Subject: Re: [bitcoindev] P2QRH / BIP-360 Update

From: Hunter Beast <[email-masked]>
Date: Mon, 3 Mar 2025 12:57:14 -0800 (PST)

I don't disagree with your points, but I do want to point out that the BIP 
provides the option of using SLH-DSA, which is hash-based.

On Thursday, February 27, 2025 at 7:41:08 PM UTC-7 Matt Corallo wrote:

> I think you're approaching this from the wrong stance.
>
> If our goal is to "make bitcoin Quantum-secure", its gonna take a decade 
> for the state of PQ 
> research to build something that's ready for us to "just switch to". I 
> don't buy that there's a 
> world where we get serious about adding something lattice-based to Bitcoin 
> for a longggg time (I'm 
> not sure I've ever heard a serious cryptographer suggest that 
> lattice-based systems are a good idea, 
> rather than "a good thing to layer on top of a traditional non-PQC 
> scheme").
>
> In the short-term, the only (remotely-practical) thing we can do is add 
> something that we have high 
> confidence will still be secure in two decades (which basically is only 
> hash-based schemes) and get 
> wallets to include it in their taproot outputs. That gives wallets created 
> today the possibility of 
> being robust in a QC world, but, indeed, it would require tough decisions 
> in the future.
>
> If your view is that Bitcoin would simply be fine if we didn't confiscate 
> any coins in response to a 
> practical QC stealing 5% of total supply, I'm not really convinced, but we 
> can also make it a 
> version-2 segwit output ("taproot but a future softfork can freely freeze 
> the keypath spends") if 
> you really feel strongly.
>
> TBH the whole "would we confiscate if the time comes" question I think 
> simply cannot be answered 
> today because it depends very, very much on specific details (eg lets say 
> we did the above proposal 
> and its been around for 30 years and ~all wallets support it, that's a 
> very very different world 
> from, for example, deploying some PQC scheme under threat where a QC could 
> realistically steal coins 
> in five years). The only thing we can really do today is create the option 
> in the future, we cannot 
> decide for the future what to do.
>
> Matt
>
> On 2/23/25 3:33 PM, Hunter Beast wrote:
> > Hi Matt,
> > 
> > The only problem with that approach is that SLH-DSA signatures are quite 
> large. NIST has also 
> > approved ML-DSA and FN-DSA, which, while both are based on lattice 
> cryptography, they're not only 
> > standardized, but becoming widely supported. One consideration is 
> hardware acceleration, and I 
> > believe those three algorithms will have the best chance of having 
> hardware implementations as PQC 
> > extensions are added to CPUs and SoCs.
> > 
> > As for gating P2TR, the problem with that approach is that keypath 
> spends would need to be disabled 
> > and that has a confiscatory effect that I'm seeking to avoid in this BIP.
> > 
> > An additional opcode should not be necessary if multisig capability is 
> built into the attestation.
> > 
> > I agree with your statement on full BIP-32 compatibility. BIP-360 is 
> just a starting point, and 
> > maybe you're right, it's best thought of as a "break glass" 
> implementation. It's not ideal, it's 
> > full of compromises, not everyone is 100% happy with it, and that's 
> probably okay, because bitcoin 
> > isn't perfect-- but it doesn't have to be in order to work.
> > 
> > Thank you for your thoughts.
> > 
> > Hunter
> > 
> > On Friday, February 21, 2025 at 3:18:21 AM UTC-7 Matt Corallo wrote:
> > 
> > If we want to do something like this in the short to medium term, IMO we 
> should strip out all the
> > signature schemes that are anything more than quite straightforward in 
> their security assumptions
> > (i.e. only keep hash-based signatures, maybe just SPHINCS+), only embed 
> them in a taproot leaf, and
> > call it a day.
> > 
> > BIP 32 compatibility isn't a really huge deal if we're talking about an 
> "emergency break glass"
> > kinda setup - most wallets are set up with a root key and can just embed 
> the same PQ pubkey in all
> > of their outputs. The privacy cost is only realized in a break glass 
> case, and long before then
> > hopefully whatever we do today is replaced with something better, with 
> the knowledge that we'll
> > gain
> > on the way to "then". We'd still want to do it in an opcode so that we 
> can do multisig, though.
> > 
> > Matt
> > 
> > On 2/19/25 10:40 AM, Hunter Beast wrote:
> > > Dear Bitcoin Dev Community,
> > >
> > >
> > > A bit over six months after introducing the P2QRH proposal (now 
> BIP-360), I'm writing to share
> > > significant developments and request additional feedback on our 
> post-quantum roadmap, and I'd
> > also
> > > like to mention a potential P2TRH post-quantum mitigation strategy.
> > >
> > >
> > > First, now that there's a BIP number assigned, you can find the update 
> BIP here:
> > >
> > > https://github.com/cryptoquick/bips/blob/p2qrh/bip-0360.mediawiki <
> https://github.com/
> > cryptoquick/bips/blob/p2qrh/bip-0360.mediawiki> <
> https://github.com/cryptoquick/ <https://
> > github.com/cryptoquick/>
> > > bips/blob/p2qrh/bip-0360.mediawiki>
> > >
> > >
> > > The revised BIP-360 draft reflects substantial changes since initial 
> publication, particularly
> > > regarding algorithm selection. While we originally considered SQIsign, 
> it has 15,000x slower
> > > verification compared to ECC [1]. If it takes 1 second to verify a 
> fully ECC block, it would
> > take 4
> > > hours to validate a block filled with SQIsign transactions. This has 
> obvious and concerning DDoS
> > > implications.
> > >
> > >
> > > While it would take a long time to signmany thousands of SQIsign 
> transactions as well, the
> > increased
> > > time needed to sign the transactions likely won’t affect the 
> practicality of DDoS attacks--
> > another
> > > concern which has been brought to my attention. As such, I've decided 
> to deprecate SQIsign
> > from the BIP.
> > >
> > >
> > > It's worth mentioning because it was brought up in the PR, there's a 
> new class of algorithms
> > that
> > > support signature aggregation, but they generally result in signatures 
> that are still quite
> > large.
> > > Chipmunk and RACCOON are good examples [2], [3]. I do expect that to 
> improve with time. It
> > might be
> > > worthwhile to shorten the list by making signature aggregation a 
> requirement, so as not to
> > regress
> > > too far from Schnorr signatures. That said, I think those capabilities 
> should be introduced in a
> > > separate BIP once they're more mature and worthwhile.
> > >
> > >
> > > Our current shortlist prioritizes FALCON for its signature aggregation 
> potential, with
> > SPHINCS+ and
> > > CRYSTALS-Dilithium as secondary candidates. However, major technical 
> challenges remain,
> > particularly
> > > BIP-32 compatibility issues affecting xpub generation in watch-only 
> wallets, as detailed by
> > > conduition in another mailing list discussion [4], and also, how we 
> should handle multisig
> > wallets.
> > >
> > >
> > > Additionally, I think it's worthwhile to restrict BIP-360 to 
> NIST-approved algorithms to
> > maintain
> > > FIPS compliance. That's because HSMs such as those provided by 
> Securosys already have support
> > for
> > > all three algorithms [5], which is essential for secure deployment of 
> federated L2 treasuries.
> > >
> > >
> > > Presently, for multisigs, we have a merkle tree configuration defined 
> for encumbering the output
> > > with multiple keys. While that's efficient, it's a novel construction. 
> I'm not certain we should
> > > proceed with the merkle tree commitment scheme-- it needs more 
> scrutiny. We could use a sort
> > of P2SH
> > > approach, just modifying the semantics of OP_CHECKMULTISIG in a 
> witness script to alias to
> > public
> > > keys in the attestation. But that could introduce additional overhead 
> in a signature scheme that
> > > already uses a lot more space. Without this, however, we do not yet 
> have a way specified to
> > indicate
> > > thresholds or a locking script for the attestation, as it is designed 
> to be purposely
> > limited, so as
> > > specified it is only capable of n/n multisig. I consider m/n multisigs 
> to be the single largest
> > > obvious omission in the spec right now. It definitely needs more 
> thought and I'm open to
> > > suggestions. Perhaps two additional bytes at the top level of the 
> SegWit v3 output hash could be
> > > provided to indicate PQC signature threshold and total, and those 
> would be hashed and
> > committed to
> > > in the output, then provided in a field in the attestation once spent.
> > >
> > >
> > > While finalizing PQC selections, I've also drafted P2TRH as an interim 
> solution to secure
> > Taproot
> > > keypath spends without disabling them, as Matthew Corallo proposes in 
> the aforementioned mailing
> > > list thread [4]. The P2TRH approach hashes public keys rather than 
> exposing them directly,
> > > particularly benefiting:
> > >
> > >
> > > - MuSig2 Lightning channel implementations
> > >
> > > - FROST-based MPC vaults
> > >
> > > - High-value transactions using private pools that don't reveal the 
> block template
> > >
> > >
> > > For those interested, take a look at the draft BIP for P2TRH here: 
> https://github.com/
> > cryptoquick/ <https://github.com/cryptoquick/>
> > > bips/blob/p2trh/bip-p2trh.mediawiki <
> https://github.com/cryptoquick/bips/blob/p2trh/bip-
> > p2trh.mediawiki <
> https://github.com/cryptoquick/bips/blob/p2trh/bip-p2trh.mediawiki>>
> > >
> > >
> > > I have my hands full with P2QRH advocacy and development and would 
> prefer to focus on that,
> > but I
> > > wanted to introduce P2TRH in case that is attractive as the 
> community's preferred solution-- at
> > > least for Taproot quantum security. The tradeoff is that it adds 8.25 
> vB of overhead per
> > input, and
> > > key tweaking might have slightly less utility for some applications, 
> and it also doesn't protect
> > > against short exposure quantum attacks as defined in BIP-360.
> > >
> > >
> > > Returning to P2QRH and what's needed to push it across the finish 
> line...
> > >
> > >
> > > I still need to finish the test vectors. I'm implementing these using 
> a fork of rust-bitcoin and
> > > modeling them after Steven Roose's work on BIP-346. I've been told 
> that's not a blocker for
> > merging
> > > the draft, but if it isn't merged by the time I'm finished, hopefully 
> that will provide some
> > > additional impetus behind it.
> > >
> > >
> > > One concern Murch brought up is that introducing four new algorithms 
> into the network was too
> > many--
> > > adding too much complexity to the network and to wallets and other 
> applications-- and I agree.
> > >
> > >
> > > Hopefully this is addressed to some degree by removing SQIsign 
> (especially in its current state
> > > lacking implementation maturity), and will help push the BIP below a 
> certain complexity
> > threshold,
> > > making it somewhat easier to review.
> > >
> > > I think it's still important to include multiple signature algorithm 
> options for users to select
> > > their desired level of security. It's not 100% certain that all of 
> these algorithms will remain
> > > quantum resistant for all time, so redundancy here is… key.
> > >
> > >
> > > Another concern is that NIST level V is overkill. I have less 
> conviction on this since secp256k1
> > > technically has 128 bits of security due to Pollard's rho attacks. But 
> if the intention was
> > for 256
> > > bits of security, should level V security be the default? It's 
> difficult for me to say.
> > Perhaps both
> > > level V and level I implementations could be included, but this would 
> be a deviation from the
> > BIP as
> > > presently specified, which defaults to level V security. The 
> disadvantage of including level I
> > > support for each algorithm is that it essentially doubles the 
> complexity of libbitcoinpqc.
> > >
> > >
> > > Ultimately, I hope the default of NIST V and selection of 3 mature 
> NIST-approved algorithms
> > > demonstrate a focused, polished, and conservative proposal.
> > >
> > >
> > > At this point, the major call to action I would like to highlight is 
> simply the need for more
> > > feedback from the community. Please review and provide feedback here: 
> https://github.com/
> > bitcoin/ <https://github.com/bitcoin/>
> > > bips/pull/1670 <https://github.com/bitcoin/bips/pull/1670 <
> https://github.com/bitcoin/bips/
> > pull/1670>>
> > >
> > >
> > > I look forward to feedback and opinions on P2QRH and P2TRH.
> > >
> > >
> > > P.S. I'll be advocating for BIP-360 at OP_NEXT in VA, btc++ in Austin, 
> Consensus in Toronto,
> > and BTC
> > > 25 in Las Vegas, and later this year, TABConf in Atlanta.
> > >
> > >
> > >
> > > [1] https://pqshield.github.io/nist-sigs-zoo <
> https://pqshield.github.io/nist-sigs-zoo>
> > >
> > > [2] https://eprint.iacr.org/2023/1820.pdf <
> https://eprint.iacr.org/2023/1820.pdf>
> > >
> > > [3] https://eprint.iacr.org/2024/1291.pdf <
> https://eprint.iacr.org/2024/1291.pdf>
> > >
> > > [4] 
> https://groups.google.com/g/bitcoindev/c/8O857bRSVV8/m/7uu4dZNgAwAJ 
> <https://
> > groups.google.com/g/bitcoindev/c/8O857bRSVV8/m/7uu4dZNgAwAJ>
> > >
> > > [5] 
> https://docs.securosys.com/tsb/Tutorials/Post-Quantum-Cryptography/pqc-release-overview
> > <
> https://docs.securosys.com/tsb/Tutorials/Post-Quantum-Cryptography/pqc-release-overview
> >
> > >
> > >
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