<pre>
  BIP: 373
  Layer: Applications
  Title: MuSig2 PSBT Fields
  Author: Ava Chow <me@achow101.com>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0373
  Status: Draft
  Type: Standards Track
  Created: 2024-01-15
  License: CC0-1.0
</pre>

==Introduction==

===Abstract===

This document proposes additional fields for BIP 174 PSBTv0 and BIP 370 PSBTv2 that allow for BIP
327 MuSig2 Multi-Signature data to be included in a PSBT of any version. These will be fields for
the participants' keys, the public nonces, and the partial signatures produced with MuSig2.

===Copyright===

This BIP is licensed under the Creative Commons CC0 1.0 Universal license.

===Motivation===

BIP 327 specifies a way to create BIP 340 compatible public keys and signatures using the MuSig2
Multi-Signature scheme. The existing PSBT fields are unable to support MuSig2 as it introduces new
concepts and additional rounds of communication. Therefore new fields must be defined to allow PSBTs
to carry the information necessary to produce a valid signature with MuSig2.

==Specification==

The new per-input types are defined as follows:

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><valuedata></tt>
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| rowspan="2"|MuSig2 Participant Public Keys
| rowspan="2"|<tt>PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS = 0x1a</tt>
| <tt><33 byte aggregate pubkey (compressed)></tt>
| <tt><33 byte participant pubkey (compressed)>*</tt>
| rowspan="2"|
| rowspan="2"|
| rowspan="2"| 0, 2
|-
| The MuSig2 aggregate public key (compressed) <ref>'''Why the compressed aggregate public key instead of x-only?'''
BIP 32 requires public keys to include their evenness byte. Aggregate public keys are expected to be
derived from, following [[bip-0328.mediawiki|BIP 328]], and therefore will
need to include the evenness. Furthermore, PSBT_IN_TAP_BIP32_DERIVATION fields include fingerprints
to identify master keys, and these fingerprints require y-coordinate of the public key,
so x-only serialization can't be used. By including
the aggregate key as a full public key, signers that are unaware of the MuSig2 outside of the PSBT
will still be able to identify which keys are derived from the aggregate key by computing and then
comparing the fingerprints. This is necessary for the signer to apply the correct tweaks to their
partial signature.</ref> from the <tt>KeyAgg</tt> algorithm. This key may or may not appear
(as x-only) in the Taproot output key, the internal key, or in a script. It may instead be a parent public
key from which the Taproot output key, internal key, or keys in a script were derived.
| A list of the compressed public keys of the participants in the MuSig2 aggregate key in the order
required for aggregation. If sorting was done, then the keys must be in the sorted order.
|-
| rowspan="2"|MuSig2 Public Nonce
| rowspan="2"|<tt>PSBT_IN_MUSIG2_PUB_NONCE = 0x1b</tt>
| <tt><33 byte participant pubkey (compressed)> <33 byte aggregate pubkey (compressed)> <32 byte hash or omitted></tt>
| <tt><66 byte public nonce></tt>
| rowspan="2"|
| rowspan="2"|
| rowspan="2"| 0, 2
|-
| The compressed public key of the participant providing this nonce, followed by the compressed aggregate public
key the participant is providing the nonce for, followed by the BIP 341 tapleaf hash of
the Taproot leaf script that will be signed. If the aggregate key is the Taproot internal key or the
Taproot output key, then the tapleaf hash must be omitted. The compressed participant public key must be
the Taproot output key or found in a script. It is not the internal key nor the aggregate public key that
it was derived from, if it was derived from an aggregate key.
| The public nonce produced by the <tt>NonceGen</tt> algorithm.
|-
| rowspan="2"|MuSig2 Participant Partial Signature
| rowspan="2"|<tt>PSBT_IN_MUSIG2_PARTIAL_SIG = 0x1c</tt>
| <tt><33 byte participant pubkey (compressed)> <33 byte aggregate pubkey (compressed)> <32 byte hash or omitted></tt>
| <tt><32 byte partial signature></tt>
| rowspan="2"|
| rowspan="2"|
| rowspan="2"| 0, 2
|-
| The compressed public key of the participant providing this partial signature, followed by the
compressed public key the participant is providing the signature for, followed by the BIP 341 tapleaf hash
of the Taproot leaf script that will be signed. If the aggregate key is the Taproot internal key or
the Taproot output key, then the tapleaf hash must be omitted. Note that the compressed participant public key must be
the Taproot output key or found in a script. It is not the internal key nor the aggregate public key that
it was derived from, if it was derived from an aggregate key.
| The partial signature produced by the <tt>Sign</tt> algorithm.
|}

The new per-output types are defined as follows:

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><valuedata></tt>
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| rowspan="2"|MuSig2 Participant Public Keys
| rowspan="2"|<tt>PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS = 0x08</tt>
| <tt><33 byte aggregate pubkey (compressed)></tt>
| <tt><33 byte participant pubkey (compressed)>*</tt>
| rowspan="2"|
| rowspan="2"|
| rowspan="2"|0, 2
|-
| The MuSig2 compressed aggregate public key from the <tt>KeyAgg</tt> algorithm. This key may or may not
appear (as x-only) in the Taproot output key, the internal key, or in a script. It may instead be a parent
public key from which the Taproot output key, internal key, or keys in a script were derived.
| A list of the compressed public keys of the participants in the MuSig2 aggregate key in the order
required for aggregation. If sorting was done, then the keys must be in the sorted order.
|}

==Roles==

===Updater===

When an updater observes a Taproot output which involves a MuSig2 aggregate public key that it is
aware if, it can add a <tt>PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS</tt> field containing the public keys
of the participants. This aggregate public key may be directly in the script, the Taproot internal
key, the Taproot output key, or a public key from which the key in the script was derived from.

An aggregate public key that appears directly in the script or internal key may be from the result
of deriving child pubkeys from participant xpubs. If the updater has this derivation information, it
should also add <tt>PSBT_IN_TAP_BIP32_DERIVATION</tt> for each participant public key.

If the public key found was derived from an aggregate public key, then all MuSig2 PSBT fields for
that public key should contain the aggregate public key rather than the found pubkey itself. The
updater should also add <tt>PSBT_IN_TAP_BIP32_DERIVATION</tt> that contains the derivation path used
to derive the found pubkey from the aggregate pubkey.
Derivation from the aggregate pubkey can be assumed to follow [[bip-0328.mediawiki|BIP 328]]
if there is no <tt>PSBT_IN_GLOBAL_XPUB</tt> that specifies the synthetic xpub for the aggregate
public key.

Updaters should add <tt>PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS</tt> and
<tt>PSBT_OUT_TAP_BIP32_DERIVATION</tt> similarly to inputs to aid in change detection.

===Signer===

To determine whether a signer is a participant in the MuSig2 aggregate key, the signer should first
look at all <tt>PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS</tt> and see if any key which it knows the
private key for appears as a participant in any aggregate pubkey. Signers should also check whether
any of the keys in <tt>PSBT_IN_TAP_BIP32_DERIVATION</tt> belong to it, and if any of those keys
appear in as a participant in <tt>PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS</tt>.

For each aggregate public key that the signer is a participant of that it wants
to produce a signature for, if the signer does not find an existing
<tt>PSBT_IN_MUSIG2_PUB_NONCE</tt> field for its key, then it should add one using
the <tt>NonceGen</tt> algorithm (or one of its variations) to produce a public
nonce that is added in a <tt>PSBT_IN_MUSIG2_PUB_NONCE</tt> field. However
signers must keep in mind that '''improper nonce usage can compromise private
keys.''' Please see BIP 327 for best practices on nonce generation and usage.

Once all signers have added their <tt>PSBT_IN_MUSIG2_PUB_NONCE</tt> fields, each signer will perform
the <tt>NonceAgg</tt> algorithm followed by the <tt>Sign</tt> algorithm in order to produce the
partial signature for their key. The result will be added to the PSBT in a
<tt>PSBT_IN_MUSIG2_PARTIAL_SIG</tt> field.

Signers must remember to apply any relevant tweaks such as a tweak that is the result of performing
BIP 32 unhardened dervation with the aggregate public key as the parent key.

If all other signers have provided a <tt>PSBT_IN_MUSIG2_PARTIAL_SIG</tt>, then the final signer may
perform the <tt>PartialSigAgg</tt> algorithm and produce a BIP 340 compatible signature that can be
placed into a <tt>PSBT_IN_TAP_KEY_SIG</tt> or a <tt>PSBT_IN_TAP_SCRIPT_SIG</tt>.

===Finalizer===

A finalizer may perform the same <tt>PartialSigAgg</tt> step as the final signer if it has not
already been done.

Otherwise, the resulting signature is a BIP 340 compatible signature and finalizers should treat it
as such.

==Backwards Compatibility==

These are simply new fields added to the existing PSBT format. Because PSBT is designed to be
extensible, old software will ignore the new fields.

Reusing <tt>PSBT_IN_TAP_BIP32_DERIVATION</tt> to provide derivation paths for participant public
keys may cause software unaware of MuSig2 to produce a signature for that public key. This is still
safe. If that public key does not directly appear in the leaf script that was signed, then the
signature produced will not be useful and so cannot be replayed. If the public key does directly
appear in the leaf script, then the signer will have validated the script as if it did not involve a
MuSig2 and will have found it acceptable in order for it to have produced a signature. In either
case, producing a signature does not give rise to the possibility of losing funds.

==Test Vectors==

TBD

==Rationale==

<references/>

==Reference implementation==

The reference implementation of the PSBT format is available at TBD.

==Acknowledgements==

Thanks to Sanket Kanjalkar whose notes on this topic formed the initial basis of this BIP. Also
thanks to Pieter Wuille, Jonas Nick, Tim Ruffing, Marko Bencun, Salvatore Ingala, and all others who
have participated in discussions about these fields.
