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

From: "'Antoine Poinsot' via Bitcoin Development Mailing List" <[email-masked]>
Date: Sun, 16 Mar 2025 18:50:45 +0000

> 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 <[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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