Date: Mon, 15 Jan 2024 23:29:46 +0000
To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
 bitcoindev@groups.io
From: Ava Chow <lists@achow101.com>
Message-ID: <5d299fc4-8809-4f32-a9b8-17e353d6ff30@achow101.com>
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Subject: [bitcoin-dev] MuSig2 derivation, descriptor, and PSBT field BIPs
Precedence: list

Hi All,

In October I sent the MuSig2 descriptor and PSBT field BIPs to the list.=20
Since then, I've made a few changes to the BIPs and am looking for=20
feedback on these.

The most significant change is the addition of third BIP which describes=20
how the synthetic xpubs are constructed and derived. This is split from=20
the descriptors BIP as I felt that the PSBT fields BIP needed to=20
reference this process too, and referencing the descriptors BIP for that=20
seemed a bit odd.

Otherwise, the descriptors BIP is unchanged, although I am open to=20
Salvatore's suggestion of dropping the ranged derivation within the=20
expression and only allow ranged derivation of the aggregate pubkey itself.

I've also made a change to the PSBT fields BIP where the aggregate=20
pubkey is included as a plain pubkey rather than as xonly. I think this=20
change is necessary for to make discovering derived keys easier. The=20
derivation paths for derived keys contain the fingerprint of the parent=20
(i.e. the aggregate pubkey) and the fingerprint requires the evenness=20
bit to be serialized. So the aggregate pubkey in the PSBT fields need to=20
contain that evenness information in order for something looking at only=20
the PSBT to be able to determine whether a key is derived from an=20
aggregate pubkey also specified in the PSBT.

The full text of the BIPs can be found at the following:
* Derivation:=20
https://github.com/achow101/bips/blob/musig2/bip-musig2-derivation.mediawik=
i
* Descriptors:=20
https://github.com/achow101/bips/blob/musig2/bip-musig2-descriptors.mediawi=
ki
* PSBT:=20
https://github.com/achow101/bips/blob/musig2/bip-musig2-psbt.mediawiki

Thanks,
Ava Chow