Introduce TaprootSigner trait.

For Taproot support, we need to define an alternative trait to
EcdsaChannelSigner. This trait will be implemented by all signers
that wish to support Taproot channels.

lightning/src/sign/mod.rs

@@ -61,6 +61,9 @@ use crate::util::invoice::construct_invoice_preimage;
 
 pub(crate) mod type_resolver;
 
+#[cfg(taproot)]
+pub mod taproot;
+
 /// Used as initial key material, to be expanded into multiple secret keys (but not to be used
 /// directly). This is used within LDK to encrypt/decrypt inbound payment data.
 ///

lightning/src/sign/taproot.rs

@@ -0,0 +1,149 @@
+//! Defines a Taproot-specific signer type.
+
+use bitcoin::blockdata::transaction::Transaction;
+use bitcoin::secp256k1;
+use bitcoin::secp256k1::{PublicKey, schnorr::Signature, Secp256k1, SecretKey};
+
+use musig2::types::{PartialSignature, PublicNonce};
+
+use crate::ln::chan_utils::{ClosingTransaction, CommitmentTransaction, HolderCommitmentTransaction, HTLCOutputInCommitment};
+use crate::ln::msgs::PartialSignatureWithNonce;
+use crate::ln::PaymentPreimage;
+use crate::sign::{ChannelSigner, HTLCDescriptor};
+
+/// A Taproot-specific signer type that defines signing-related methods that are either unique to
+/// Taproot or have argument or return types that differ from the ones an ECDSA signer would be
+/// expected to have.
+pub trait TaprootChannelSigner: ChannelSigner {
+	/// Generate a local nonce pair, which requires committing to ahead of time.
+	/// The counterparty needs the public nonce generated herein to compute a partial signature.
+	fn generate_local_nonce_pair(&self, commitment_number: u64, secp_ctx: &Secp256k1<secp256k1::All>) -> PublicNonce;
+
+	/// Create a signature for a counterparty's commitment transaction and associated HTLC transactions.
+	///
+	/// Note that if signing fails or is rejected, the channel will be force-closed.
+	///
+	/// Policy checks should be implemented in this function, including checking the amount
+	/// sent to us and checking the HTLCs.
+	///
+	/// The preimages of outgoing HTLCs that were fulfilled since the last commitment are provided.
+	/// A validating signer should ensure that an HTLC output is removed only when the matching
+	/// preimage is provided, or when the value to holder is restored.
+	///
+	/// Note that all the relevant preimages will be provided, but there may also be additional
+	/// irrelevant or duplicate preimages.
+	//
+	// TODO: Document the things someone using this interface should enforce before signing.
+	fn partially_sign_counterparty_commitment(&self, counterparty_nonce: PublicNonce,
+		commitment_tx: &CommitmentTransaction, preimages: Vec<PaymentPreimage>,
+		secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<(PartialSignatureWithNonce, Vec<Signature>), ()>;
+
+	/// Creates a signature for a holder's commitment transaction.
+	///
+	/// This will be called
+	/// - with a non-revoked `commitment_tx`.
+	/// - with the latest `commitment_tx` when we initiate a force-close.
+	///
+	/// This may be called multiple times for the same transaction.
+	///
+	/// An external signer implementation should check that the commitment has not been revoked.
+	///
+	// TODO: Document the things someone using this interface should enforce before signing.
+	fn finalize_holder_commitment(&self, commitment_number: u64,
+		commitment_tx: &HolderCommitmentTransaction,
+		counterparty_partial_signature: PartialSignatureWithNonce,
+		secp_ctx: &Secp256k1<secp256k1::All>
+	) -> Result<PartialSignature, ()>;
+
+	/// Create a signature for the given input in a transaction spending an HTLC transaction output
+	/// or a commitment transaction `to_local` output when our counterparty broadcasts an old state.
+	///
+	/// A justice transaction may claim multiple outputs at the same time if timelocks are
+	/// similar, but only a signature for the input at index `input` should be signed for here.
+	/// It may be called multiple times for same output(s) if a fee-bump is needed with regards
+	/// to an upcoming timelock expiration.
+	///
+	/// Amount is value of the output spent by this input, committed to in the BIP 341 signature.
+	///
+	/// `per_commitment_key` is revocation secret which was provided by our counterparty when they
+	/// revoked the state which they eventually broadcast. It's not a _holder_ secret key and does
+	/// not allow the spending of any funds by itself (you need our holder `revocation_secret` to do
+	/// so).
+	fn sign_justice_revoked_output(&self, justice_tx: &Transaction, input: usize, amount: u64,
+		per_commitment_key: &SecretKey, secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()>;
+
+	/// Create a signature for the given input in a transaction spending a commitment transaction
+	/// HTLC output when our counterparty broadcasts an old state.
+	///
+	/// A justice transaction may claim multiple outputs at the same time if timelocks are
+	/// similar, but only a signature for the input at index `input` should be signed for here.
+	/// It may be called multiple times for same output(s) if a fee-bump is needed with regards
+	/// to an upcoming timelock expiration.
+	///
+	/// `amount` is the value of the output spent by this input, committed to in the BIP 341
+	/// signature.
+	///
+	/// `per_commitment_key` is revocation secret which was provided by our counterparty when they
+	/// revoked the state which they eventually broadcast. It's not a _holder_ secret key and does
+	/// not allow the spending of any funds by itself (you need our holder revocation_secret to do
+	/// so).
+	///
+	/// `htlc` holds HTLC elements (hash, timelock), thus changing the format of the witness script
+	/// (which is committed to in the BIP 341 signatures).
+	fn sign_justice_revoked_htlc(&self, justice_tx: &Transaction, input: usize, amount: u64,
+		per_commitment_key: &SecretKey, htlc: &HTLCOutputInCommitment,
+		secp_ctx: &Secp256k1<secp256k1::All>) -> Result<Signature, ()>;
+
+	/// Computes the signature for a commitment transaction's HTLC output used as an input within
+	/// `htlc_tx`, which spends the commitment transaction at index `input`. The signature returned
+	/// must be be computed using [`TapSighashType::Default`].
+	///
+	/// Note that this may be called for HTLCs in the penultimate commitment transaction if a
+	/// [`ChannelMonitor`] [replica](https://github.com/lightningdevkit/rust-lightning/blob/main/GLOSSARY.md#monitor-replicas)
+	/// broadcasts it before receiving the update for the latest commitment transaction.
+	///
+	///
+	/// [`TapSighashType::Default`]: bitcoin::sighash::TapSighashType::Default
+	/// [`ChannelMonitor`]: crate::chain::channelmonitor::ChannelMonitor
+	fn sign_holder_htlc_transaction(&self, htlc_tx: &Transaction, input: usize,
+		htlc_descriptor: &HTLCDescriptor, secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()>;
+
+	/// Create a signature for a claiming transaction for a HTLC output on a counterparty's commitment
+	/// transaction, either offered or received.
+	///
+	/// Such a transaction may claim multiples offered outputs at same time if we know the
+	/// preimage for each when we create it, but only the input at index `input` should be
+	/// signed for here. It may be called multiple times for same output(s) if a fee-bump is
+	/// needed with regards to an upcoming timelock expiration.
+	///
+	/// `witness_script` is either an offered or received script as defined in BOLT3 for HTLC
+	/// outputs.
+	///
+	/// `amount` is value of the output spent by this input, committed to in the BIP 341 signature.
+	///
+	/// `per_commitment_point` is the dynamic point corresponding to the channel state
+	/// detected onchain. It has been generated by our counterparty and is used to derive
+	/// channel state keys, which are then included in the witness script and committed to in the
+	/// BIP 341 signature.
+	fn sign_counterparty_htlc_transaction(&self, htlc_tx: &Transaction, input: usize, amount: u64,
+		per_commitment_point: &PublicKey, htlc: &HTLCOutputInCommitment,
+		secp_ctx: &Secp256k1<secp256k1::All>) -> Result<Signature, ()>;
+
+	/// Create a signature for a (proposed) closing transaction.
+	///
+	/// Note that, due to rounding, there may be one "missing" satoshi, and either party may have
+	/// chosen to forgo their output as dust.
+	fn partially_sign_closing_transaction(&self, closing_tx: &ClosingTransaction,
+		secp_ctx: &Secp256k1<secp256k1::All>) -> Result<PartialSignature, ()>;
+
+	/// Computes the signature for a commitment transaction's anchor output used as an
+	/// input within `anchor_tx`, which spends the commitment transaction, at index `input`.
+	fn sign_holder_anchor_input(
+		&self, anchor_tx: &Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()>;
+
+	// TODO: sign channel announcement
+}