Implement Script for Witness and Add Tweak in Partially Signed Bitcoin Transaction to make it a single data source needed for a Signer to produce valid signatures.

Author: yellowred

lightning/src/sign/mod.rs

@@ -18,7 +18,7 @@ use bitcoin::blockdata::script::{Script, ScriptBuf, Builder};
 use bitcoin::blockdata::opcodes;
 use bitcoin::ecdsa::Signature as EcdsaSignature;
 use bitcoin::network::constants::Network;
-use bitcoin::psbt::PartiallySignedTransaction;
+use bitcoin::psbt::{PartiallySignedTransaction, raw};
 use bitcoin::bip32::{ExtendedPrivKey, ExtendedPubKey, ChildNumber};
 use bitcoin::sighash;
 use bitcoin::sighash::EcdsaSighashType;
@@ -43,7 +43,7 @@ use crate::util::ser::{Writeable, Writer, Readable, ReadableArgs};
 use crate::chain::transaction::OutPoint;
 use crate::ln::channel::ANCHOR_OUTPUT_VALUE_SATOSHI;
 use crate::ln::{chan_utils, PaymentPreimage};
-use crate::ln::chan_utils::{HTLCOutputInCommitment, make_funding_redeemscript, ChannelPublicKeys, HolderCommitmentTransaction, ChannelTransactionParameters, CommitmentTransaction, ClosingTransaction};
+use crate::ln::chan_utils::{HTLCOutputInCommitment, make_funding_redeemscript, ChannelPublicKeys, HolderCommitmentTransaction, ChannelTransactionParameters, CommitmentTransaction, ClosingTransaction, get_revokeable_redeemscript};
 use crate::ln::channel_keys::{DelayedPaymentBasepoint, DelayedPaymentKey, HtlcKey, HtlcBasepoint, RevocationKey, RevocationBasepoint};
 use crate::ln::msgs::{UnsignedChannelAnnouncement, UnsignedGossipMessage};
 #[cfg(taproot)]
@@ -149,6 +149,7 @@ pub struct StaticPaymentOutputDescriptor {
 	/// Added as optional, but always `Some` if the descriptor was produced in v0.0.117 or later.
 	pub channel_transaction_parameters: Option<ChannelTransactionParameters>,
 }
+
 impl StaticPaymentOutputDescriptor {
 	/// Returns the `witness_script` of the spendable output.
 	///
@@ -304,7 +305,7 @@ impl SpendableOutputDescriptor {
 	///
 	/// This is not exported to bindings users as there is no standard serialization for an input.
 	/// See [`Self::create_spendable_outputs_psbt`] instead.
-	pub fn to_psbt_input(&self) -> bitcoin::psbt::Input {
+	pub fn to_psbt_input<T: secp256k1::Signing>(&self, secp_ctx: &Secp256k1<T>, delayed_payment_basepoint: Option<&DelayedPaymentBasepoint>) -> bitcoin::psbt::Input {
 		match self {
 			SpendableOutputDescriptor::StaticOutput { output, .. } => {
 				// Is a standard P2WPKH, no need for witness script
@@ -314,14 +315,25 @@ impl SpendableOutputDescriptor {
 				}
 			},
 			SpendableOutputDescriptor::DelayedPaymentOutput(descriptor) => {
-				// TODO we could add the witness script as well
+				let witness_script = delayed_payment_basepoint.map(|basepoint| {
+					let payment_key = DelayedPaymentKey::from_basepoint(
+						secp_ctx,
+						basepoint,
+						&descriptor.per_commitment_point,
+					);
+					get_revokeable_redeemscript(
+						&descriptor.revocation_pubkey,
+						descriptor.to_self_delay,
+						&payment_key,
+					)
+				});
 				bitcoin::psbt::Input {
 					witness_utxo: Some(descriptor.output.clone()),
+					witness_script: witness_script,
 					..Default::default()
 				}
 			},
 			SpendableOutputDescriptor::StaticPaymentOutput(descriptor) => {
-				// TODO we could add the witness script as well
 				bitcoin::psbt::Input {
 					witness_utxo: Some(descriptor.output.clone()),
 					..Default::default()
@@ -330,6 +342,7 @@ impl SpendableOutputDescriptor {
 		}
 	}
 
+
 	/// Creates an unsigned [`PartiallySignedTransaction`] which spends the given descriptors to
 	/// the given outputs, plus an output to the given change destination (if sufficient
 	/// change value remains). The PSBT will have a feerate, at least, of the given value.
@@ -346,11 +359,14 @@ impl SpendableOutputDescriptor {
 	/// does not match the one we can spend.
 	///
 	/// We do not enforce that outputs meet the dust limit or that any output scripts are standard.
-	pub fn create_spendable_outputs_psbt(descriptors: &[&SpendableOutputDescriptor], outputs: Vec<TxOut>, change_destination_script: ScriptBuf, feerate_sat_per_1000_weight: u32, locktime: Option<LockTime>) -> Result<(PartiallySignedTransaction, u64), ()> {
+	pub fn create_spendable_outputs_psbt(descriptors: &[&SpendableOutputDescriptor], outputs: Vec<TxOut>, change_destination_script: ScriptBuf, feerate_sat_per_1000_weight: u32, locktime: Option<LockTime>, delayed_payment_basepoint: Option<&DelayedPaymentBasepoint>) -> Result<(PartiallySignedTransaction, u64), ()> {
+		let secp_ctx = Secp256k1::new();
 		let mut input = Vec::with_capacity(descriptors.len());
 		let mut input_value = 0;
 		let mut witness_weight = 0;
 		let mut output_set = HashSet::with_capacity(descriptors.len());
+		// Required to derive signing key: privkey = basepoint_secret + SHA256(per_commitment_point || basepoint)
+		let mut add_tweak: Option<Vec<u8>> = None;
 		for outp in descriptors {
 			match outp {
 				SpendableOutputDescriptor::StaticPaymentOutput(descriptor) => {
@@ -387,6 +403,8 @@ impl SpendableOutputDescriptor {
 					#[cfg(feature = "grind_signatures")]
 					{ witness_weight -= 1; } // Guarantees a low R signature
 					input_value += descriptor.output.value;
+
+					add_tweak = delayed_payment_basepoint.and_then(|basepoint| Some(derive_add_tweak(&descriptor.per_commitment_point, &basepoint)));
 				},
 				SpendableOutputDescriptor::StaticOutput { ref outpoint, ref output, .. } => {
 					if !output_set.insert(*outpoint) { return Err(()); }
@@ -413,20 +431,43 @@ impl SpendableOutputDescriptor {
 		let expected_max_weight =
 			transaction_utils::maybe_add_change_output(&mut tx, input_value, witness_weight, feerate_sat_per_1000_weight, change_destination_script)?;
 
-		let psbt_inputs = descriptors.iter().map(|d| d.to_psbt_input()).collect::<Vec<_>>();
+		let psbt_inputs = descriptors.iter().map(|d| d.to_psbt_input(&secp_ctx, delayed_payment_basepoint)).collect::<Vec<_>>();
 		let psbt = PartiallySignedTransaction {
 			inputs: psbt_inputs,
 			outputs: vec![Default::default(); tx.output.len()],
 			unsigned_tx: tx,
 			xpub: Default::default(),
 			version: 0,
-			proprietary: Default::default(),
+			proprietary: add_tweak.map(|add_tweak| {vec![(
+				raw::ProprietaryKey {
+					prefix: "spendable_output".as_bytes().to_vec(),
+					subtype: 0,
+					key: "add_tweak".as_bytes().to_vec(),
+				},
+				add_tweak,
+			)].into_iter().collect()}).or(Some(Default::default())).expect("proprietary fields"),
 			unknown: Default::default(),
 		};
 		Ok((psbt, expected_max_weight))
 	}
 }
 
+/// Derives a per-commitment-transaction (eg an htlc key or delayed_payment key) private key addition tweak
+/// from a delayed payment basepoint and a per_commitment_point:
+/// `privkey = basepoint_secret + SHA256(per_commitment_point || basepoint)`
+/// TODO(oleg): refactor after migration to LDK v119
+pub fn derive_add_tweak(
+    per_commitment_point: &PublicKey,
+    basepoint: &DelayedPaymentBasepoint,
+) -> Vec<u8> {
+    let mut sha = Sha256::engine();
+    sha.input(&per_commitment_point.serialize());
+    sha.input(&basepoint.to_public_key().serialize());
+    let res = Sha256::from_engine(sha).to_byte_array();
+    res.to_vec()
+}
+
+
 /// The parameters required to derive a channel signer via [`SignerProvider`].
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct ChannelDerivationParameters {
@@ -1615,7 +1656,7 @@ impl KeysManager {
 	/// May panic if the [`SpendableOutputDescriptor`]s were not generated by channels which used
 	/// this [`KeysManager`] or one of the [`InMemorySigner`] created by this [`KeysManager`].
 	pub fn spend_spendable_outputs<C: Signing>(&self, descriptors: &[&SpendableOutputDescriptor], outputs: Vec<TxOut>, change_destination_script: ScriptBuf, feerate_sat_per_1000_weight: u32, locktime: Option<LockTime>, secp_ctx: &Secp256k1<C>) -> Result<Transaction, ()> {
-		let (mut psbt, expected_max_weight) = SpendableOutputDescriptor::create_spendable_outputs_psbt(descriptors, outputs, change_destination_script, feerate_sat_per_1000_weight, locktime)?;
+		let (mut psbt, expected_max_weight) = SpendableOutputDescriptor::create_spendable_outputs_psbt(descriptors, outputs, change_destination_script, feerate_sat_per_1000_weight, locktime, None)?;
 		psbt = self.sign_spendable_outputs_psbt(descriptors, psbt, secp_ctx)?;
 
 		let spend_tx = psbt.extract_tx();