* Re: [bitcoindev] BIP draft: CISA for Taproot Key Path Spends
2026-07-18 18:45 ` 'conduition' via Bitcoin Development Mailing List
@ 2026-07-19 21:57 ` 'Fabian' via Bitcoin Development Mailing List
0 siblings, 0 replies; 3+ messages in thread
From: 'Fabian' via Bitcoin Development Mailing List @ 2026-07-19 21:57 UTC (permalink / raw)
To: conduition; +Cc: Bitcoin Development Mailing List
[-- Attachment #1: Type: text/plain, Size: 8154 bytes --]
Hi conduition,
Thanks a lot for checking out the proposal and raising those
questions!
One caveat up front: I have not spent much time on the PQ side of
things yet, so I can not give you a very serious answer on those parts
and I do not want to make strong claims here. But I am interested to
invest more time there now that the proposal is published and explore
how it can be adapted for different PQ scenarios.
> reserving segwit version 2 conflicts with BIP-360 which proposes to
> use the same.
Right, this was pointed out in an inline comment as well, please see my
answer there:
https://github.com/fjahr/bips/pull/6#discussion_r3609171527
> 1. Can the framework you use for verifying aggregated sigs be
> generalized to support future aggregated PQ signatures in the future?
I would expect that the transaction-level structure should generalize:
the markers, the mode commitment in the signature message, the group
formation, and the final input carrying the aggregate should not care
which scheme is used (again, take it with a grain of salt since I have not
studied any PQ schemes in real depth). We would still need a new witness
version but there would be need for a new output type anyway in this
scenario, I guess. I certainly hope in any scenario that the framework
and related learnings will be useful in a PQ world as well.
> 2. How do you see this proposal mixing with BIP360?
I do not have a concrete good answer here yet. The trade-off you describe
is certainly real: aggregation needs all public keys as input to verification,
so hiding them costs 32 bytes per input which halves full-agg savings
and makes half-agg ~useless. This proposal deliberately picks the
taproot security model, bare keys give us maximum savings. If
P2TRv2 materializes as the migration output type, I could see CISA
certainly being defined on top of it. The witness rules here should carry
over and since script path spending follows BIP 341/342 unchanged,
committing PQ fallback leaves would work the same way as proposedthere. That definitely seems worth exploring.
A hash-hidden variant like your option b should be possible within the
same framework. This could be specified as its own output type if the
ecosystem likes this trade-off and I think that is a very interesting idea to
explore. Both outputs types could coexist and users could pick based on
their quantum fear index while the implementation would share most of
the logic/code. Even shared aggregation groups seem possible cleanly
if we add them to this proposal, but this likely wouldn't be great for
CoinJoin privacy guarantees.
But I think even the proposal as-is can provide a lot of value in many
conceivable scenarios of CRQC arrival and I think you were hinting at this
with the migration argument as well. If the network, for example, comes
to consensus that CRQCs will arrive in 1-2 years and a PQ scheme is about
to be deployed, which will be bigger and more expensive than BIP340, we
should see a lot of demand for consolidation and CoinJoins and having
CISA availability would provide a nice (though temporary) effect. But of course
this is also limited to the coins that have moved already by that time.
Best,Fabian
On Saturday, July 18th, 2026 at 8:50 PM, 'conduition' via Bitcoin Development Mailing List <bitcoindev@googlegroups.com> wrote:
> Sending this again because I think the first time didn't work.
>
> Thanks Fabian, this is very cool. I haven't read the entire BIP in detail yet, but first thing I notice on a quick skim is that reserving segwit version 2 conflicts with BIP-360 which proposes to use the same.
>
> I'm particularly interested in how to dovetail this proposal with PQ migration efforts. I have two questions:
>
> - Can the framework you use for verifying aggregated sigs be generalized to support future aggregated PQ signatures in the future? (assuming we someday have a PQ sig algorithm that admits aggregation)
>
> - How do you see this proposal mixing with BIP360? The major hurdle I see is that your proposed output type puts a bare EC pubkey on-chain in the script-pubkey. Unfortunately [Boris' EC recovery trick](https://delvingbitcoin.org/t/public-key-recovery-for-ec-leaves-in-p2mr-bip-360/2603) does not seem to work in this context: Because signatures are aggregated we can't recover pubkeys, so we can't hide them behind a hash. We either have to put them in the SPK, or the witness. So at face value, it seems there are two options:
>
> - Couple CISA tightly with P2TRv2. On one hand this would be good because it would provide a concrete incentive for users to migrate to a wallet that supports PQC. This also seems like the easier option from an engineering perspective. However it comes with all the baggage of P2TRv2 (not actually quantum-safe by default, need to solve the Q-day timing problem).
> - Hide the EC pubkeys behind a hash in the SPK (e.g. P2MR or P2TRH), and attach the EC pubkey in the witness of every aggregated TX input, so that the verifier can run the aggregated sig-verify algorithms. This would be more secure and relaxes the Q-day timing constraints, but we don't get as much relative savings.
>
> Napkin math of the witness weight of an aggregated input, assuming full-aggregation is used:
>
> - with P2TRv2:
>
> - 1 byte for the marker
> - total: 1 byte
> - with P2TRH:
>
> - 1 byte for the marker
> - 32 bytes for the EC pubkey
> - total: 33 weight units
> - with P2MR:
>
> - 1 byte for the marker (witness)
> - 32 bytes for the merkle sibling node (witness)
> - 32 bytes for the EC pubkey (witness)
> - total: 65 weight units
>
> regards,conduition
>
> On Saturday, July 18th, 2026 at 7:44 AM, 'Fabian' via Bitcoin Development Mailing List <bitcoindev@googlegroups.com> wrote:
>
>> Hi list,
>>
>> I would like to share a BIP draft for transaction-wide cross-input
>> signature aggregation (CISA). It introduces a new witness version,
>> which enables Taproot-style key path spending where inputs can
>> aggregate their signatures.
>>
>> Each input chooses between half-aggregation, full-aggregation, or an
>> explicit opt-out via a marker byte in its witness.
>> The aggregation schemes themselves are specified in the previously
>> shared BIP 458 (half-aggregation) and BIP 459 (full-aggregation)
>> drafts. Script path spending follows BIP 341/342 unchanged.
>>
>> The BIP draft text can be found here:
>> https://github.com/fjahr/bips/blob/cf0d4f2142cd0504b16e86739167b1f7ab9a3a06/bip-XXXX.mediawiki
>>
>> For inline comments, I have opened a mock pull request on my BIPs
>> repo fork:
>> https://github.com/fjahr/bips/pull/6
>>
>> Feedback of any kind is much appreciated.
>>
>> Best,Fabian
>>
>> --
>> 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/Qc_VmkfXgE8KHBfUnUB-hdWF1QOYv6zshzI7WEv_hvqJ6O_4zssJ4X2T2Di79I3TqHQvRDXsFtD8os5M44wrbB61M1LD_-GDRGYEagoHsV4%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/IrJGniZYNo7x0fr4wABg7SsdqJoJWs5yozmiFLGzJkxOwtHU01_O8swpJMKVi_uEkdqAfFUTELpOTja53HCrgmBgQBFYup1NQ4LCZHMRcFQ%3D%40proton.me.
--
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/Uk5a8rhAaNaCC8sif05lv1rmZLRYBCedbNVxPwDvHsWAA9Gbm36vGOw4XFvh7XG_MAJtiYBaD6BSEIr_KYWIxS5Ccm0OhwEIZo2gFeFfXvs%3D%40protonmail.com.
[-- Attachment #2: Type: text/html, Size: 18302 bytes --]
^ permalink raw reply [flat|nested] 3+ messages in thread