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390: Additional clarifications on KEY expression restrictions

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Ava Chow 2025-06-18 15:04:33 -07:00
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@ -9,7 +9,7 @@
Type: Informational Type: Informational
Created: 2024-06-04 Created: 2024-06-04
License: CC0-1.0 License: CC0-1.0
Requires: 380 Requires: 380, 328
</pre> </pre>
==Abstract== ==Abstract==
@ -30,6 +30,9 @@ in a MuSig2 aggregate key to be expressed in descriptors so that wallets can mor
A new key expression is defined: <tt>musig()</tt>. A new key expression is defined: <tt>musig()</tt>.
In the following sections, the term <tt>KEY</tt> refers to key expressions as defined in BIPs 380
and 389.
===<tt>musig(KEY, KEY, ..., KEY)</tt>=== ===<tt>musig(KEY, KEY, ..., KEY)</tt>===
The <tt>musig(KEY, KEY, ..., KEY)</tt> expression can only be used inside of a <tt>tr()</tt> The <tt>musig(KEY, KEY, ..., KEY)</tt> expression can only be used inside of a <tt>tr()</tt>
@ -49,16 +52,17 @@ backed up, or guess, the correct order of keys.</ref>.
===<tt>musig(KEY, KEY, ..., KEY)/NUM/.../*</tt>=== ===<tt>musig(KEY, KEY, ..., KEY)/NUM/.../*</tt>===
<tt>musig(KEY, KEY, ..., KEY)/NUM/.../*</tt> expressions are also allowed, with the same usage <tt>musig(KEY, KEY, ..., KEY)/NUM/.../*</tt> expressions are also allowed if no KEY expression
restrictions as in the previous section. The aggregate public key contains child derivation as specified by <tt>/*</tt> or multipath as specified with
is first computed as described above, with the keys also being sorted after all derivation and prior <tt>/<NUM;NUM;...></tt>, in addition to the same usage restrictions as in the previous section. The KEY expressions
to aggregation. Then further BIP 32 derivation will be performed on the aggregate public key as described in additionally must be xpubs or derived from xpubs. The aggregate public key is first computed as
[[bip-0328.mediawiki|BIP 328]]. As there is no aggregate private key, described above, with the keys also being sorted after all derivation and prior to aggregation.
only unhardened derivation from the aggregate public key is allowed, and thus the derivation steps Then further BIP 32 derivation will be performed on the aggregate public key as described in
following the <tt>musig()</tt> expression cannot contain [[bip-0328.mediawiki|BIP 328]]. The <tt>/NUM/.../*</tt> following the <tt>musig()</tt> specifies
<tt>/NUMh</tt> or <tt>/NUM'</tt> derivation steps nor <tt>/*h</tt>, or <tt>/*'</tt> child derivation. the derivation path to be used when deriving from the aggregate public key. <tt>/*</tt> is also
For these <tt>musig()</tt> expressions, the KEY expressions contained within must be xpubs or derived from optional. As there is no aggregate private key, only unhardened derivation from the aggregate public
xpubs, and cannot contain child derivation as specified by a <tt>/*</tt>, <tt>/*'</tt>, <tt>/*h</tt>, or multipath as specified with <tt>/<NUM;NUM;...></tt>. key is allowed. Thus these derivation steps cannot contain <tt>/NUMh</tt> or <tt>/NUM'</tt>
nor can child derivation be specified as <tt>/*h</tt>, or <tt>/*'</tt>.
==Test Vectors== ==Test Vectors==
@ -99,6 +103,7 @@ Invalid descriptors
* Cannot have multipath participants if <tt>musig()</tt> is also multipath: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/<0;1>,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/<2;3>)</tt> * Cannot have multipath participants if <tt>musig()</tt> is also multipath: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/<0;1>,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/<2;3>)</tt>
* <tt>musig()</tt> cannot have hardened derivation steps: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0h/*)</tt> * <tt>musig()</tt> cannot have hardened derivation steps: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0h/*)</tt>
* <tt>musig()</tt> cannot have hardened child derivation: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*h)</tt> * <tt>musig()</tt> cannot have hardened child derivation: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*h)</tt>
* <tt>musig()</tt> cannot have participants with child derivation when <tt>musig()</tt> has derivation steps: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/*,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/*)/1/2</tt>
==Backwards Compatibility== ==Backwards Compatibility==