Hashed keys are actually fully quantum secure

Hashed keys are actually fully quantum secure
2025-03-16 18:50 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
                   ` (3 more replies)
  
From: Martin Habovštiak @ 2025-03-16 18:25 UTC (permalink / raw)
  To: Bitcoin Development Mailing List

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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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2025-03-16 18:50 ` 
                   ` 
  
From: Martin Habovštiak @ 2025-03-16 18:25 UTC (permalink / raw)
  To: Bitcoin Development Mailing List

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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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* 
  2025-03-16 18:25 [bitcoindev] Hashed keys are actually fully quantum secure Martin Habovštiak

  2025-03-18 16:48   ` Erik Aronesty
  2025-03-24  0:24   ` Lloyd Fournier
  2025-03-16 19:03 ` 
                   ` 
  3 siblings, 2 replies; 11+ messages in thread
From: 'Antoine Poinsot' via Bitcoin Development Mailing List @ 2025-03-16 18:50 UTC (permalink / raw)
  To: Martin Habovštiak;  Bitcoin Development Mailing List

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> 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.
On Sunday, March 16th, 2025 at 2:25 PM, Martin Habovštiak <martin.habovstiak@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
>
> --
> 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 email to bitcoindev+unsubscribe@googlegroups•com.
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2025-03-16 19:03 ` Agustin Cruz

  
From: Martin Habovštiak @ 2025-03-16 20:52 UTC (permalink / raw)
  To: Agustin Cruz;  Bitcoin Development Mailing List

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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 <agustin.cruz@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
>
> El dom, 16 de mar de 2025, 3:31 p. m., Martin Habovštiak <
> martin.habovstiak@gmail•com> escribió:
>
>> 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
>>
>> --
>> 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
>> email to bitcoindev+unsubscribe@googlegroups•com.
>> To view this discussion visit
>> https://groups.google.com/d/msgid/bitcoindev/CALkkCJY%3Ddv6cZ_HoUNQybF4-byGOjME3Jt2DRr20yZqMmdJUnQ%40mail.gmail.com
>> <https://groups.google.com/d/msgid/bitcoindev/CALkkCJY%3Ddv6cZ_HoUNQybF4-byGOjME3Jt2DRr20yZqMmdJUnQ%40mail.gmail.com?utm_medium=email&utm_source=footer>
>> .
>>
>

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2025-03-16 18:25 [bitcoindev] Hashed keys are actually fully quantum secure Martin Habovštiak
  2025-03-16 18:50 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
  2025-03-16 19:03 ` Agustin Cruz

  2025-03-17 11:07   ` Martin Habovštiak
  2025-03-30 15:41 ` David A. Harding
  
From: Lloyd Fournier @ 2025-03-17 10:44 UTC (permalink / raw)
  To: Martin Habovštiak;  Bitcoin Development Mailing List

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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

On Mon, 17 Mar 2025 at 05:31, Martin Habovštiak <martin.habovstiak@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
>
> --
> 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
> email to bitcoindev+unsubscribe@googlegroups•com.
> To view this discussion visit
> https://groups.google.com/d/msgid/bitcoindev/CALkkCJY%3Ddv6cZ_HoUNQybF4-byGOjME3Jt2DRr20yZqMmdJUnQ%40mail.gmail.com
> <https://groups.google.com/d/msgid/bitcoindev/CALkkCJY%3Ddv6cZ_HoUNQybF4-byGOjME3Jt2DRr20yZqMmdJUnQ%40mail.gmail.com?utm_medium=email&utm_source=footer>
> .
>

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2025-03-17 10:44 ` Lloyd Fournier

  
From: Martin Habovštiak @ 2025-03-17 11:07 UTC (permalink / raw)
  To: Lloyd Fournier;  Bitcoin Development Mailing List

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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.fourn@gmail•com> napísal(a):

> 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
>
> On Mon, 17 Mar 2025 at 05:31, Martin Habovštiak <
> martin.habovstiak@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
>>
>> --
>> 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
>> email to bitcoindev+unsubscribe@googlegroups•com.
>> To view this discussion visit
>> https://groups.google.com/d/msgid/bitcoindev/CALkkCJY%3Ddv6cZ_HoUNQybF4-byGOjME3Jt2DRr20yZqMmdJUnQ%40mail.gmail.com
>> <https://groups.google.com/d/msgid/bitcoindev/CALkkCJY%3Ddv6cZ_HoUNQybF4-byGOjME3Jt2DRr20yZqMmdJUnQ%40mail.gmail.com?utm_medium=email&utm_source=footer>
>> .
>>
>

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2025-03-16 18:50 ` 'Antoine Poinsot' via Bitcoin Development Mailing List

  2025-03-24  0:24   ` Lloyd Fournier
  
From: Erik Aronesty @ 2025-03-18 16:48 UTC (permalink / raw)
  To: Bitcoin Development Mailing List


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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).


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2025-03-16 18:50 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
  2025-03-18 16:48   ` Erik Aronesty

  2025-03-30 20:16     ` Martin Habovštiak
  
From: Lloyd Fournier @ 2025-03-24  0:24 UTC (permalink / raw)
  To: Antoine Poinsot;  Martin Habovštiak, Bitcoin Development Mailing List

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On Tue, 18 Mar 2025 at 00:48, 'Antoine Poinsot' via Bitcoin Development
Mailing List <bitcoindev@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.habovstiak@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
>
> --
> 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
> email to bitcoindev+unsubscribe@googlegroups•com.
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> .
>
>
> --
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> <https://groups.google.com/d/msgid/bitcoindev/XHIL8Z4i4hji8LhbJ0AiKQ4eago2evXwjTGUOqqyAye_2nM3QicDpHo6KkcznBAHPUrIWSLj_GuiTQ_97KPjxcOrG8pE0rgcXucK2-4txKE%3D%40protonmail.com?utm_medium=email&utm_source=footer>
> .
>

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2025-03-16 18:25 [bitcoindev] Hashed keys are actually fully quantum secure Martin Habovštiak
                   ` (2 preceding siblings ...)
  2025-03-17 10:44 ` Lloyd Fournier

  2025-03-30 20:11   ` Martin Habovštiak
  
From: David A. Harding @ 2025-03-30 15:41 UTC (permalink / raw)
  To: Martin Habovštiak;  Bitcoin Development Mailing List

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/1518710367.3550.111.camel@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).

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 permalink raw reply	[|nested]
2025-03-30 15:41 ` David A. Harding

  
From: Martin Habovštiak @ 2025-03-30 20:11 UTC (permalink / raw)
  To: David A. Harding;  Bitcoin Development Mailing List

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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 <dave@dtrt•org> napísal(a):

> 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/1518710367.3550.111.camel@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).
>

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2025-03-24  0:24   ` Lloyd Fournier

  
From: Martin Habovštiak @ 2025-03-30 20:16 UTC (permalink / raw)
  To: Lloyd Fournier;  Antoine Poinsot, Bitcoin Development Mailing List

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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.fourn@gmail•com> napísal(a):

>
>
> On Tue, 18 Mar 2025 at 00:48, 'Antoine Poinsot' via Bitcoin Development
> Mailing List <bitcoindev@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.habovstiak@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
>>
>> --
>> 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
>> email to bitcoindev+unsubscribe@googlegroups•com.
>> To view this discussion visit
>> https://groups.google.com/d/msgid/bitcoindev/CALkkCJY%3Ddv6cZ_HoUNQybF4-byGOjME3Jt2DRr20yZqMmdJUnQ%40mail.gmail.com
>> .
>>
>>
>> --
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>> "Bitcoin Development Mailing List" group.
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>> email to bitcoindev+unsubscribe@googlegroups•com.
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>> https://groups.google.com/d/msgid/bitcoindev/XHIL8Z4i4hji8LhbJ0AiKQ4eago2evXwjTGUOqqyAye_2nM3QicDpHo6KkcznBAHPUrIWSLj_GuiTQ_97KPjxcOrG8pE0rgcXucK2-4txKE%3D%40protonmail.com
>> <https://groups.google.com/d/msgid/bitcoindev/XHIL8Z4i4hji8LhbJ0AiKQ4eago2evXwjTGUOqqyAye_2nM3QicDpHo6KkcznBAHPUrIWSLj_GuiTQ_97KPjxcOrG8pE0rgcXucK2-4txKE%3D%40protonmail.com?utm_medium=email&utm_source=footer>
>> .
>>
>

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