Implement Script for Witness and Add Tweak in Spendable Output to Partially Signed Bitcoin Transaction Input converter.

Adding Witness Script and key tweaks makes PSBT a single data source needed for a Signer to produce valid signatures.
A Signer is not required to be able to generate L2 keys, e.g delayed payment basepoint.

Author: yellowred

lightning/src/chain/channelmonitor.rs

@@ -4255,6 +4255,7 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 						revocation_pubkey: broadcasted_holder_revokable_script.2,
 						channel_keys_id: self.channel_keys_id,
 						channel_value_satoshis: self.channel_value_satoshis,
+						channel_transaction_parameters: Some(self.onchain_tx_handler.channel_transaction_parameters.clone()),
 					}));
 				}
 			}

lightning/src/ln/channel_keys.rs

@@ -37,14 +37,30 @@ macro_rules! basepoint_impl {
 			pub fn to_public_key(&self) -> PublicKey {
 				self.0
 			}
+
+			/// Derives a per-commitment-transaction (eg an htlc key or delayed_payment key) private key addition tweak
+			/// from a basepoint and a per_commitment_point:
+			/// `privkey = basepoint_secret + SHA256(per_commitment_point || basepoint)`
+			/// This calculates the hash part in the tweak derivation process, which is used to ensure
+			/// that each key is unique and cannot be guessed by an external party. It is equivalent
+			/// to the `from_basepoint` method, but without the addition operation, providing just the
+			/// tweak from the hash of the per_commitment_point and the basepoint.
+			pub fn derive_add_tweak(
+				&self,
+				per_commitment_point: &PublicKey,
+			) -> [u8; 32] {
+				let mut sha = Sha256::engine();
+				sha.input(&per_commitment_point.serialize());
+				sha.input(&self.to_public_key().serialize());
+				Sha256::from_engine(sha).to_byte_array()
+			}
 		}
 
 		impl From<PublicKey> for $BasepointT {
 			fn from(value: PublicKey) -> Self {
 				Self(value)
 			}
 		}
-
 	}
 }
 macro_rules! key_impl {

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;
@@ -37,13 +37,13 @@ use bitcoin::secp256k1::ecdsa::{RecoverableSignature, Signature};
 use bitcoin::secp256k1::schnorr;
 use bitcoin::{secp256k1, Sequence, Witness, Txid};
 
+use crate::ln::channel::ANCHOR_OUTPUT_VALUE_SATOSHI;
 use crate::util::transaction_utils;
 use crate::crypto::utils::{hkdf_extract_expand_twice, sign, sign_with_aux_rand};
 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)]
@@ -103,7 +103,13 @@ pub struct DelayedPaymentOutputDescriptor {
 	pub channel_keys_id: [u8; 32],
 	/// The value of the channel which this output originated from, possibly indirectly.
 	pub channel_value_satoshis: u64,
+	/// The channel public keys and other parameters needed to generate a spending transaction or to provide to a re-derived signer through
+	/// [`ChannelSigner::provide_channel_parameters`].
+	///
+	/// Added as optional, but always `Some` if the descriptor was produced in v0.0.121 or later.
+	pub channel_transaction_parameters: Option<ChannelTransactionParameters>,
 }
+
 impl DelayedPaymentOutputDescriptor {
 	/// The maximum length a well-formed witness spending one of these should have.
 	/// Note: If you have the grind_signatures feature enabled, this will be at least 1 byte
@@ -121,6 +127,7 @@ impl_writeable_tlv_based!(DelayedPaymentOutputDescriptor, {
 	(8, revocation_pubkey, required),
 	(10, channel_keys_id, required),
 	(12, channel_value_satoshis, required),
+	(14, channel_transaction_parameters, option),
 });
 
 pub(crate) const P2WPKH_WITNESS_WEIGHT: u64 = 1 /* num stack items */ +
@@ -149,21 +156,24 @@ 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.
 	///
 	/// Note that this will only return `Some` for [`StaticPaymentOutputDescriptor`]s that
 	/// originated from an anchor outputs channel, as they take the form of a P2WSH script.
 	pub fn witness_script(&self) -> Option<ScriptBuf> {
 		self.channel_transaction_parameters.as_ref()
-			.and_then(|channel_params|
-				 if channel_params.channel_type_features.supports_anchors_zero_fee_htlc_tx() {
-					let payment_point = channel_params.holder_pubkeys.payment_point;
+			.and_then(|channel_params| {
+				// Use simplified derivation, assuming `option_static_remotekey` is negotiated.
+				// `remote_payment_basepoint` is used as Payment Key.
+				let payment_point = channel_params.holder_pubkeys.payment_point;
+				if channel_params.channel_type_features.supports_anchors_zero_fee_htlc_tx() {
 					Some(chan_utils::get_to_countersignatory_with_anchors_redeemscript(&payment_point))
-				 } else {
-					 None
-				 }
-			)
+				} else {
+					Some(ScriptBuf::new_p2pkh(&bitcoin::PublicKey::new(payment_point).pubkey_hash()))
+				}
+			})
 	}
 
 	/// The maximum length a well-formed witness spending one of these should have.
@@ -304,7 +314,7 @@ 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>) -> bitcoin::psbt::Input {
 		match self {
 			SpendableOutputDescriptor::StaticOutput { output, .. } => {
 				// Is a standard P2WPKH, no need for witness script
@@ -314,16 +324,47 @@ impl SpendableOutputDescriptor {
 				}
 			},
 			SpendableOutputDescriptor::DelayedPaymentOutput(descriptor) => {
-				// TODO we could add the witness script as well
+				let delayed_payment_basepoint = descriptor.channel_transaction_parameters.as_ref().map(|params| params.holder_pubkeys.delayed_payment_basepoint);
+
+				let (witness_script, add_tweak) = if let Some(basepoint) = delayed_payment_basepoint.as_ref() {
+					let payment_key = DelayedPaymentKey::from_basepoint(
+						secp_ctx,
+						basepoint,
+						&descriptor.per_commitment_point,
+					);
+					// Required to derive signing key: privkey = basepoint_secret + SHA256(per_commitment_point || basepoint)
+					let add_tweak = basepoint.derive_add_tweak(&descriptor.per_commitment_point);
+					(Some(get_revokeable_redeemscript(
+						&descriptor.revocation_pubkey,
+						descriptor.to_self_delay,
+						&payment_key,
+					)), Some(add_tweak))
+				} else {
+				   (None, None)
+				};
+				
 				bitcoin::psbt::Input {
 					witness_utxo: Some(descriptor.output.clone()),
+					witness_script,
+					proprietary: add_tweak.map(|add_tweak| {vec![(
+						raw::ProprietaryKey {
+							prefix: "LDK_spendable_output".as_bytes().to_vec(),
+							subtype: 0,
+							key: "add_tweak".as_bytes().to_vec(),
+						},
+						add_tweak.to_vec(),
+					)].into_iter().collect()}).unwrap_or_default(),
 					..Default::default()
 				}
 			},
 			SpendableOutputDescriptor::StaticPaymentOutput(descriptor) => {
-				// TODO we could add the witness script as well
+				let witness_script = descriptor.witness_script();
+				
+				// With simplified derivation, the private payment key is equal to private payment basepoint, 
+				// so add tweak is not needed.
 				bitcoin::psbt::Input {
 					witness_utxo: Some(descriptor.output.clone()),
+					witness_script,
 					..Default::default()
 				}
 			},
@@ -347,6 +388,7 @@ impl SpendableOutputDescriptor {
 	///
 	/// 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), ()> {
+		let secp_ctx = Secp256k1::new();
 		let mut input = Vec::with_capacity(descriptors.len());
 		let mut input_value = 0;
 		let mut witness_weight = 0;
@@ -413,7 +455,7 @@ 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)).collect::<Vec<_>>();
 		let psbt = PartiallySignedTransaction {
 			inputs: psbt_inputs,
 			outputs: vec![Default::default(); tx.output.len()],