Re: [bitcoindev] In defense of a PQ output type

Matt Corallo
2026-04-10T00:34:07Z

On 4/9/26 2:58 PM, 'Antoine Poinsot' via Bitcoin Development Mailing List wrote:
> Many of us appear to be in favour of introducing post-quantum signatures to
> Bitcoin via a new Tapscript operation, conditioning the CRQC resistance on a
> future invalidation of Taproot key spends.I would like to offer an argument in
> favour of introducing such post-quantum signatures as a new output type instead,
> that does not depend on invalidating a spending path on existing outputs.
>
> First of all, it's important to clarify what we are trying to achieve. We need
> to accept that, by virtue of being faced with an uncertain existential threat to
> the network, there are scenarios, however unlikely, in which the network does
> not survive. Not all plausible futures are worth optimizing for. For instance,
> one in which PoW ends up broken only a few years after EC crypto, or one where
> the entire Bitcoin userbase *must* migrate within a handful of years.
>
> I think there are two futures worth optimizing for primarily:
> - a CRQC never materializes and users can continue benefiting from the
> properties of a Bitcoin network relying on classical cryptographic
> assumptions;
> - a CRQC materializes on a long enough timeframe that PQ signature schemes that
> maintain today's properties can be designed, vetted and adopted, and the vast
> majority of the userbase migrated.
>
> And because hope is not a strategy, it's important to also have a "break glass"
> emergency plan in case a CRQC emerges on a shorter (yet still reasonable)
> timeframe. I think the current proposals for hash-based PQ schemes fit this
> category. If they became the only safe option available, it would certainly make
> Bitcoin a lot less attractive. But having them around is good risk mitigation
> *regardless* of whether a CRQC emerges.
>
> It's often argued that a freeze will be necessary anyways, therefore we might as
> well disable the Taproot keyspend path simultaneously and simply introduce the
> PQ scheme today in Tapscript. I personally reject the premise, but more
> importantly i think we should separate the concerns of 1) making a PQ scheme
> available and 2) freezing vulnerable coins. Yet introducing a PQ scheme inside
> vulnerable Taproot outputs locks us onto the path of eventually freezing
> vulnerable coins, as it will inevitably turn users of the PQ scheme into
> supporters of a freeze.

You've missed the much-more-important thing that cannot be extricated from disabling insecure spend
paths - the ability to recover from a seedphrase. In any world where a CRQC exists, whether it is
next year or a century from now, there will be a million or two bitcoin in lost-key-wallets and a
nontrivial amount in wallets people haven't touched in ten years but still have keys for. Given a
goal of any migration strategy should be to enable the absolute maximum number of coins to be
retained by their owners (I think this is basically the *only* goal?), enabling seedphrase proof
recovery is pretty critical. Sadly the only way that can be done is through disabling insecure spend
proofs.

But you've also confused unrelated concerns here - whether a hash-based signature is added as a
tapscript opcode or not is not strictly tied to whether a new output type is created. If BIP 360 is
the way bitcoin goes, it *still* needs a new hash-based opcode in tapscript. Maybe more
interestingly, a new taproot "version" could be added which has identical semantics to today's
taproot but signals through an alternative version number (or maybe there's a cleverer way to encode
a bit somewhere that we should prefer) that a PQC pubkey exist in a taproot leaf somewhere so the
insecure spend path should be disabled.

> This approach would tie the availability of a PQ scheme to reaching consensus on
> a future freeze. Frankly, i do not believe the latter is achievable, let alone
> at this stage with so little evidence that a CRQC will materialize anytime soon.
> By contrast, there is a much stronger case for introducing a PQ scheme in the
> near term purely as a risk mitigation measure. Coupling the two decisions would
> necessarily delay the deployment of a PQ scheme, unnecessarily exacerbating
> risks whether or not CRQCs become a reality.

Adding a PQ output type which no one will use (eg one where use of the hash-based signature is
mandatory, which drives fees up hugely and has all the drawbacks you mention) is not a risk
mitigation strategy - it does not materially allow for any migration and doesn't accomplish much of
anything. But as mentioned above I do not see why any addition of hash based signatures to tapscript
should require any kind of community consensus on future disablement of insecure spend paths - not
only is it a likely prerequisite for an alternative output type, but its also (obviously?) not
possible to have any kind of "consensus" on what the future bitcoin community will do. Thus it would
be rather impossible to do *anything* if that were a requirement.

> Another drawback of the PQ output type approach is that it would make those
> outputs distinguishable from Taproot ones, which is suboptimal in the event that
> a CRQC never materializes. But i would argue that even in this case, the cost is
> minimal. The users most likely to adopt PQ outputs today (those securing large
> amounts of BTC with a small set of keys) already have vastly different usage
> patterns from Taproot users: they often reuse addresses and use legacy output
> types (and show little interest in upgrading).
>
> Best,
> Antoine Poinsot
>

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