MuSig2 power

[Sjors]

Combines the following:

• wallet: derivehdkey RPC to get xpub at arbitrary path bitcoin/bitcoin#32784
• wallet: allow skipping script paths bitcoin/bitcoin#32857
• Have createwalletdescriptor auto-detect an unused(KEY) bitcoin/bitcoin#32861
• wallet: relax external_signer flag constraints, add musig2 test (partial) bitcoin/bitcoin#33112

The MuSig2-specific commits integrate with
bitcoin-core/HWI#794.

It:

• derives the ordinary multipath descriptor from the wallet's separate receive
  and change descriptors, replacing /0/* and /1/* with /<0;1>/*;
• passes that descriptor and a display name to HWI's registerdescriptor
  command;
• stores the opaque registration returned by each external signer, keyed by
  name and fingerprint;
• supplies the registration to HWI when displaying an address or signing a
  PSBT.

HWI may translate the descriptor to a device-specific representation, such as
a BIP388 wallet policy for Ledger, but that is not exposed to Bitcoin Core.

The general workflow is:

• create an external-signer wallet and import an active
  tr(musig(...)/<0;1>/*) descriptor;
• register it with
  bitcoin-cli -rpcwallet=<wallet> registerdescriptor "MuSig wallet";
• verify a receive address with walletdisplayaddress;
• fund the wallet;
• spend with a single send call.

Bitcoin Core performs both MuSig2 signing rounds in-process. It collects public
nonces, retries signing once the nonce set is complete, collects partial
signatures, aggregates the final Schnorr signature, and broadcasts.

Descriptor registration and registered signing are fanned out to all connected
external signers. This supports wallets whose MuSig2 participants live on
multiple hardware devices. Registrations can be inspected through
getwalletinfo.external_signer_registrations.

If a signer fails after a local MuSig2 contribution was added, the returned
PSBT preserves that contribution so signing can be resumed later.

Rebuild instructions:

git switch -c 2025/06/musig2-power origin/master
git merge --no-ff sjors/2025/06/gethdkey
git merge --no-ff sjors/2025/07/smart-createwalletdescriptor
git merge --no-ff sjors/2025/07/no_script_path
git merge --no-ff sjors/2025/07/external-signer-relax
git cherry-pick # the rest

Tests:

build/bin/test_bitcoin \
  --run_test=descriptor_tests,script_tests,transaction_tests,txvalidationcache_tests,wallet_tests,walletload_tests,psbt_wallet_tests,wallet_rpc_tests

python3 build/test/functional/test_runner.py \
  wallet_hd.py \
  wallet_derivehdkey.py \
  wallet_createwalletdescriptor.py \
  wallet_importdescriptors.py \
  wallet_multisig_descriptor_psbt.py \
  wallet_signer.py \
  wallet_signer_musig2.py \
  wallet_taproot.py

./ci/lint.py

The HWI end-to-end test also exercises this branch against the Ledger Bitcoin
app under Speculos: descriptor registration, address display, funding, both
MuSig2 signing rounds, broadcast, and confirmation.

[Sjors]

Fresh rebase after bitcoin#31244 landed.

[Sjors]

Added bitcoin#33135 and the latest change to bitcoin#33008 (storing one hmac record per policy and fingerprint combination).

[Sjors]

The registerpolicy will now derive the BIP388 policy and perform registration (for some basic descriptor forms). Spending still requires manually passing the PSBT(s) to HWI with bitcoin-core/HWI#794. You should no longer need MooSig, except to display an address.

The fixup commits reflect changes made to the original branches. I'll up the commit history occasionally when one of its dependencies lands.

[Sjors]

bitcoin#29675 landed! Waiting for bitcoin#29136 to get a rebase so I can update the stack here.

[Sjors]

Rebased! bitcoin#32821 and bitcoin#33135 also landed. Down from 67 commits to 44. I haven't had a chance to manually retest the whole flow though.

[Sjors]

Rebased after bitcoin#32471 landed. Down to 37 commits.

[Sjors]

Added #110 which (very roughly) implements BIP388 policy support for displaying addresses.

[Sjors]

Added #111 for signing support.

[Sjors]

Rebased and updated with the new approach taken by HWI. It shrinks the implementation a bit, because with bitcoin-core/HWI#842 HWI takes care of converting the descriptor to BIP388.

For easier testing, I added a temporary commit that migrates wallet registrations made before today to the new format.

[Sjors]

I added a temporary commit that migrates wallet registrations

Dropped, because it's useless due to the switch from /** to <0;1>/* in bitcoin-core/HWI#842 (comment). Cherry-pick 69965c8 and combine with the (also dropped) temporary commit in bitcoin-core/HWI#794 (comment) if you want to try anyway.