390: clarifications on KEY expression restrictions #1871

pull achow101 wants to merge 2 commits into bitcoin:master from achow101:desc-clarifications changing 10 files +25 −10

achow101 commented at 10:06 pm on June 18, 2025: member

Clarifies the restrictions on KEY expressions around when child derivation is allowed.

Also added a Requires header to all of the descriptor BIPs since they require reading 380 first.

descriptors: Require BIP 380

All of BIPs describing new descriptors require BIP 380

bc66fdb166

achow101 force-pushed on Jun 18, 2025
murchandamus commented at 10:58 pm on June 18, 2025: contributor

Thanks, looks good to me.
jonatack added the label Proposed BIP modification on Jun 18, 2025

in bip-0390.mediawiki:64 in 23e78b68df outdated

69+described above, with the keys also being sorted after all derivation and prior to aggregation.
70+Then further BIP 32 derivation will be performed on the aggregate public key as described in
71+[[bip-0328.mediawiki|BIP 328]]. The <tt>/NUM/.../*</tt> following the <tt>musig()</tt> specifies
72+the derivation path to be used when deriving from the aggregate public key. <tt>/*</tt> is also
73+optional. As there is no aggregate private key, only unhardened derivation from the aggregate public
74+key is allowed. Thus these derivation steps cannot contain be <tt>/NUMh</tt> or <tt>/NUM'</tt>

jonatack commented at 11:54 pm on June 18, 2025:

s/contain be/contain/ ?

achow101 commented at 0:13 am on June 19, 2025:

Done

in bip-0390.mediawiki:61 in 23e78b68df outdated

66+contains child derivation as specified by <tt>/*</tt> or multipath as specified with
67+<tt>/<NUM;NUM;...></tt>, in addition to the same usage restrictions as in the previous section. The KEY expressions
68+additionally must be xpubs or derived from xpubs. The aggregate public key is first computed as
69+described above, with the keys also being sorted after all derivation and prior to aggregation.
70+Then further BIP 32 derivation will be performed on the aggregate public key as described in
71+[[bip-0328.mediawiki|BIP 328]]. The <tt>/NUM/.../*</tt> following the <tt>musig()</tt> specifies

jonatack commented at 11:55 pm on June 18, 2025:

(in passing, should 390 require 328?)

achow101 commented at 0:14 am on June 19, 2025:

I guess so. added

390: Additional clarifications on KEY expression restrictions c754a4d095
in bip-0389.mediawiki:12 in 23e78b68df outdated
8@@ -9,6 +9,7 @@ 9 Type: Informational 10 Created: 2022-07-26 11 License: BSD-2-Clause 12+ Requires: 380
jonatack commented at 11:59 pm on June 18, 2025:

Checked that 388 (omitted from this change) does not also require 380
achow101 force-pushed on Jun 19, 2025
in bip-0390.mediawiki:63 in c754a4d095
68+additionally must be xpubs or derived from xpubs. The aggregate public key is first computed as 69+described above, with the keys also being sorted after all derivation and prior to aggregation. 70+Then further BIP 32 derivation will be performed on the aggregate public key as described in 71+[[bip-0328.mediawiki|BIP 328]]. The <tt>/NUM/.../*</tt> following the <tt>musig()</tt> specifies 72+the derivation path to be used when deriving from the aggregate public key. <tt>/*</tt> is also 73+optional. As there is no aggregate private key, only unhardened derivation from the aggregate public
rkrux commented at 11:49 am on June 19, 2025:

/* is also optional.

So musig(xpub.../1,xpub.../2)/3/4 will be treated as musig(xpub.../1,xpub.../2)/3/4/*? If yes, I understand it’s being provided to reduce verbosity but one with an explicit /* at the end would get rid of any possibility of a doubt in the mind of a reader imho.

rkrux commented at 1:35 pm on June 19, 2025:

So musig(xpub…/1,xpub…/2)/3/4 will be treated as musig(xpub…/1,xpub…/2)/3/4/*?

On second thought, I think this is not what this means, they both will be treated separately. Sorry I didn’t mean to add more confusion.
rkrux commented at 11:51 am on June 19, 2025: contributor

The new verbiage is better and mostly not confusing - asked one question.
jonatack commented at 2:19 am on June 20, 2025: member

ACK c754a4d095704228de928deb817cda47a6499e3e
jonatack merged this on Jun 20, 2025
jonatack closed this on Jun 20, 2025

Contributors
achow101 murchandamus jonatack rkrux

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Proposed BIP modification