Merge bitcoin/bitcoin#24043: Add (sorted)multi_a descriptor for k-of-n multisig inside tr

4828d53eccd52a67631c64cef0ba7df90dff138d Add (sorted)multi_a descriptors to doc/descriptors.md (Pieter Wuille)
b5f33ac1f82aea290b4653af36ac2ad1bf1cce7b Simplify wallet_taproot.py functional test (Pieter Wuille)
eb0667ea96d52db9135514a5e95ab943f6abd8a6 Add tests for (sorted)multi_a derivation/signing (Pieter Wuille)
c17c6aa08df81aa0086d80b50187c8cd60ecc222 Add signing support for (sorted)multi_a scripts (Pieter Wuille)
3eed6fca57d1fa7544f372e6e7de0a9ae1b5715a Add multi_a descriptor inference (Pieter Wuille)
79728c4a3d8a74f276daf1e72abbdecdab85a5d8 Add (sorted)multi_a descriptor and script derivation (Pieter Wuille)
25e95f9ff89a97b87ce218f28274c3c821b2d54d Merge/generalize IsValidMultisigKeyCount/GetMultisigKeyCount (Pieter Wuille)

Pull request description:

  This adds a new `multi_a(k,key_1,key_2,...,key_n)` (and corresponding `sortedmulti_a`) descriptor for k-of-n policies inside `tr()`. Semantically it is very similar to the existing `multi()` descriptor, but with the following changes:
  * The corresponding script is `<key1> OP_CHECKSIG <key2> OP_CHECKSIGADD <key3> OP_CHECKSIGADD ... <key_n> OP_CHECKSIGADD <k> OP_NUMEQUAL`, rather than the traditional `OP_CHECKMULTISIG`-based script, making it usable inside the `tr()` descriptor.
  * The keys can optionally be specified in x-only notation.
  * Both the number of keys and the threshold can be as high as 999; this is the limit due to the consensus stacksize=1000 limit

  I expect that this functionality will later be replaced with a miniscript-based implementation, but I don't think it's necessary to wait for that.

  Limitations:
  * The wallet code will for not estimate witness size incorrectly for script path spends, which may result in a (dramatic) fee underpayment with large multi_a scripts.
  * The multi_a script construction is (slightly) suboptimal for n-of-n (where a `<key1> OP_CHECKSIGVERIFY ... <key_n-1> OP_CHECKSIGVERIFY <key_n> OP_CHECKSIG` would be better). Such a construction is not included here.

ACKs for top commit:
  achow101:
    ACK 4828d53eccd52a67631c64cef0ba7df90dff138d
  gruve-p:
    ACK https://github.com/bitcoin/bitcoin/pull/24043/commits/4828d53eccd52a67631c64cef0ba7df90dff138d
  sanket1729:
    code review ACK 4828d53eccd52a67631c64cef0ba7df90dff138d
  darosior:
    Code review ACK 4828d53eccd52a67631c64cef0ba7df90dff138d

Tree-SHA512: 5dcd434b79585f0ff830f7d501d27df5e346f5749f47a3109ec309ebf2cbbad0e1da541eec654026d911ab67fd7cf7793fab0f765628d68d81b96ef2a4d234ce

1 files changed, 5 insertions, 1 deletions

diff --git a/doc/descriptors.md b/doc/descriptors.md
index 318d065fdb..ab2face4f0 100644
--- a/doc/descriptors.md
+++ b/doc/descriptors.md
@@ -33,6 +33,7 @@ Output descriptors currently support:
 - Pay-to-taproot outputs (P2TR), through the `tr` function.
 - Multisig scripts, through the `multi` function.
 - Multisig scripts where the public keys are sorted lexicographically, through the `sortedmulti` function.
+- Multisig scripts inside taproot script trees, through the `multi_a` (and `sortedmulti_a`) function.
 - Any type of supported address through the `addr` function.
 - Raw hex scripts through the `raw` function.
 - Public keys (compressed and uncompressed) in hex notation, or BIP32 extended pubkeys with derivation paths.
@@ -56,6 +57,7 @@ Output descriptors currently support:
 - `wsh(multi(1,xpub661MyMwAqRbcFW31YEwpkMuc5THy2PSt5bDMsktWQcFF8syAmRUapSCGu8ED9W6oDMSgv6Zz8idoc4a6mr8BDzTJY47LJhkJ8UB7WEGuduB/1/0/*,xpub69H7F5d8KSRgmmdJg2KhpAK8SR3DjMwAdkxj3ZuxV27CprR9LgpeyGmXUbC6wb7ERfvrnKZjXoUmmDznezpbZb7ap6r1D3tgFxHmwMkQTPH/0/0/*))` describes a set of *1-of-2* P2WSH multisig outputs where the first multisig key is the *1/0/`i`* child of the first specified xpub and the second multisig key is the *0/0/`i`* child of the second specified xpub, and `i` is any number in a configurable range (`0-1000` by default).
 - `wsh(sortedmulti(1,xpub661MyMwAqRbcFW31YEwpkMuc5THy2PSt5bDMsktWQcFF8syAmRUapSCGu8ED9W6oDMSgv6Zz8idoc4a6mr8BDzTJY47LJhkJ8UB7WEGuduB/1/0/*,xpub69H7F5d8KSRgmmdJg2KhpAK8SR3DjMwAdkxj3ZuxV27CprR9LgpeyGmXUbC6wb7ERfvrnKZjXoUmmDznezpbZb7ap6r1D3tgFxHmwMkQTPH/0/0/*))` describes a set of *1-of-2* P2WSH multisig outputs where one multisig key is the *1/0/`i`* child of the first specified xpub and the other multisig key is the *0/0/`i`* child of the second specified xpub, and `i` is any number in a configurable range (`0-1000` by default). The order of public keys in the resulting witnessScripts is determined by the lexicographic order of the public keys at that index.
 - `tr(c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5,{pk(fff97bd5755eeea420453a14355235d382f6472f8568a18b2f057a1460297556),pk(e493dbf1c10d80f3581e4904930b1404cc6c13900ee0758474fa94abe8c4cd13)})` describes a P2TR output with the `c6...` x-only pubkey as internal key, and two script paths.
+- `tr(c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5,sortedmulti_a(2,2f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,5cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc))` describes a P2TR output with the `c6...` x-only pubkey as internal key, and a single `multi_a` script that needs 2 signatures with 2 specified x-only keys, which will be sorted lexicographically.
 
 ## Reference
 
@@ -68,8 +70,10 @@ Descriptors consist of several types of expressions. The top level expression is
 - `pkh(KEY)` (not inside `tr`): P2PKH output for the given public key (use `addr` if you only know the pubkey hash).
 - `wpkh(KEY)` (top level or inside `sh` only): P2WPKH output for the given compressed pubkey.
 - `combo(KEY)` (top level only): an alias for the collection of `pk(KEY)` and `pkh(KEY)`. If the key is compressed, it also includes `wpkh(KEY)` and `sh(wpkh(KEY))`.
-- `multi(k,KEY_1,KEY_2,...,KEY_n)` (not inside `tr`): k-of-n multisig script.
+- `multi(k,KEY_1,KEY_2,...,KEY_n)` (not inside `tr`): k-of-n multisig script using OP_CHECKMULTISIG.
 - `sortedmulti(k,KEY_1,KEY_2,...,KEY_n)` (not inside `tr`): k-of-n multisig script with keys sorted lexicographically in the resulting script.
+- `multi_a(k,KEY_1,KEY_2,...,KEY_N)` (only inside `tr`): k-of-n multisig script using OP_CHECKSIG, OP_CHECKSIGADD, and OP_NUMEQUAL.
+- `sortedmulti_a(k,KEY_1,KEY_2,...,KEY_N)` (only inside `tr`): similar to `multi_a`, but the (x-only) public keys in it will be sorted lexicographically.
 - `tr(KEY)` or `tr(KEY,TREE)` (top level only): P2TR output with the specified key as internal key, and optionally a tree of script paths.
 - `addr(ADDR)` (top level only): the script which ADDR expands to.
 - `raw(HEX)` (top level only): the script whose hex encoding is HEX.