From: "'conduition' via Bitcoin Development Mailing List" <bitcoindev@googlegroups.com>
To: Fabian <fjahr@protonmail.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] BIP draft: CISA for Taproot Key Path Spends
Date: Sat, 18 Jul 2026 18:45:06 +0000 [thread overview]
Message-ID: <IrJGniZYNo7x0fr4wABg7SsdqJoJWs5yozmiFLGzJkxOwtHU01_O8swpJMKVi_uEkdqAfFUTELpOTja53HCrgmBgQBFYup1NQ4LCZHMRcFQ=@proton.me> (raw)
In-Reply-To: <Qc_VmkfXgE8KHBfUnUB-hdWF1QOYv6zshzI7WEv_hvqJ6O_4zssJ4X2T2Di79I3TqHQvRDXsFtD8os5M44wrbB61M1LD_-GDRGYEagoHsV4=@protonmail.com>
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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:
1. 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)
2. 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 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:
1. 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).
2. 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
>
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next prev parent reply other threads:[~2026-07-18 18:50 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-18 12:39 [bitcoindev] BIP draft: CISA for Taproot Key Path Spends 'Fabian' via Bitcoin Development Mailing List
2026-07-18 18:45 ` 'conduition' via Bitcoin Development Mailing List [this message]
2026-07-19 21:57 ` 'Fabian' via Bitcoin Development Mailing List
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