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

Antoine Riard
2026-04-11T01:28:34Z

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Hi,

Thanks for rolling up the ball forward on this topic.

I'm +1 on disentangling the introduction of a PQ safe scheme from
the more fuzzy idea of freezing coins based on output types.

Even the idea of "freezing" coins, the goal of why is still unclear.
It sounds the motivations are blurred between ensuring coins are
staying in the hands of their legitimate owners, a goal I can share
but I don't see how freezing help here, from the more loose idea of
ensuring there is no crash in the bitcoin price vs fiat in the face
of CQRC-enabled attacks, which sounds to me a pandora box.

Even in this eventuality, if there is a general concern on the network
disruptions that might be induced by CRQC attacks (e.g chain instability
due to reorgs by competing CRQC attackers), I believe there are still
intermediary technical solutions, e.g rate-limiting the number of output
types that can be spent by difficulty periods to minimize the risks of
disruptions, while not technically confiscating anyone coin.

Back to introducing a PQ safe scheme, I don't think in this thread
the question is raised to enable to secure one's coin under double
classic cryptogrraphic assumption and PQ assumption, i.e "hybrid"
security (more for the risk of a cryptanalysis break of any PQ safe
scheme that would be introduced at the consensus-level). It might
more a real engineering burden, though I believe it's giving more
flexibility for technically savy bitcoin users to secure one's stack.

Anyway, I think it's good to have a scheme ready early on given
the development cycle to have stuff available on HW wallets and
HSMs. E.g BIP32 support was added in 2018 on Gemalto's HSM i.e a
mere 6 years after the standard introduction (which is not that
bad given that blockchain were recents actors in the hardware
industry at the time).

Best,
Antoine
OTS hash: 6d7c2f5ab01bcdda4ec27d4c21198a9b13ce1dfd138c4a2e6dfaedee9458f6c0

Le Saturday, April 11, 2026 à 2:06:55 AM UTC+1, Hayashi a écrit :

> Hi Conduition, Matt and Ethan
>
> > an ownership proof used for non-BIP32 hashed addresses
> I’m concerned that shared xpubs could become an attack vector if we allow
> ZKP of hash preimages for unused addresses (excluding P2PK/P2TR). Given
> that, are there alternative methods for publishing proof of ownership that
> we should consider?
>
>
> It seems the current default stance is effectively "do not freeze,"
> because preserving the status quo is the only path if we cannot reach
> consensus (and if we do not chose hardfork). However, by formalizing a
> freezing plan—either through a new BIP or an amendment to BIP361—I believe
> we gain several strategic advantages:
>
> *Clarity on P2MR discussion*: It would clarify the ongoing P2MR and P2TR
> discussions by defining how P2TR will be treated (I personally prefer P2MR).
>
> *Incentivized Migration*: Establishing a clear future plan encourages
> users to migrate to BIP32-hardened addresses with longer time period which
> eventually maximize recovery.
>
> *Advance Planning for CRQCs*: We will not panic on the edge case scenario
> that CRQCs arrive earlier than PQ signature scheme adoption or when we find
> out we cannot allow enough migration period after PQ signature scheme
> adoption (I strongly believe we also have to prepare for this future).
>
> While further R&D is required, we likely have sufficient information to
> formalize a framework now. We can also disable or modify the defined
> freezing plan if the threat landscape changes significantly.
>
> Hayashi
> 2026年4月11日土曜日 8:33:54 UTC+8 Ethan Heilman:
>
>> > IMO even something like P2MR's additional cost will strongly
>> discourage adoption.
>>
>> I don't agree.
>>
>> Over time as quantum attacks become a bigger and bigger concern for
>> holders, wallets will want to show that they can offer security against
>> CRQCs. This is especially true for wallets focused on high value Bitcoin
>> outputs. Even if someone thinks there is only a 2% chance they lose all
>> their Bitcoin because of a quantum computer, that 2% chance will keep them
>> up at night.
>>
>> P2MR would have 17.25 more vBytes, an 11% overhead.
>>
>> P2TR 1 input, 2 output - key path spend. 154 vbytes
>> P2MR 1 input, 2 output - spending a schnorr sig leaf of a P2MR output
>> with two leafs: 1. PQ sig leaf and 2. Schnorr sig leaf. 171.25 vbytes
>>
>> I'm stacking the deck against P2MR here. Under some circumstances P2MR
>> has lower fees than P2TR.
>>
>> It is hard to imagine someone holding significant quantities of Bitcoin
>> not wanting to pay 50 sats to ensure their Bitcoin isn't stolen by a
>> quantum computer.
>>
>>
>> On Fri, Apr 10, 2026 at 7:10 PM Matt Corallo <lf-l...@mattcorallo.com>
>> wrote:
>>
>>>
>>>
>>> On 4/10/26 1:03 PM, conduition wrote:
>>> >> 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
>>> >
>>> > I think Antoine's point here is that if we introduce a PQC opcode to
>>> tapscript but choose NOT to deploy P2MR, and then encourage people to use
>>> that opcode in P2TR script leaves, then we are locking ourselves into the
>>> assumption that the community will later disable P2TR key-path spending -
>>> otherwise those addresses will be compromised by a CRQC and the PQC leaf
>>> script is useless.
>>>
>>> Right, but you cut my quote off and appear to be responding to a point I
>>> didn't make? The very next
>>> few words that you cut were "not only is it a likely prerequisite for an
>>> alternative output type".
>>> Yes, we have to figure out what kind of output type we want, whether
>>> P2MR (360), P2TRv2 or just
>>> P2TR. There are strong arguments for each. But none of that has any
>>> bearing on whether we add hash
>>> based signatures to tapscript. We have to add hash based signatures to
>>> tapscript first no matter
>>> what output type we want!
>>>
>>> >> 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
>>> >
>>> > I don't think anyone is suggesting deployment of an output type with
>>> mandatory hash-based signatures. That would be borderline unusable for
>>> anyone but large companies and wealthy elites.
>>> >
>>> > Every decent proposal I've seen has suggested using PQC in tandem with
>>> ECC across multiple tapscript leaves, whether in some bastardized variant
>>> of P2TR, or in BIP360's P2MR.
>>>
>>> IMO even something like P2MR's additional cost will strongly discourage
>>> adoption. We have a very
>>> long history with Bitcoin wallets not only refusing to adopt new
>>> features but actively making some
>>> of the worst possible design decisions from a Bitcoin PoV. IMO we should
>>> very strongly not give them
>>> any excuse, even if that's just fees.
>>>
>>> Matt
>>>
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>>> .
>>>
>>

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