MuSig2

[Sjors]

This is based on:

• Add register command for BIP388 policies #791
• ledger: handle script path (use SignPsbtYieldedObject) #793
• Optionally pass BIP388 policy to signtx #792

To be used with, and developed in parallel to:

• MuSig2 power (with BIP388) Sjors/bitcoin#91

The general workflow is:

• create a Bitcoin Core wallet with an external signer and a tr(musig(...)) descriptor
  • MuSig2 power (with BIP388) Sjors/bitcoin#91 should have everything you need
• register the policy: bitcoin-cli registerpolicy "MuSigTest" (Core calls hwi register and stores the hmac)
• verify a receive address on the device with bitcoin-cli walletdisplayaddress (Core calls hwi displayaddress --policy-name ...)
• send coins to it
• spend with a single call: bitcoin rpc send '[{"bc1p...": 0.000001}]' fee_rate=1
  • Core runs both MuSig2 rounds in-process: round 1 exchanges pub nonces (hot cosigners + hwi signtx --policy-name ... per device), round 2 collects partial signatures, then Core aggregates and broadcasts
  • if the device is unplugged mid-flow, the PSBT with the partial contributions is returned; a later walletprocesspsbt resumes the dance
• hwi signtx/walletprocesspsbt round-tripping by hand still works if you prefer to drive the rounds yourself

To work around a Ledger quirk, we stash other participants' pubnonce and partial sig before sending it to the device and merge them back afterwards.

Works with the Ledger Bitcoin app 2.4.1, i.e. this does not depend on #819.

CI runs an end-to-end scenario (register → display → fund → 2-round spend → confirm) against a bitcoind built from the Core branch above, plus device tests for register/displayaddress against speculos.

Potential follow-ups:

• ColdCard support (needs a firmware bump)
• multi-device scenario (e.g. 3-of-3 across several simulators)

[Sjors]

Cherry-picked the latest version of all the independent PRs. Other than displaying an address this should be all you need to use MuSig2.

[Sjors]

Somehow I thought I already implemented sending, but it seems I didn't. Done now. Tested with key path as well as script path.

[Sjors]

Did a complete overhaul to match Sjors/async-hwi#2 (though just Ledger, we'll leave ColdCard for a followup). Much vibed, but manually tested. I'll extract smaller patches as separate PR's later.

This PR is orthogonal to #819, however I would prefer to use the newer Ledger app version from it, since it contains various MuSig2 related fixes.

[achow101]

Testing this out, I noticed that it seems like if the ledger is the last signer to add a pubnonce, and I pass that PSBT right back to signtx, nothing happens.

[Sjors]

Rebased and probably fixed the ledger is the last signer issue. I can't test this myself at the moment.