Merge bitcoin/bitcoin#29154: tests: improve wallet multisig descriptor test and docs

d93b794709 tests: improve wallet multisig descriptor test and docs (Michael Dietz)

Pull request description:

  It is best to store all key origin information
  (master key fingerprint and all derivation steps)
  in the multisig descriptor. Being explicit with
  this information should be beneficial if this approach is used with other wallets/signers (whether hardware or software). There is no harm including all of this with xpubs (if anything it simplifies the test code) and makes this example/docs more complete and safer incase it is referenced by others.

ACKs for top commit:
  S3RK:
    Code Review ACK d93b794709
  achow101:
    ACK d93b794709

Tree-SHA512: 0e5c4d13f060489405e6cf50c8a09911f5a0cee71023649235afd80a5e3aae38d52c6e12ad4660205b9357b09f45596941391bdcf6fceccbe07c4e5a1592a482

doc/descriptors.md

@@ -63,6 +63,7 @@ Output descriptors currently support:
 - `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.
+- `wsh(sortedmulti(2,[6f53d49c/44h/1h/0h]tpubDDjsCRDQ9YzyaAq9rspCfq8RZFrWoBpYnLxK6sS2hS2yukqSczgcYiur8Scx4Hd5AZatxTuzMtJQJhchufv1FRFanLqUP7JHwusSSpfcEp2/0/*,[e6807791/44h/1h/0h]tpubDDAfvogaaAxaFJ6c15ht7Tq6ZmiqFYfrSmZsHu7tHXBgnjMZSHAeHSwhvjARNA6Qybon4ksPksjRbPDVp7yXA1KjTjSd5x18KHqbppnXP1s/0/*,[367c9cfa/44h/1h/0h]tpubDDtPnSgWYk8dDnaDwnof4ehcnjuL5VoUt1eW2MoAed1grPHuXPDnkX1fWMvXfcz3NqFxPbhqNZ3QBdYjLz2hABeM9Z2oqMR1Gt2HHYDoCgh/0/*))#av0kxgw0` describes a *2-of-3* multisig. For brevity, the internal "change" descriptor accompanying the above external "receiving" descriptor is not included here, but it typically differs only in the xpub derivation steps, ending in `/1/*` for change addresses.
 
 ## Reference
 
@@ -167,9 +168,9 @@ The basic steps are:
      the participant's signer wallet. Avoid reusing this wallet for any purpose other than signing transactions from the
      corresponding multisig we are about to create. Hint: extract the wallet's xpubs using `listdescriptors` and pick the one from the
      `pkh` descriptor since it's least likely to be accidentally reused (legacy addresses)
-  2. Create a watch-only descriptor wallet (blank, private keys disabled). Now the multisig is created by importing the two descriptors:
+  2. Create a watch-only descriptor wallet (blank, private keys disabled). Now the multisig is created by importing the external and internal descriptors:
      `wsh(sortedmulti(<M>,XPUB1/0/*,XPUB2/0/*,…,XPUBN/0/*))` and `wsh(sortedmulti(<M>,XPUB1/1/*,XPUB2/1/*,…,XPUBN/1/*))`
-     (one descriptor w/ `0` for receiving addresses and another w/ `1` for change). Every participant does this
+     (one descriptor w/ `0` for receiving addresses and another w/ `1` for change). Every participant does this. All key origin information (master key fingerprint and all derivation steps) should be included with xpubs for proper support of hardware devices / external signers
   3. A receiving address is generated for the multisig. As a check to ensure step 2 was done correctly, every participant
      should verify they get the same addresses
   4. Funds are sent to the resulting address

test/functional/wallet_multisig_descriptor_psbt.py

@@ -7,7 +7,6 @@
 This is meant to be documentation as much as functional tests, so it is kept as simple and readable as possible.
 """
 
-from test_framework.address import base58_to_byte
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_approx,
@@ -30,10 +29,12 @@ class WalletMultisigDescriptorPSBTTest(BitcoinTestFramework):
         self.skip_if_no_sqlite()
 
     @staticmethod
-    def _get_xpub(wallet):
+    def _get_xpub(wallet, internal):
         """Extract the wallet's xpubs using `listdescriptors` and pick the one from the `pkh` descriptor since it's least likely to be accidentally reused (legacy addresses)."""
-        descriptor = next(filter(lambda d: d["desc"].startswith("pkh"), wallet.listdescriptors()["descriptors"]))
-        return descriptor["desc"].split("]")[-1].split("/")[0]
+        pkh_descriptor = next(filter(lambda d: d["desc"].startswith("pkh(") and d["internal"] == internal, wallet.listdescriptors()["descriptors"]))
+        # Keep all key origin information (master key fingerprint and all derivation steps) for proper support of hardware devices
+        # See section 'Key origin identification' in 'doc/descriptors.md' for more details...
+        return pkh_descriptor["desc"].split("pkh(")[1].split(")")[0]
 
     @staticmethod
     def _check_psbt(psbt, to, value, multisig):
@@ -47,19 +48,13 @@ class WalletMultisigDescriptorPSBTTest(BitcoinTestFramework):
                 amount += vout["value"]
         assert_approx(amount, float(value), vspan=0.001)
 
-    def participants_create_multisigs(self, xpubs):
+    def participants_create_multisigs(self, external_xpubs, internal_xpubs):
         """The multisig is created by importing the following descriptors. The resulting wallet is watch-only and every participant can do this."""
-        # some simple validation
-        assert_equal(len(xpubs), self.N)
-        # a sanity-check/assertion, this will throw if the base58 checksum of any of the provided xpubs are invalid
-        for xpub in xpubs:
-            base58_to_byte(xpub)
-
         for i, node in enumerate(self.nodes):
             node.createwallet(wallet_name=f"{self.name}_{i}", blank=True, descriptors=True, disable_private_keys=True)
             multisig = node.get_wallet_rpc(f"{self.name}_{i}")
-            external = multisig.getdescriptorinfo(f"wsh(sortedmulti({self.M},{f'/0/*,'.join(xpubs)}/0/*))")
-            internal = multisig.getdescriptorinfo(f"wsh(sortedmulti({self.M},{f'/1/*,'.join(xpubs)}/1/*))")
+            external = multisig.getdescriptorinfo(f"wsh(sortedmulti({self.M},{f','.join(external_xpubs)}))")
+            internal = multisig.getdescriptorinfo(f"wsh(sortedmulti({self.M},{f','.join(internal_xpubs)}))")
             result = multisig.importdescriptors([
                 {  # receiving addresses (internal: False)
                     "desc": external["descriptor"],
@@ -93,10 +88,10 @@ class WalletMultisigDescriptorPSBTTest(BitcoinTestFramework):
         }
 
         self.log.info("Generate and exchange xpubs...")
-        xpubs = [self._get_xpub(signer) for signer in participants["signers"]]
+        external_xpubs, internal_xpubs = [[self._get_xpub(signer, internal) for signer in participants["signers"]] for internal in [False, True]]
 
         self.log.info("Every participant imports the following descriptors to create the watch-only multisig...")
-        participants["multisigs"] = list(self.participants_create_multisigs(xpubs))
+        participants["multisigs"] = list(self.participants_create_multisigs(external_xpubs, internal_xpubs))
 
         self.log.info("Check that every participant's multisig generates the same addresses...")
         for _ in range(10):  # we check that the first 10 generated addresses are the same for all participant's multisigs