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BIP-373: denote different public key types/purposes consistently #1705

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Dec 9, 2024
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BIP-373: denote different public key types/purposes consistently #1705

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27 changes: 14 additions & 13 deletions bip-0373.mediawiki
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Original file line number Diff line number Diff line change
Expand Up @@ -45,17 +45,18 @@ The new per-input types are defined as follows:
|-
| rowspan="2"|MuSig2 Participant Public Keys
| rowspan="2"|<tt>PSBT_IN_MUSIG2_PARTICIPANT_PUBKEYS = 0x1a</tt>
| <tt><33 byte plain aggregate pubkey></tt>
| <tt><33 byte compressed pubkey>*</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 plain public key<ref>'''Why the plain aggregate public key instead of x-only?'''
| 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 full compressed public keys. By including
to identify master keys, and these fingerprints require y-coordinate of the public key,
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nit

Suggested change
to identify master keys, and these fingerprints require y-coordinate of the public key,
to identify master keys, and these fingerprints require the y-coordinate of the public key,

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The previous sentence is confusing as well:

"Aggregate public keys are expected to be derived from, following BIP 328, and therefore [...]"

To a layman (me!) it reads like there's a word missing ("derived from... what?"). But I see now that it's referring to them as the starting point ("from" the aggregate public key to a BIP32 public key). Perhaps clearer to say:

"BIP 32 public keys are expected to be derived from aggregate public keys (per BIP 328) and therefore..."

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
Expand All @@ -67,32 +68,32 @@ required for aggregation. If sorting was done, then the keys must be in the sort
|-
| rowspan="2"|MuSig2 Public Nonce
| rowspan="2"|<tt>PSBT_IN_MUSIG2_PUB_NONCE = 0x1b</tt>
| <tt><33 byte compressed pubkey> <33 byte plain pubkey> <32 byte hash or omitted></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 plain public
| 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 plain public key must be
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 compressed pubkey> <33 byte plain pubkey> <32 byte hash or omitted></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
plain public key the participant is providing the signature for, followed by the BIP 341 tapleaf hash
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 plain public key must be
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.
Expand All @@ -111,13 +112,13 @@ The new per-output types are defined as follows:
|-
| rowspan="2"|MuSig2 Participant Public Keys
| rowspan="2"|<tt>PSBT_OUT_MUSIG2_PARTICIPANT_PUBKEYS = 0x08</tt>
| <tt><33 byte compressed pubkey></tt>
| <tt><33 byte compressed pubkey>*</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 plain public key from the <tt>KeyAgg</tt> algorithm. This key may or may not
| 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
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