Hey friends, Even if we can require a pre-quantum output to be paired with a QR output when spending in this way, and even if the QR output must be at least X blocks old... What prevents an attacker from just pre-minting a whole bunch of QR outputs, aging them for a while, and then lying in wait to steal? A well-prepared QC attacker's QR outputs may even be significantly older than an honest user's QR outputs. An aged QR output committing to a QR signature proves nothing about the ownership of an unrelated pre-quantum UTXO. The QR output must prove historical ownership of the vulnerable EC key-hashed output. To fix this, we must change this line in OP: > 2. the user creates a transaction that, aside from having a usual > spendable output also commits to a signature of QR public key. This transaction must be fully protected by QR signing. It must commit to, but not reveal, the EC public key, while also proving ownership. I would correct this description to: > 2. the user creates a transaction with at least one QR input which, > aside from having a usual spendable output also commits to > *a signature from the legacy EC pubkey.* This TX might have an OP_RETURN output or an inscription which embeds SHA256(ec_signature).  Or, like taproot, the QR output script might itself contain a hidden commitment to that hash.  A few blocks after this transaction is mined, the honest user can spend the QR and legacy UTXOs together, opening the EC signature commitment. Validating nodes would have to check the QR output is old enough, but also check that it committed to the correct pubkey+signature. A QC attacker shouldn't be able to break this unless the legacy EC pubkey has already been revealed prior to the commitment TX. Only the authentic user could've pre-committed to that signature. If we assume the QC attacker can't roll-back the chain more than X blocks, they can't go back and insert an EC sig commitment retroactively. I suspect this might've been Martin's intent, judging from the way he was writing? regards, conduition On Sunday, March 23rd, 2025 at 8:24 PM, Lloyd Fournier wrote: > > > On Tue, 18 Mar 2025 at 00:48, 'Antoine Poinsot' via Bitcoin Development Mailing List 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 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. > > > > -- > > 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/XHIL8Z4i4hji8LhbJ0AiKQ4eago2evXwjTGUOqqyAye_2nM3QicDpHo6KkcznBAHPUrIWSLj_GuiTQ_97KPjxcOrG8pE0rgcXucK2-4txKE%3D%40protonmail.com. > > -- > 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/CAH5Bsr0muoF27besnoQh32vL-keujeR%2Bd-_JurE0%2ByXY5gPKQg%40mail.gmail.com. -- 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/Rgj4DeSKQkdEWMRTmqYYLas84WIDyRftEKqmwlw0C9-ur4Tx9_d6g7SzTU_WBspYbezLDTMpgIFXon1_cpFSjgYOMtHlQJNS_utF2dZQ4ig%3D%40proton.me.