Subject: Re: [bitcoindev] Hashed keys are actually fully quantum secure

From: Martin Habovštiak <[email-masked]>
Date: Sun, 30 Mar 2025 22:16:08 +0200

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 <[email-masked]> napísal(a):

>
>
> On Tue, 18 Mar 2025 at 00:48, 'Antoine Poinsot' via Bitcoin Development
> Mailing List <[email-masked]> 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 <
>> [email-masked]> 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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