  BIP: 390
  Layer: Applications
  Title: musig() Descriptor Key Expression
  Author: Ava Chow <me@achow101.com>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0390
  Status: Draft
  Type: Informational
  Created: 2024-06-04
  License: CC0-1.0
  Requires: 380, 328

Abstract

This document specifies a musig() key expression for output script descriptors. musig() expressions take multiple keys and produce an aggregate public key using BIP-327.

Copyright

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

Motivation

BIP-327 introduces the MuSig2 Multi-Signature scheme. It is useful to have a way for keys to be used in a MuSig2 aggregate key to be expressed in descriptors so that wallets can more easily use MuSig2.

Specification

A new key expression is defined: musig().

In the following sections, the term KEY refers to key expressions as defined in BIP-380 and BIP-389.

`musig(KEY, KEY, ..., KEY)`

The musig(KEY, KEY, ..., KEY) expression can only be used inside of a tr() expression as a key expression. It additionally cannot be nested within another musig() expression. Participant public keys may be repeated. The aggregate public key is produced by using the KeyAgg algorithm on all KEYs specified in the expression after performing all specified derivation. As with script expressions, KEY can contain child derivation specified by /*. A new aggregate public key will be computed for each child index. Keys must be sorted with the KeySort algorithm after all derivation and prior to aggregation^[1].

`musig(KEY, KEY, ..., KEY)/NUM/.../*`

musig(KEY, KEY, ..., KEY)/NUM/.../* expressions are also allowed if no KEY expression contains child derivation as specified by /* or multipath as specified with /<NUM;NUM;...>, in addition to the same usage restrictions as in the previous section. The KEY expressions additionally must be xpubs or derived from xpubs. The aggregate public key is first computed as described above, with the keys also being sorted after all derivation and prior to aggregation. Then further BIP-32 derivation will be performed on the aggregate public key as described in BIP-328. The /NUM/.../* following the musig() specifies the derivation path to be used when deriving from the aggregate public key. /* is also optional. As there is no aggregate private key, only unhardened derivation from the aggregate public key is allowed. Thus these derivation steps cannot contain /NUMh or /NUM' nor can child derivation be specified as /*h, or /*'.

Test Vectors

Valid descriptors containing followed by the scripts they produce. Descriptors involving derived child keys will have the 0th, 1st, and 2nd scripts listed.

rawtr(musig(KwDiBf89QgGbjEhKnhXJuH7LrciVrZi3qYjgd9M7rFU74sHUHy8S,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
- 5120789d937bade6673538f3e28d8368dda4d0512f94da44cf477a505716d26a1575
tr(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
- 512079e6c3e628c9bfbce91de6b7fb28e2aec7713d377cf260ab599dcbc40e542312
rawtr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*)
- 51209508c08832f3bb9d5e8baf8cb5cfa3669902e2f2da19acea63ff47b93faa9bfc
- 51205ca1102663025a83dd9b5dbc214762c5a6309af00d48167d2d6483808525a298
- 51207dbed1b89c338df6a1ae137f133a19cae6e03d481196ee6f1a5c7d1aeb56b166
tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*,pk(f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9))
- 51201d377b637b5c73f670f5c8a96a2c0bb0d1a682a1fca6aba91fe673501a189782
- 51208950c83b117a6c208d5205ffefcf75b187b32512eb7f0d8577db8d9102833036
- 5120a49a477c61df73691b77fcd563a80a15ea67bb9c75470310ce5c0f25918db60d
tr(f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,pk(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*))
- 512068983d461174afc90c26f3b2821d8a9ced9534586a756763b68371a404635cc8
- 5120368e2d864115181bdc8bb5dc8684be8d0760d5c33315570d71a21afce4afd43e
- 512097a1e6270b33ad85744677418bae5f59ea9136027223bc6e282c47c167b471d5
tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/1,xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/1)/2)
- 5120a17ceacd6422bd5ffd9f165807b254b7d68ad39f179cc4f11545a6835227e97c

Invalid descriptors

musig() is not allowed in top-level pk(): pk(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
musig() is not allowed in top-level pkh(): pkh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
musig() is not allowed in wpkh(): wpkh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
musig() is not allowed in combo(): combo(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
musig() is not allowed in sh(wpkh()): sh(wpkh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66)))
musig() is not allowed in sh(wsh()): sh(wsh(pk(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))))
musig() is not allowed in wsh(): wsh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
musig() is not allowed in sh(): sh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))
Ranged musig() requires all participants to be xpubs: tr(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66)/0/0)
Cannot have ranged participants if musig() is also ranged: tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/*,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*)
Cannot have multipath participants if musig() is also multipath: tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/<0;1>,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/<2;3>)
musig() cannot have hardened derivation steps: tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0h/*)
musig() cannot have hardened child derivation: tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*h)
musig() cannot have participants with child derivation when musig() has derivation steps: tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/*,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/*)/1/2

Backwards Compatibility

musig() expressions use the format and general operation specified in BIP-380. As these are a set of wholly new expressions, they are not compatible with any implementation. However the keys are produced using a standard process so existing software are likely to be familiar with them.

Rationale

^ Why must the keys be sorted prior to aggregation? Although the descriptor's written form sets an order for the keys that could be used for aggregation, the order should not matter as MuSig2 philosophically operates over a set of keys, with the order merely being an implementation detail in aggregation itself. Requiring sorting of keys prior to aggregation enforces this philosophy as keys can be written in the descriptor in any order with the end result still being the same. Furthermore, this aids with recovery where the descriptor was not backed up as users will not need to also have backed up, or guess, the correct order of keys.

Reference Implementation

The reference implementation is available in Bitcoin Core PR #31244.

Acknowledgements

Thanks to Pieter Wuille, Andrew Poelstra, Sanket Kanjalkar, Salvatore Ingala, and all others who participated in discussions on this topic.

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