Multi and Sortedmulti descriptors should support up to 20 keys instead of 16 inside P2WSH context #20620

Currently, the multisig descriptor constructions are limited to 16 keys only possibly because of template parsing reasons. Under p2sh context, such scripts would be invalid anyway because of 520-byte rule, but we can support up to 20 keys for CHECKMULTISIG in witness script context.

I think that even under witness context such Script are non standard as the small-integerness is checked in MatchMultisig: https://github.com/bitcoin/bitcoin/blob/bc8ada1c15345d14e324aee68488c8aa8a75cae0/src/script/standard.cpp#L92-L108

I don't follow you completly. Yes, such scripts are non-standard now, but should be considered standard. And hence the issue :)

Ok, i thought you were talking about descriptor parsing. Can propose a patch to make them standard if others find it reasonable

@darosior That is only relevant for bare multisig though (which isn't standard at all by default). Descriptors could well start supporting 17-20 key multisig when wrapped in wsh(), without affecting any network policy (also there is no strict reason descriptors can't support nonstandard things too, but I think we want to avoid that if possible). @sanket1729 I believe v17-v20 multisig inside P2WSH is standard, as inside P2SH/P2WSH template matching isn't used to determine standardness (otherwise ~all of miniscript would be nonstandard...).

Is this an issue? If yes, what are the steps to reproduce?

@MarcoFalke this is an issue, but with Descriptors rather than TX fees and policy. As outlined by sipa above 17-20 pubkeys multisig are standard under witness context, it's just that our current descriptor implementation does not parse them. Here is a minimal example:

getdescriptorinfo 'sh(wsh(multi(20,03669b8afcec803a0d323e9a17f3ea8e68e8abe5a278020a929adbec52421adbd0,0260b2003c386519fc9eadf2b5cf124dd8eea4c4e68d5e154050a9346ea98ce600,0362a74e399c39ed5593852a30147f2959b56bb827dfa3e60e464b02ccf87dc5e8,0261345b53de74a4d721ef877c255429961b7e43714171ac06168d7e08c542a8b8,02da72e8b46901a65d4374fe6315538d8f368557dda3a1dcf9ea903f3afe7314c8,0318c82dd0b53fd3a932d16e0ba9e278fcc937c582d5781be626ff16e201f72286,0297ccef1ef99f9d73dec9ad37476ddb232f1238aff877af19e72ba04493361009,02e502cfd5c3f972fe9a3e2a18827820638f96b6f347e54d63deb839011fd5765d,03e687710f0e3ebe81c1037074da939d409c0025f17eb86adb9427d28f0f7ae0e9,02c04d3a5274952acdbc76987f3184b346a483d43be40874624b29e3692c1df5af,02ed06e0f418b5b43a7ec01d1d7d27290fa15f75771cb69b642a51471c29c84acd,036d46073cbb9ffee90473f3da429abc8de7f8751199da44485682a989a4bebb24,02f5d1ff7c9029a80a4e36b9a5497027ef7f3e73384a4a94fbfe7c4e9164eec8bc,02e41deffd1b7cce11cde209a781adcffdabd1b91c0ba0375857a2bfd9302419f3,02d76625f7956a7fc505ab02556c23ee72d832f1bac391bcd2d3abce5710a13d06,0399eb0a5487515802dc14544cf10b3666623762fbed2ec38a3975716e2c29c232,02bc2feaa536991d269aae46abb8f3772a5b3ad592314945e51543e7da84c4af6e,0318bf32e5217c1eb771a6d5ce1cd39395dff7ff665704f175c9a5451d95a2f2ca,02c681a6243f16208c2004bb81f5a8a67edfdd3e3711534eadeec3dcf0b010c759,0249fdd6b69768b8d84b4893f8ff84b36835c50183de20fcae8f366a45290d01fd)))'

FWIW i'm working on this