ark/vtxo/policy/

clause.rs

use bitcoin::absolute::LockTime;
use bitcoin::hashes::sha256;
use bitcoin::key::constants::SCHNORR_SIGNATURE_SIZE;
use bitcoin::secp256k1::schnorr;
use bitcoin::taproot::{self, ControlBlock};
use bitcoin::{Sequence, VarInt, Witness};
use bitcoin::{secp256k1::PublicKey, ScriptBuf};

use bitcoin_ext::{BlockDelta, BlockHeight};

use crate::lightning::{PaymentHash, Preimage};
use crate::vtxo::policy::Policy;
use crate::Vtxo;
use crate::scripts;

/// A trait describing a VTXO policy clause.
///
/// It can be used when creating the VTXO, specifying the script pubkey,
/// and check the satisfaction weight when spending it.
pub trait TapScriptClause: Sized + Clone {
	/// The type of witness data required to sign the clause.
	type WitnessData;

	/// Returns the tapscript for the clause.
	fn tapscript(&self) -> ScriptBuf;

	/// Construct the taproot control block for spending the VTXO using this clause
	fn control_block<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> ControlBlock {
		vtxo.output_taproot()
			.control_block(&(self.tapscript(), taproot::LeafVersion::TapScript))
			.expect("clause is not in taproot tree")
	}

	/// Computes the total witness size in bytes for spending via this clause.
	fn witness_size<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> usize;

	/// Constructs the witness for the clause.
	fn witness(
		&self,
		data: &Self::WitnessData,
		control_block: &ControlBlock,
	) -> Witness;
}

/// A clause that allows to sign and spend the UTXO after a relative
/// timelock.
#[derive(Debug, Clone)]
pub struct DelayedSignClause {
	pub pubkey: PublicKey,
	pub block_delta: BlockDelta,
}

impl DelayedSignClause {
	/// Returns the input sequence value for this clause.
	pub fn sequence(&self) -> Sequence {
		Sequence::from_height(self.block_delta)
	}
}

impl TapScriptClause for DelayedSignClause {
	type WitnessData = schnorr::Signature;

	fn tapscript(&self) -> ScriptBuf {
		scripts::delayed_sign(self.block_delta, self.pubkey.x_only_public_key().0)
	}

	fn witness(
		&self,
		signature: &Self::WitnessData,
		control_block: &ControlBlock,
	) -> Witness {
		Witness::from_slice(&[
			&signature[..],
			self.tapscript().as_bytes(),
			&control_block.serialize()[..],
		])
	}


	fn witness_size<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> usize {
		let cb_size = self.control_block(vtxo).size();
		let tapscript_size = self.tapscript().as_bytes().len();

		1 // byte for the number of witness elements
		+ 1  // schnorr signature size byte
		+ SCHNORR_SIGNATURE_SIZE // schnorr sig bytes
		+ VarInt::from(tapscript_size).size()  // tapscript size bytes
		+ tapscript_size // tapscript bytes
		+ VarInt::from(cb_size).size()  // control block size bytes
		+ cb_size // control block bytes
	}
}

impl Into<VtxoClause> for DelayedSignClause {
	fn into(self) -> VtxoClause {
		VtxoClause::DelayedSign(self)
	}
}

/// A clause that allows to sign and spend the UTXO after an absolute
/// timelock.
#[derive(Debug, Clone)]
pub struct TimelockSignClause {
	pub pubkey: PublicKey,
	pub timelock_height: BlockHeight,
}

impl TimelockSignClause {
	/// Returns the absolute locktime for this clause.
	pub fn locktime(&self) -> LockTime {
		LockTime::from_height(self.timelock_height).expect("timelock height is valid")
	}
}

impl TapScriptClause for TimelockSignClause {
	type WitnessData = schnorr::Signature;

	fn tapscript(&self) -> ScriptBuf {
		scripts::timelock_sign(self.timelock_height, self.pubkey.x_only_public_key().0)
	}

	fn witness(
		&self,
		signature: &Self::WitnessData,
		control_block: &ControlBlock,
	) -> Witness {
		Witness::from_slice(&[
			&signature[..],
			self.tapscript().as_bytes(),
			&control_block.serialize()[..],
		])
	}

	fn witness_size<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> usize {
		let cb_size = self.control_block(vtxo).size();
		let tapscript_size = self.tapscript().as_bytes().len();

		1 // byte for the number of witness elements
		+ 1  // schnorr signature size byte
		+ SCHNORR_SIGNATURE_SIZE // schnorr sig bytes
		+ VarInt::from(tapscript_size).size()  // tapscript size bytes
		+ tapscript_size // tapscript bytes
		+ VarInt::from(cb_size).size()  // control block size bytes
		+ cb_size // control block bytes
	}
}

impl Into<VtxoClause> for TimelockSignClause {
	fn into(self) -> VtxoClause {
		VtxoClause::TimelockSign(self)
	}
}

/// A clause that allows to sign and spend the UTXO after a relative
/// timelock, with an additional absolute one.
#[derive(Debug, Clone)]
pub struct DelayedTimelockSignClause {
	pub pubkey: PublicKey,
	pub timelock_height: BlockHeight,
	pub block_delta: BlockDelta,
}

impl DelayedTimelockSignClause {
	/// Returns the input sequence for this clause.
	pub fn sequence(&self) -> Sequence {
		Sequence::from_height(self.block_delta)
	}

	/// Returns the absolute locktime for this clause.
	pub fn locktime(&self) -> LockTime {
		LockTime::from_height(self.timelock_height).expect("timelock height is valid")
	}
}

impl TapScriptClause for DelayedTimelockSignClause {
	type WitnessData = schnorr::Signature;

	fn tapscript(&self) -> ScriptBuf {
		scripts::delay_timelock_sign(
			self.block_delta,
			self.timelock_height,
			self.pubkey.x_only_public_key().0,
		)
	}

	fn witness(
		&self,
		signature: &Self::WitnessData,
		control_block: &ControlBlock,
	) -> Witness {
		Witness::from_slice(&[
			&signature[..],
			self.tapscript().as_bytes(),
			&control_block.serialize()[..],
		])
	}

	fn witness_size<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> usize {
		let cb_size = self.control_block(vtxo).size();
		let tapscript_size = self.tapscript().as_bytes().len();

		1 // byte for the number of witness elements
		+ 1  // schnorr signature size byte
		+ SCHNORR_SIGNATURE_SIZE // schnorr sig bytes
		+ VarInt::from(tapscript_size).size()  // tapscript size bytes
		+ tapscript_size // tapscript bytes
		+ VarInt::from(cb_size).size()  // control block size bytes
		+ cb_size // control block bytes
	}
}

impl Into<VtxoClause> for DelayedTimelockSignClause {
	fn into(self) -> VtxoClause {
		VtxoClause::DelayedTimelockSign(self)
	}
}

/// A clause that allows to sign and spend the UTXO after a relative
/// timelock, if preimage matching the hash is provided.
#[derive(Debug, Clone)]
pub struct HashDelaySignClause {
	pub pubkey: PublicKey,
	pub hash: sha256::Hash,
	pub block_delta: BlockDelta,
}

impl HashDelaySignClause {
	/// Returns the input sequence for this clause.
	pub fn sequence(&self) -> Sequence {
		Sequence::from_height(self.block_delta)
	}

	/// Try to extract the preimage from a witness that spends this clause.
	///
	/// Witness layout: `[signature, preimage, tapscript, control_block]`.
	/// Returns the preimage if it is 32 bytes and hashes to the given payment hash.
	pub fn extract_preimage_from_witness(
		witness: &Witness,
		payment_hash: PaymentHash,
	) -> Option<Preimage> {
		if witness.len() != 4 {
			return None;
		}

		let bytes = witness.nth(1)?;
		let bytes: [u8; 32] = bytes.try_into().ok()?;

		let preimage = Preimage::from(bytes);
		if preimage.compute_payment_hash() != payment_hash {
			return None;
		}

		Some(preimage)
	}
}

impl TapScriptClause for HashDelaySignClause {
	type WitnessData = (schnorr::Signature, [u8; 32]);

	fn tapscript(&self) -> ScriptBuf {
		scripts::hash_delay_sign(
			self.hash,
			self.block_delta,
			self.pubkey.x_only_public_key().0,
		)
	}

	fn witness(
		&self,
		data: &Self::WitnessData,
		control_block: &ControlBlock,
	) -> Witness {
		let (signature, preimage) = data;
		Witness::from_slice(&[
			&signature[..],
			&preimage[..],
			self.tapscript().as_bytes(),
			&control_block.serialize()[..],
		])
	}

	fn witness_size<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> usize {
		let cb_size = self.control_block(vtxo).size();
		let tapscript_size = self.tapscript().as_bytes().len();

		1 // byte for the number of witness elements
		+ 1  // schnorr signature size byte
		+ SCHNORR_SIGNATURE_SIZE // schnorr sig bytes
		+ 1  // preimage size byte
		+ 32 // preimage bytes
		+ VarInt::from(tapscript_size).size()  // tapscript size bytes
		+ tapscript_size // tapscript bytes
		+ VarInt::from(cb_size).size()  // control block size bytes
		+ cb_size // control block bytes
	}
}

impl Into<VtxoClause> for HashDelaySignClause {
	fn into(self) -> VtxoClause {
		VtxoClause::HashDelaySign(self)
	}
}

/// A clause that allows spending by revealing a preimage and providing a signature.
///
/// This is used for the unlock clause in hArk leaf outputs, where the aggregate
/// pubkey of user+server must sign, and a preimage must be revealed.
#[derive(Debug, Clone)]
pub struct HashSignClause {
	pub pubkey: PublicKey,
	pub hash: sha256::Hash,
}

impl TapScriptClause for HashSignClause {
	type WitnessData = (schnorr::Signature, [u8; 32]);

	fn tapscript(&self) -> ScriptBuf {
		scripts::hash_and_sign(self.hash, self.pubkey.x_only_public_key().0)
	}

	fn witness(
		&self,
		data: &Self::WitnessData,
		control_block: &ControlBlock,
	) -> Witness {
		let (signature, preimage) = data;
		Witness::from_slice(&[
			&signature[..],
			&preimage[..],
			self.tapscript().as_bytes(),
			&control_block.serialize()[..],
		])
	}

	fn witness_size<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> usize {
		let cb_size = self.control_block(vtxo).size();
		let tapscript_size = 57;

		debug_assert_eq!(tapscript_size, self.tapscript().as_bytes().len());

		1 // byte for the number of witness elements
		+ 1  // schnorr signature size byte
		+ SCHNORR_SIGNATURE_SIZE // schnorr sig bytes
		+ 1  // preimage size byte
		+ 32 // preimage bytes
		+ VarInt::from(tapscript_size).size()  // tapscript size bytes
		+ tapscript_size // tapscript bytes
		+ VarInt::from(cb_size).size()  // control block size bytes
		+ cb_size // control block bytes
	}
}

impl Into<VtxoClause> for HashSignClause {
	fn into(self) -> VtxoClause {
		VtxoClause::HashSign(self)
	}
}

#[derive(Debug, Clone)]
pub enum VtxoClause {
	DelayedSign(DelayedSignClause),
	TimelockSign(TimelockSignClause),
	DelayedTimelockSign(DelayedTimelockSignClause),
	HashDelaySign(HashDelaySignClause),
	HashSign(HashSignClause),
}

impl VtxoClause {
	/// Returns the public key associated with this clause.
	pub fn pubkey(&self) -> PublicKey {
		match self {
			Self::DelayedSign(c) => c.pubkey,
			Self::TimelockSign(c) => c.pubkey,
			Self::DelayedTimelockSign(c) => c.pubkey,
			Self::HashDelaySign(c) => c.pubkey,
			Self::HashSign(c) => c.pubkey,
		}
	}


	/// Returns the tapscript for this clause.
	pub fn tapscript(&self) -> ScriptBuf {
		match self {
			Self::DelayedSign(c) => c.tapscript(),
			Self::TimelockSign(c) => c.tapscript(),
			Self::DelayedTimelockSign(c) => c.tapscript(),
			Self::HashDelaySign(c) => c.tapscript(),
			Self::HashSign(c) => c.tapscript(),
		}
	}

	/// Returns the input sequence for this clause, if applicable.
	pub fn sequence(&self) -> Option<Sequence> {
		match self {
			Self::DelayedSign(c) => Some(c.sequence()),
			Self::TimelockSign(_) => None,
			Self::DelayedTimelockSign(c) => Some(c.sequence()),
			Self::HashDelaySign(c) => Some(c.sequence()),
			Self::HashSign(_) => None,
		}
	}

	/// Computes the total witness size in bytes for spending the VTXO via this clause.
	pub fn control_block<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> ControlBlock {
		match self {
			Self::DelayedSign(c) => c.control_block(vtxo),
			Self::TimelockSign(c) => c.control_block(vtxo),
			Self::DelayedTimelockSign(c) => c.control_block(vtxo),
			Self::HashDelaySign(c) => c.control_block(vtxo),
			Self::HashSign(c) => c.control_block(vtxo),
		}
	}

	/// Computes the total witness size in bytes for spending the VTXO via this clause.
	pub fn witness_size<G, P: Policy>(&self, vtxo: &Vtxo<G, P>) -> usize {
		match self {
			Self::DelayedSign(c) => c.witness_size(vtxo),
			Self::TimelockSign(c) => c.witness_size(vtxo),
			Self::DelayedTimelockSign(c) => c.witness_size(vtxo),
			Self::HashDelaySign(c) => c.witness_size(vtxo),
			Self::HashSign(c) => c.witness_size(vtxo),
		}
	}
}

#[cfg(test)]
mod tests {
	use std::str::FromStr;

	use bitcoin::taproot::TaprootSpendInfo;
	use bitcoin::{Amount, OutPoint, Transaction, TxIn, TxOut, Txid, sighash};
	use bitcoin::hashes::Hash;
	use bitcoin::key::Keypair;
	use bitcoin_ext::{TaprootSpendInfoExt, fee};

	use crate::{SECP, musig};
	use crate::test_util::verify_tx;

	use super::*;

	lazy_static! {
		static ref USER_KEYPAIR: Keypair = Keypair::from_str("5255d132d6ec7d4fc2a41c8f0018bb14343489ddd0344025cc60c7aa2b3fda6a").unwrap();
		static ref SERVER_KEYPAIR: Keypair = Keypair::from_str("1fb316e653eec61de11c6b794636d230379509389215df1ceb520b65313e5426").unwrap();
	}

	#[allow(unused)]
	fn all_clause_tested(clause: VtxoClause) -> bool {
		// NB: matcher to ensure all clauses are tested
		match clause {
			VtxoClause::DelayedSign(_) => true,
			VtxoClause::TimelockSign(_) => true,
			VtxoClause::DelayedTimelockSign(_) => true,
			VtxoClause::HashDelaySign(_) => true,
			VtxoClause::HashSign(_) => true,
		}
	}

	fn transaction() -> Transaction {
		let address = bitcoin::Address::from_str("tb1q00h5delzqxl7xae8ufmsegghcl4jwfvdnd8530")
			.unwrap().assume_checked();

		Transaction {
			version: bitcoin::transaction::Version(3),
			lock_time: bitcoin::absolute::LockTime::ZERO,
			input: vec![],
			output: vec![TxOut {
				script_pubkey: address.script_pubkey(),
				value: Amount::from_sat(900_000),
			}, fee::fee_anchor()]
		}
	}

	fn taproot_material(clause_spk: ScriptBuf) -> (TaprootSpendInfo, ControlBlock) {
		let user_pubkey = USER_KEYPAIR.public_key();
		let server_pubkey = SERVER_KEYPAIR.public_key();

		let combined_pk = musig::combine_keys([user_pubkey, server_pubkey])
			.x_only_public_key().0;
		let taproot = taproot::TaprootBuilder::new()
			.add_leaf(0, clause_spk.clone()).unwrap()
			.finalize(&SECP, combined_pk).unwrap();

		let cb = taproot
			.control_block(&(clause_spk.clone(), taproot::LeafVersion::TapScript))
			.expect("script is in taproot");

		(taproot, cb)
	}

	fn signature(tx: &Transaction, input: &TxOut, clause_spk: ScriptBuf) -> schnorr::Signature {
		let leaf_hash = taproot::TapLeafHash::from_script(
			&clause_spk,
			taproot::LeafVersion::TapScript,
		);

		let mut shc = sighash::SighashCache::new(tx);
		let sighash = shc.taproot_script_spend_signature_hash(
			0, &sighash::Prevouts::All(&[input.clone()]), leaf_hash, sighash::TapSighashType::Default,
		).expect("all prevouts provided");

		SECP.sign_schnorr(&sighash.into(), &*USER_KEYPAIR)
	}

	#[test]
	fn test_delayed_sign_clause() {
		let clause = DelayedSignClause {
			pubkey: USER_KEYPAIR.public_key(),
			block_delta: 100,
		};

		// We compute taproot material for the clause
		let (taproot, cb) = taproot_material(clause.tapscript());
		let tx_in = TxOut {
			script_pubkey: taproot.script_pubkey(),
			value: Amount::from_sat(1_000_000),
		};

		// We build transaction spending input containing clause
		let mut tx = transaction();
		tx.input.push(TxIn {
			previous_output: OutPoint::new(Txid::all_zeros(), 0),
			script_sig: ScriptBuf::default(),
			sequence: clause.sequence(),
			witness: Witness::new(),
		});

		// We compute the signature for the transaction
		let signature = signature(&tx, &tx_in, clause.tapscript());
		tx.input[0].witness = clause.witness(&signature, &cb);

		// We verify the transaction
		verify_tx(&[tx_in], 0, &tx).expect("transaction is invalid");
	}

	#[test]
	fn test_timelock_sign_clause() {
		let clause = TimelockSignClause {
			pubkey: USER_KEYPAIR.public_key(),
			timelock_height: 100,
		};

		// We compute taproot material for the clause
		let (taproot, cb) = taproot_material(clause.tapscript());
		let tx_in = TxOut {
			script_pubkey: taproot.script_pubkey(),
			value: Amount::from_sat(1_000_000),
		};

		// We build transaction spending input containing clause
		let mut tx = transaction();
		tx.lock_time = clause.locktime();
		tx.input.push(TxIn {
			previous_output: OutPoint::new(Txid::all_zeros(), 0),
			script_sig: ScriptBuf::default(),
			sequence: Sequence::ZERO,
			witness: Witness::new(),
		});

		// We compute the signature for the transaction
		let signature = signature(&tx, &tx_in, clause.tapscript());
		tx.input[0].witness = clause.witness(&signature, &cb);

		// We verify the transaction
		verify_tx(&[tx_in], 0, &tx).expect("transaction is invalid");
	}

	#[test]
	fn test_delayed_timelock_clause() {
		let clause = DelayedTimelockSignClause {
			pubkey: USER_KEYPAIR.public_key(),
			timelock_height: 100,
			block_delta: 24,
		};

		// We compute taproot material for the clause
		let (taproot, cb) = taproot_material(clause.tapscript());
		let tx_in = TxOut {
			script_pubkey: taproot.script_pubkey(),
			value: Amount::from_sat(1_000_000),
		};

		// We build transaction spending input containing clause
		let mut tx = transaction();
		tx.lock_time = clause.locktime();
		tx.input.push(TxIn {
			previous_output: OutPoint::new(Txid::all_zeros(), 0),
			script_sig: ScriptBuf::default(),
			sequence: clause.sequence(),
			witness: Witness::new(),
		});

		// We compute the signature for the transaction
		let signature = signature(&tx, &tx_in, clause.tapscript());
		tx.input[0].witness = clause.witness(&signature, &cb);

		// We verify the transaction
		verify_tx(&[tx_in], 0, &tx).expect("transaction is invalid");
	}

	#[test]
	fn test_hash_delay_clause() {
		let preimage = [0; 32];

		let clause = HashDelaySignClause {
			pubkey: USER_KEYPAIR.public_key(),
			hash: sha256::Hash::hash(&preimage),
			block_delta: 24,
		};

		// We compute taproot material for the clause
		let (taproot, cb) = taproot_material(clause.tapscript());
		let tx_in = TxOut {
			script_pubkey: taproot.script_pubkey(),
			value: Amount::from_sat(1_000_000),
		};

		// We build transaction spending input containing clause
		let mut tx = transaction();
		tx.input.push(TxIn {
			previous_output: OutPoint::new(Txid::all_zeros(), 0),
			script_sig: ScriptBuf::default(),
			sequence: clause.sequence(),
			witness: Witness::new(),
		});

		// We compute the signature for the transaction
		let signature = signature(&tx, &tx_in, clause.tapscript());
		tx.input[0].witness = clause.witness(&(signature, preimage), &cb);

		// We verify the transaction
		verify_tx(&[tx_in], 0, &tx).expect("transaction is invalid");
	}

	#[test]
	fn test_extract_preimage_from_witness() {
		let preimage_bytes = [42u8; 32];
		let payment_hash = sha256::Hash::hash(&preimage_bytes);

		let clause = HashDelaySignClause {
			pubkey: USER_KEYPAIR.public_key(),
			hash: payment_hash,
			block_delta: 24,
		};

		// Build a valid witness via the clause
		let (taproot, cb) = taproot_material(clause.tapscript());
		let tx_in = TxOut {
			script_pubkey: taproot.script_pubkey(),
			value: Amount::from_sat(1_000_000),
		};

		let mut tx = transaction();
		tx.input.push(TxIn {
			previous_output: OutPoint::new(Txid::all_zeros(), 0),
			script_sig: ScriptBuf::default(),
			sequence: clause.sequence(),
			witness: Witness::new(),
		});

		let sig = signature(&tx, &tx_in, clause.tapscript());
		let witness = clause.witness(&(sig, preimage_bytes), &cb);

		// Extract should succeed with correct payment hash
		let extracted = HashDelaySignClause::extract_preimage_from_witness(
			&witness,
			payment_hash.into(),
		);
		assert!(extracted.is_some());
		assert_eq!(extracted.unwrap().as_ref(), &preimage_bytes);

		// Extract should fail with wrong payment hash
		let wrong_hash = sha256::Hash::hash(&[0u8; 32]);
		let extracted = HashDelaySignClause::extract_preimage_from_witness(
			&witness,
			wrong_hash.into(),
		);
		assert!(extracted.is_none());

		// Extract should fail with wrong witness length
		let short_witness = Witness::from_slice(&[&sig[..], &preimage_bytes[..]]);
		let extracted = HashDelaySignClause::extract_preimage_from_witness(
			&short_witness,
			payment_hash.into(),
		);
		assert!(extracted.is_none());
	}

	#[test]
	fn test_hash_sign_clause() {
		let preimage = [0u8; 32];
		let hash = sha256::Hash::hash(&preimage);

		// HashSignClause uses an x-only aggregate public key
		let agg_pk = musig::combine_keys([USER_KEYPAIR.public_key(), SERVER_KEYPAIR.public_key()]);

		let clause = HashSignClause {
			pubkey: agg_pk,
			hash,
		};

		// We compute taproot material for the clause
		let (taproot, cb) = taproot_material(clause.tapscript());
		let tx_in = TxOut {
			script_pubkey: taproot.script_pubkey(),
			value: Amount::from_sat(1_000_000),
		};

		// We build transaction spending input containing clause
		let mut tx = transaction();
		tx.input.push(TxIn {
			previous_output: OutPoint::new(Txid::all_zeros(), 0),
			script_sig: ScriptBuf::default(),
			sequence: Sequence::ZERO, // HashSignClause has no relative timelock
			witness: Witness::new(),
		});

		// For HashSignClause, we need a MuSig signature from both parties
		let leaf_hash = taproot::TapLeafHash::from_script(
			&clause.tapscript(),
			taproot::LeafVersion::TapScript,
		);

		let mut shc = sighash::SighashCache::new(&tx);
		let sighash = shc.taproot_script_spend_signature_hash(
			0, &sighash::Prevouts::All(&[tx_in.clone()]), leaf_hash, sighash::TapSighashType::Default,
		).expect("all prevouts provided");

		// Create MuSig signature
		let (user_sec_nonce, user_pub_nonce) = musig::nonce_pair(&*USER_KEYPAIR);
		let (server_pub_nonce, server_part_sig) = musig::deterministic_partial_sign(
			&*SERVER_KEYPAIR,
			[USER_KEYPAIR.public_key()],
			&[&user_pub_nonce],
			sighash.to_byte_array(),
			None,
		);
		let agg_nonce = musig::nonce_agg(&[&user_pub_nonce, &server_pub_nonce]);

		let (_user_part_sig, final_sig) = musig::partial_sign(
			[USER_KEYPAIR.public_key(), SERVER_KEYPAIR.public_key()],
			agg_nonce,
			&*USER_KEYPAIR,
			user_sec_nonce,
			sighash.to_byte_array(),
			None,
			Some(&[&server_part_sig]),
		);
		let final_sig = final_sig.expect("should have final signature");

		tx.input[0].witness = clause.witness(&(final_sig, preimage), &cb);

		// We verify the transaction
		verify_tx(&[tx_in], 0, &tx).expect("transaction is invalid");
	}
}