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| <pre> | ||
| BIP: 373 | ||
| Layer: Applications | ||
| Title: MuSig2 PSBT Fields | ||
| Author: Ava Chow <[email protected]> | ||
| 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> | ||
|
|
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| ==Introduction== | ||
|
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||
| ===Abstract=== | ||
|
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| 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=== | ||
|
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| This BIP is licensed under the Creative Commons CC0 1.0 Universal license. | ||
|
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| ===Motivation=== | ||
|
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| 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. | ||
|
|
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| ==Specification== | ||
|
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| The new per-input types are defined as follows: | ||
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| {| | ||
| ! 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 plain aggregate pubkey></tt> | ||
| | <tt><33 byte compressed pubkey>*</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?''' | ||
| 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 | ||
| 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 | ||
| be in the script directly (as x-only). It may instead be a parent public key from which the public keys in the | ||
| 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 compressed pubkey> <33 byte plain pubkey> <32 byte hash or omitted></tt> | ||
| | <tt><66 byte public nonces</tt> | ||
| | rowspan="2"| | ||
| | rowspan="2"| | ||
| | rowspan="2"| 0, 2 | ||
| |- | ||
| | The compressed public key of the participant providing this nonce, followed by the plain public | ||
| key the participant is providing the nonce for, followed by the hash of 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 | ||
| the key found in the script and not the aggregate public key that it was derived from, if it was | ||
| derived from an aggregate key. | ||
| | The public nonces 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><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 | ||
| 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 key found in the script and not 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 compressed pubkey></tt> | ||
| | <tt><33 byte compressed pubkey>*</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 | ||
| be in the script directly. It may instead be a parent public key from which the public keys in the | ||
| 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. | ||
| |} | ||
|
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| ==Roles== | ||
|
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| ===Updater=== | ||
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| 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. | ||
|
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| 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. | ||
|
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| 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. | ||
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| 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. | ||
|
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| ===Signer=== | ||
|
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| 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>. | ||
|
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| 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. | ||
|
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| 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. | ||
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| 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. | ||
|
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| 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>. | ||
|
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| ===Finalizer=== | ||
|
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| A finalizer may perform the same <tt>PartialSigAgg</tt> step as the final signer if it has not | ||
| already been done. | ||
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| Otherwise, the resulting signature is a BIP 340 compatible signature and finalizers should treat it | ||
| as such. | ||
|
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| ==Compatibility== | ||
|
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| 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. | ||
|
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| 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. | ||
|
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| ==Test Vectors== | ||
|
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| TBD | ||
|
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| ==Rationale== | ||
|
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| <references/> | ||
|
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| ==Reference implementation== | ||
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| The reference implementation of the PSBT format is available at TBD. | ||
|
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| ==Acknowledgements== | ||
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| 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. |