons. I don't think these details matter much today. But it's certainly possible.
2. of course, soft fork would be required but it will be anyway to deploy a QR signing algo. And I don't think anything saving coins from certain loss will be contentious. :)
The changes would need to identify inputs using secp256k1 verification and look up the commitments in the other inputs. Also they'd need to check how deep the spent inputs are.

Dňa ne 16. 3. 2025, 20:03 Agustin Cruz <agusti...@gmail.com> napísal(a):

Hi Martin,

Your approach of using a committed QR signature to “anchor” the spending of hashed keys is intriguing. If I understand correctly, the idea is:
- A user commits to a QR signature in a first transaction (Tx1), proving ownership of the QR private key without exposing vulnerable data.
- Later, they spend both the old P2PKH output and the QR output together (Tx2), revealing the QR signature, with rules ensuring the old output can’t be spent independently.
- This forces an attacker to either forge a QR signature (infeasible with a quantum-resistant scheme) or rewind the chain past Tx1’s confirmation (infeasible with sufficient depth).

This seems to provide a solid defense against quantum theft, assuming the QR scheme holds up and the blockchain remains secure. I also like how it mitigates the “theft vs. freeze” dilemma. Temporary freezing is indeed less catastrophic than permanent loss, and avoiding reputational damage is crucial.

To better understand how this would work, I have two questions:

1. How would the QR signature commitment be encoded and verified in the script?. Would this require new opcodes or script functionality to check the commitment when spending?

2. How would you enforce that the old P2PKH output can only be spent with the QR output? Would this need a soft fork, and if so, what consensus changes would be required?

Regards,
Agustín

--

[Agustin Cruz]

Hi Martin,

Your approach of using a committed QR signature to “anchor” the spending of hashed keys is intriguing. If I understand correctly, the idea is:
- A user commits to a QR signature in a first transaction (Tx1), proving ownership of the QR private key without exposing vulnerable data.
- Later, they spend both the old P2PKH output and the QR output together (Tx2), revealing the QR signature, with rules ensuring the old output can’t be spent independently.
- This forces an attacker to either forge a QR signature (infeasible with a quantum-resistant scheme) or rewind the chain past Tx1’s confirmation (infeasible with sufficient depth).

This seems to provide a solid defense against quantum theft, assuming the QR scheme holds up and the blockchain remains secure. I also like how it mitigates the “theft vs. freeze” dilemma. Temporary freezing is indeed less catastrophic than permanent loss, and avoiding reputational damage is crucial.

To better understand how this would work, I have two questions:

1. How would the QR signature commitment be encoded and verified in the script?. Would this require new opcodes or script functionality to check the commitment when spending?

2. How would you enforce that the old P2PKH output can only be spent with the QR output? Would this need a soft fork, and if so, what consensus changes would be required?

Regards,
Agustín

El dom, 16 de mar de 2025, 3:31 p. m., Martin Habovštiak <martin.h...@gmail.com> escribió:

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[Martin Habovštiak]

Oh, great point that while the hashing in Taproot disallows spending when null tweak is used, it's still usable to produce a proof similar to what I suggested. Also very interesting point about address reuse being "fine" with taproot.

I believe the QR signature scheme in tapleaf was already suggested but that has the problem that the scheme needs to be specified in advance. IIUC it's not currently clear which even is reasonable. My idea gives us more time to figure that out.

However, I do not think that Taproot is generally safer than p2*pkh. Comparing to millions lost coins is not valid, since those at worst decrease the price of bitcoin but economically wouldn't set it to literal zero, thus the value of one's coins just decreases, while getting stolen from means the value of one's coins goes to literal zero.

The difference wrt safety thus relies on how well one is able to avoid address reuse. Some people can avoid it completely, some can't.

The social aspect is indeed messy.

Dňa po 17. 3. 2025, 11:44 Lloyd Fournier <lloyd...@gmail.com> napísal(a):

[Antoine Poinsot]

This way, the attacker would have to revert the chain to steal which is assumed impossible.

Or just create its own "QR output"?

If your threat model assumes an attacker can promptly recover the private key from the public key then once the user broadcasts his transaction spending both the old output and his own QR output the attacker could simply create his own QR output and RBF the honest transaction.

I suppose you could in theory have, in addition to making spending old outputs invalid on their own, a rule which dictates they may only be spent along with a QR output at least X blocks old. This would give the honest user a headstart in this race, but meh.

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[Erik Aronesty]

If your threat model assumes an attacker can promptly recover the private key from the public key then once the user broadcasts his transaction spending both the old output and his own QR output the attacker could simply create his own QR output and RBF the honest transaction.

correct.   this doesn't provide protection.   and any such rule about "spent along with" would be a hard fork.   which is fine if qr ever matters (there's still not a lot of evidence that it will).

[Lloyd Fournier]

On Tue, 18 Mar 2025 at 00:48, 'Antoine Poinsot' via Bitcoin Development Mailing List <bitco...@googlegroups.com> wrote:

I suppose you could in theory have, in addition to making spending old outputs invalid on their own, a rule which dictates they may only be spent along with a QR output at least X blocks old. This would give the honest user a headstart in this race, but meh.

Yes this is how I read the OP "after sufficient number of blocks". I think this is a really nice idea. The head start can be arbitrarily large so that the attacker simply cannot compete. It's probably not too difficult to design some honest RBF mechanism either such that you can bump the fee with a new QR signature if it's taking too long.

LL

 

On Sunday, March 16th, 2025 at 2:25 PM, Martin Habovštiak <martin.h...@gmail.com> wrote:

Hello list,

this is somewhat related to Jameson's recent post but different enough to warrant a separate topic.

As you have probably heard many times and even think yourself, "hashed keys are not actually secure, because a quantum attacker can just snatch them from mempool". However this is not strictly true.

It is possible to implement fully secure recovery if we forbid spending of hashed keys unless done through the following scheme:

0. we assume we have *some* QR signing deployed, it can be done even after QC becomes viable (though not without economic cost)

1. the user obtains a small amount of bitcoin sufficient to pay for fees via external means, held on a QR script

2. the user creates a transaction that, aside from having a usual spendable output also commits to a signature of QR public key. This proves that the user knew the private key even though the public key wasn't revealed yet.

3. after sufficient number of blocks, the user spends both the old and QR output in a single transaction. Spending requires revealing the previously-committed sigature. Spending the old output alone is invalid.

This way, the attacker would have to revert the chain to steal which is assumed impossible.

The only weakness I see is that (x)pubs would effectively become private keys. However they already kinda are - one needs to protect xpubs for privacy and to avoid the risk of getting marked as "dirty" by some agencies, which can theoretically render them unspendable. And non-x-pubs generally do not leak alone (no reason to reveal them without spending).

I think that the mere possibility of this scheme has two important implications:

* the need to have "a QR scheme" ready now in case of a QC coming tomorrow is much smaller than previously thought. Yes, doing it too late has the effect of temporarily freezing coins which is costly and we don't want that but it's not nearly as bad as theft

* freezing of *these* coins would be both immoral and extremely dangerous for reputation of Bitcoin (no comments on freezing coins with revealed pubkeys, I haven't made my mind yet)

If the time comes I'd be happy to run a soft fork that implements this sanely.

Cheers

Martin

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[David A. Harding]

On 2025-03-16 08:25, Martin Habovštiak wrote:
> It is possible to implement fully secure recovery if we forbid
> spending of hashed keys unless done through the following scheme:

Hi Martin,

How does this differ from Tim Ruffing's version[1] of Guy Fawkes
signatures?[2][3]

Thanks, -Dave

[1]
https://gnusha.org/pi/bitcoindev/1518710367.3...@mmci.uni-saarland.de/
(but see also the whole thread and the links in it)

[2] https://www.cl.cam.ac.uk/archive/rja14/Papers/fawkes.pdf

[3] Amusing exploration of a blockchain cryptocurrency without
signatures by Joseph Bonneau and Andrew Miller:
https://jbonneau.com/doc/BM14-SPW-fawkescoin.pdf (never saw it before,
but it came up in my search results for a Guy Fawkes signatures
reference).

[Martin Habovštiak]

Hi,

conceptually they are the same idea, I didn't see that conversation before.

Howerver, in that conversation there seems to be a better developed scheme that is more secure as far as I can tell. I have just assumed that the chain cannot be censored profitably if the number of blocks to wait for is high enough (why: the miners are giving up fees by not including the transaction and the attacker would have to split the reward between all miners but the honest user only needs to outbid the attacker once)

That other idea seems to work even if the chain is censored for arbitrary number of blocks. In the meantime I was also thinking of a scheme that allows you to commit early and then wait for a any time you want but with the ability to detect that someone else is trying to spend it, so that you can act quickly. In some sense it was more similar but still not that good. The proposed scheme with unique decommitment is better and should be implemented instead of my idea.

Anyway, I believe even my weaker idea is still good enough to make my point given how hard it is to censor a transaction. My point wasn't saying that my specific implementation is the best but saying we can postpone the decision to implement a QR scheme to much later than what some people might think.

Have a nice day!

Martin

Dňa ne 30. 3. 2025, 17:41 David A. Harding <da...@dtrt.org> napísal(a):

[Martin Habovštiak]

Indeed, that's what I meant. Even crazy long period like two weeks is still better than losing everything. I have also assumed RBF and related rules to stay similar.

Still, a better scheme was proposed already that I was unaware of so far, thus I withdraw my proposal in favor of that one.

Dňa po 24. 3. 2025, 1:25 Lloyd Fournier <lloyd...@gmail.com> napísal(a):