MuSig2 PSBT Fields

The five-minute read

One action, walked through

1. Open BIP 373 and read the status line before the examples.
2. Write down the rule in one sentence. A fair version of that sentence is: The new per-input types are defined as follows: {| ! Name ! ! ! ! Versions Requiring Inclusion ! Versions Requiring Exclusion ! Versions Allowing Inclusion |- | rowspan="2"|MuSig2 Participant Public Keys | rowspan="2"| PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS = 0x1a | | | rowspan="2"| | rowspan="2"| | rowspan="2"| 0, 2 |- | The MuSig2 aggregate public key (compressed) '''Why the compressed aggregate public key instead of x-only?''' [[bip-0032.mediawiki|BIP 32]] public keys can be derived from a [[bip-0327.mediawiki|BIP 327]] MuSig2 aggregate public key (see: [[bip-0328.mediawiki|BIP 328]]).
3. Name the object that changes: PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS, PSBT_IN_TAP_BIP32_DERIVATION, PSBT.
4. Ask what an old client, an old contract, or an offline counterparty does. If the document is silent, the silence is part of the design.

The argument, unpacked

What has to be true

• You are implementing BIP 373 at the status the text itself states: Complete.
• The object that has to change is PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS, PSBT_IN_TAP_BIP32_DERIVATION, PSBT. A neighbouring document with a similar name is not this one.
• Wallet support is not the same as consensus validity.

What happened after the paper

The source marks this complete. That is a statement about the text, not a promise that every wallet or node has shipped it. Later documents can narrow, replace, or ignore this one. Cite the number you mean.

What to check before you use the idea

• What is covered by the signed bytes in MuSig2 PSBT Fields?
• Can the same signature be replayed on another chain, account, or message?
• Which primitive does the text require: PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS, PSBT_IN_TAP_BIP32_DERIVATION, PSBT?

Terms

BIP 373

The public text titled MuSig2 PSBT Fields.

Complete

The document's own label for how finished the text is. It is not a market fact.

The problem the paper names

A signature that does not commit to what the user saw can be replayed into a different spend, a different chain, or a different message.

What the design proposes

• The new per-input types are defined as follows: {| ! Name ! ! ! ! Versions Requiring Inclusion ! Versions Requiring Exclusion ! Versions Allowing Inclusion |- | rowspan="2"|MuSig2 Participant Public Keys | rowspan="2"| PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS = 0x1a | | | rowspan="2"| | rowspan="2"| | rowspan="2"| 0, 2 |- | The MuSig2 aggregate public key (compressed) '''Why the compressed aggregate public key instead of x-only?''' [[bip-0032.mediawiki|BIP 32]] public keys can be derived from a [[bip-0327.mediawiki|BIP 327]] MuSig2 aggregate public key (see: [[bip-0328.mediawiki|BIP 328]]).
• But since BIP 32 requires public keys to include their evenness byte, BIP 327 MuSig2 aggregate public keys must include their evenness byte as well.
• Furthermore, PSBT_IN_TAP_BIP32_DERIVATION fields include fingerprints to identify master keys, and these fingerprints require the y-coordinate of the public key, so x-only serialization can't be used.

How the mechanism is specified

• Taken from the specification, the next constraint is: But since BIP 32 requires public keys to include their evenness byte, BIP 327 MuSig2 aggregate public keys must include their evenness byte as well.
• Locate PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS, PSBT_IN_TAP_BIP32_DERIVATION, PSBT in BIP 373 and apply it to one transaction or call.
• Then check the failure the class of rule always has: a node that did not upgrade, a reverted call, a replayed signature, or a peer that does not speak the message.

What this page does not treat as proven

• A signature scheme is not a privacy system. It hides the signer only if the protocol says so.
• Wallet support is not the same as consensus validity.
• The source marks this complete. That is a statement about the text, not a promise that every wallet or node has shipped it.

Why a venture studio still reads it

Use BIP 373 when a pitch says 'MuSig2 PSBT Fields' without saying whether the rule is consensus, policy, or an interface. The number is the citation. The pitch is not.