Taproot Cosigner Fun

Disclaimer: this is for fun and for experimenting, not for production use.

A Cosigner Server that allows any client to create an aggregated key between the server and the client using MuSig2, and later to create aggregated Schnorr signatures.

It can be used to create a Taproot output where the key spend path is an aggregated key between the server and the client, while the script path is arbitrarily chosen by the client.

Once such outputs have been created and funded, the client can create PSBTs spending those. Then the server will cooperate to create the aggregated signatures for them.

This project can be used to demo a Taproot key spend.

Run the server

export XPRV_FILENAME=server-xprv-filename
cargo run

Once the server is running, for instance you can get its xpub

curl http://127.0.0.1:8000/xpub | jq .xpub

Run the tests

cargo test

You can set the env variables BITCOIND_EXEC and ELECTRS_EXEC to use different binaries.

For a e2e example, see test_tx.

Manual tests with a CLI wallet

The server is running and reachable at COSIGNER_URL (e.g. at http://127.0.0.1:8000).

The wallet CLI is available at:

cargo run --bin cli -- -h

Generate the client extended private XPRV key and create the wallet

cargo run --bin cli -- -x $XPRV descriptor

Get a wallet address

cargo run --bin cli -- -x $XPRV address

Fund the wallet, e.g. here we use Bitcoin Core

bitcoin-cli sendtoaddress 0.0001

Wait until the transaction has been received and the funds appear in the wallet

cargo run --bin cli -- -x $XPRV pending-balance
cargo run --bin cli -- -x $XPRV balance

Create the transaction

PSBTC=$(cargo run --bin cli -- -x $XPRV create $ADDRESS $SATOSHI)

Sign the transaction

PSBTS=$(cargo run --bin cli -- -x $XPRV sign $PSBTC)

Send the transaction

TXID=$(cargo run --bin cli -- -x $XPRV send $PSBTS)

Check the transcation on the explorer

curl https://fbbe.info/testnet/t/${TXID} | less

TODOs

• Clean up the code
• CLI to receive funds and spend them
• Support for non-empty bip32 paths
• Support for non-empty script paths in descriptor
• Support for aggregated xpub in descriptor
• Testnet example
• Mainnet example

Copyright

MIT

Acknowledgement

This project uses extensively secp256kfun by LLoyd Fournier. secp256kfun is a great library for this kind of experimental projects, have a look at it!