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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
Created: 2024-06-04
License: CC0-1.0
Requires: 380
Requires: 380, 328
</pre>
==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>.
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>===
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> expressions are also allowed, with the same usage
restrictions as in the previous section. 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-0328.mediawiki|BIP 328]]. As there is no aggregate private key,
only unhardened derivation from the aggregate public key is allowed, and thus the derivation steps
following the <tt>musig()</tt> expression cannot contain
<tt>/NUMh</tt> or <tt>/NUM'</tt> derivation steps nor <tt>/*h</tt>, or <tt>/*'</tt> child derivation.
For these <tt>musig()</tt> expressions, the KEY expressions contained within must be xpubs or derived from
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>.
<tt>musig(KEY, KEY, ..., KEY)/NUM/.../*</tt> expressions are also allowed if no KEY expression
contains child derivation as specified by <tt>/*</tt> or multipath as specified with
<tt>/<NUM;NUM;...></tt>, 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-0328.mediawiki|BIP 328]]. The <tt>/NUM/.../*</tt> following the <tt>musig()</tt> specifies
the derivation path to be used when deriving from the aggregate public key. <tt>/*</tt> 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 <tt>/NUMh</tt> or <tt>/NUM'</tt>
nor can child derivation be specified as <tt>/*h</tt>, or <tt>/*'</tt>.
==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>
* <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 participants with child derivation when <tt>musig()</tt> has derivation steps: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/*,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/*)/1/2</tt>
==Backwards Compatibility==