moosig

Fun with MuSig2 and Ledger devices 🎵

Do not use for real funds. Only for testing!

This repository contains a simple script to test the implementation of MuSig2 support in the BIP-388-compliant wallet policies that are supported in the Ledger bitcoin app.

Implementation details

In wallet policies, musig(...)/** or musig(...)/<NUM;NUM>/* key expressions are added in taproot script. Such wallet policies map to a subset of the proposed descriptor extensions for MuSig2 in BIP-0390.

Note that expressions like musig(@0/**,@1/**) or musig(@0/<0;1>/*,@1/<0;1>/*) are not supported, despite the corresponding descriptors being allowed in BIP-0390. Aggregate-then-derive pattern is significantly more efficient for MuSig2, especially in the context of signing devices.

For the supported policies, the implementation should be compliant with the spec in the proposed BIPs. However, it has not yet been extensively cross-tested with bitcoin-core implementation, and testing in general is still quite minimal. Therefore, bugs are likely!

Preliminaries

Ledger Bitcoin Test app with MuSig2 support

The alpha version of the MuSig2-enabled Ledger Bitcoin app can be installed from Ledger Live if you have a Nano X, Nano S+ or Stax. Support on Nano S is not planned.

Make sure that My Ledger ==> Experimental Features ==> Developer Mode setting is enabled.

Search the app called Bitcoin Test Musig. Like the Bitcoin Test app, it is compatible with all the bitcoin test networks.

Remember to close Ledger Live before continuing, as it might interfere with other programs interacting with the device simultaneously.

Install dependencies

Create a python environment:

$ python -m venv venv
$ source venv/bin/activate

Install dependencies with:

$ pip install -r requirements.txt

Note that the script depends on a still unreleased version of the ledger_bitcoin package, directly installed from the app-bitcoin-new repository.

Run

Have a Ledger device unlocked on the Bitcoin Test Musig app, and run:

$ python moosig.py

If all goes well, you will register a musig2 policy using the Ledger device for one key, and a hot cosigner for the second key. Then you will be prompted to sign a (fake) transaction¹.

This script assumes that both rounds of MuSig2 are done in rapid succession, and the device is kept connected during the entire time.

If all goes well, the final psbt produced by the script can be finalized by the bitcoin-cli -regtest finalizepsbt command (as long as the version of bitcoin-core you are using supports the descriptor you are using - but not the musig() key expressions).

Feel free to play around with the code in moosig.py to experiment with different policies.

Footnotes

1. Unfortunately, you will have to provide your own testnet/regtest coins if you want to test with a real transaction! ↩