This certification layer idea is straight from SPHINCS' own whitepaper. This is how SPHINCS authors created its many-time property without blowing up the runtime.
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Tadge, your PoQC idea is neat but it relies on at least one "good guy" QC willing to produce the proof. A self-interested QC would never willingly crack a pubkey which it knows would activate a soft-fork against its own interest. Any publicly-advertised PoQC honeypot addresses able to gather large donations would be obvious, and a rational QC would ignore them. I suppose users could sprinkle some hidden honeypot addresses around the network and hope the QC bites one of them, but I don't think that's practical - The QC would probably prioritize addresses with large balances like Satoshi's wallets. I'm not sure if I have any better ideas though, besides the also-risky option of relying on the community to act in unison, triggering activation manually if/when a QC appears to be coming soon to a blockchain near us. So a PoQC might be a good additional trigger, but we also need to be able to manually activate the post-quantum upgrade in case the PoQC is unavailable.
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Another fun aspect of QC's we haven't discussed yet is BIP32. Whatever pubkey we hide in the OP_SUCCESS tap leaf, it needs to be derived via quantum-secure means. So there can be no unhardened BIP32 anywhere in the derivation chain of the PQ-secret-key, or else xpubs can be sold to a QC for profit, even if the PQ soft fork upgrade is activated on time.
It seems like whatever upgrade path we go with, it will mean the end of BIP32 xpubs as we know them. A 3rd party won't be able to derive my post-quantum P2TR address from a regular xpub alone without also knowing the OP_SUCCESS script's tap leaf hash, which by necessity must be completely independent of the xpub. Sounds like we may need a new standard for that too. Perhaps some kind of 'wrapper' which embeds a regular BIP32 xpub, plus the tap leaf hash of the OP_SUCCESS script? That'd be enough for an untrusted 3rd party to derive a standard set of P2TR addresses with the attached PQ fallback leaf script hash.
A second wacky idea which modifies (bastardizes) BIP32 instead of replacing it: Replace the xpub's chain code with the OP_SUCCESS script's tap leaf hash before distributing. You could derive the PQ keypair from the parent xpriv, to maintain the integrity of BIP32's chained derivation, so in a it would be like inserting a little modification into BIP32's hardened derivation logic. Software which doesn't have PQ-fallback support will derive the standard P2TR addresses we all use today. Software which does have PQ-fallback support will derive the same child taproot internal keys, but tweaked with a PQ-fallback script leaf. I imagine this might make compatibility very complicated though, so i'm not sure if this is a smart choice - throwing this out there in case it inspires better ideas but i wouldn't advocate for it myself.
On Monday, December 16, 2024 at 5:22:44 PM UTC-5 Tadge Dryja wrote:
Hi everyone
(disclosure: I'm highly skeptical QCs will break secp256k1 in my lifetime, but who knows)
IMHO the activation dilemma is the trickiest part of Bitcoin
dealing with QCs. On the one hand, you want a very long term plan, many
years ahead of time, so that everyone has time to migrate and not get
their coins stolen or destroyed. But the further out a QC is, the less
likely it seems it will ever occur, and thus the less reason there is to
write software to deal with a theoretical, far off problem. (And that's
not even getting into the fact that nobody's in charge of Bitcoin so
there's no long term roadmap anyway.)
The ability to have a PQ
fallback key with zero or very low on-chain cost helps a lot with this,
whichever activation path ends up happening. Picking something and committing to it in wallets today, before any kind of activation, is a bit scary since the PQ pubkey does become an exposed private key. But I think there is a pretty good way to do it with a consensus level proof of quantum computer.
On Monday, December 16, 2024 at 7:35:47 AM UTC-5 Anthony Towns wrote:
- Disabling key path taproot spends via soft-fork is extremely
confiscatory -- for the consensus cleanup, we worry about even the
possibility of destroying funds due to transaction patterns never
seen on the network; here, shutting down key path spends would be
knowingly destroying an enormous range of utxos.
This is true, but faced with a QC you're in trouble either way: either the coins are destroyed, or the first (non-nice) entity to get a QC steals them. We can hope that if the QC ever does arrive there will be enough warning and coordination that there won't be *too* many of these utxos at that point.
But there are still a lot of lost coins where nobody knows the private keys anymore and in those cases, the lead time doesn't matter. The original owners who lost the keys would probably prefer those coins to be destroyed rather than stolen. But of course there's no way for them to reliably express that preference since they can no longer sign.
An on-chain proof of quantum computer (PoQC I guess :) ) would be a way to reduce the damage of activation forks. One way to build it: Create a NUMS point pubkey - something like described in BIP341. Send some coins to that address, then watch if it gets spent. Providing a preimage of the x-coordinate of a point, as well as a valid signature from that point means that either the hash function is broken or (what we're assuming here) the one-wayness of the EC base point multiplication has been broken. Nodes can then have code which watches for such a proof and changes consensus rules based on it.
This can be useful for the activation, or persistence of a confiscatory restriction of key path spends. For example:
Say people worry about an immanent QC. They estimate it will be built in 5-8 years. They write software which contains a temporary fork, which activates in 3 years and *de*activates in 8 years. This fork disables almost all key path spends (as well as ECDSA signatures). The only key path spends allowed are from the NUMS utxos, and if any NUMS utxo is spent, then the EC prohibition locks in to become permanent instead of reverting 3 years after initial enforcement.
In this case the soft fork is only (permanently) confiscatory if the QC provably exists and would have (presumably, sooner or later) confiscated all those coins anyway. It also could also allow people to enable PQ signatures and disable EC signatures a bit sooner than they otherwise would have, since the cost of being wrong about a QC is lessened -- losing EC operations would be painful, but even more so if it turns out the nearly finished QC never quite gets there.
An opt-in, non-confiscatory fork could also use this mechanism. A new P2TR output type (PQ2TR?) could be used which is explicitly not key-spendable post-PoQC, but the older P2TR, P2WPKH, etc output types are still EC spendable (better move em quick).
It can also work the other way: The new PQ output type can work just like P2TR, except that one opcode (the OP_PQCHECKSIG) becomes an OP_FAIL until the PoQC. Given PoQC, it's OP_SUCCESS. That way we don't have to have a consensus level definition the actual PQ signature algorithm yet; we've just put a placeholder PQ signature opcode in, and an activation method. A later soft fork can then define the signature algo. You'd want to define it pretty soon after, since wallets committing to PQ pubkeys for schemes that end up not getting implemented doesn't help. But it does allow wallets to do something soon for people who are worried and want to be "quantum safe".
-Tadge