Add holder anchor signing support to BaseSign

Author: wpaulino

lightning/src/chain/keysinterface.rs

@@ -36,6 +36,7 @@ use util::crypto::{hkdf_extract_expand_twice, sign};
 use util::ser::{Writeable, Writer, Readable, ReadableArgs};
 
 use chain::transaction::OutPoint;
+use ln::channel::ANCHOR_OUTPUT_VALUE_SATOSHI;
 use ln::{chan_utils, PaymentPreimage};
 use ln::chan_utils::{HTLCOutputInCommitment, make_funding_redeemscript, ChannelPublicKeys, HolderCommitmentTransaction, ChannelTransactionParameters, CommitmentTransaction, ClosingTransaction};
 use ln::msgs::UnsignedChannelAnnouncement;
@@ -348,6 +349,12 @@ pub trait BaseSign {
 	/// chosen to forgo their output as dust.
 	fn sign_closing_transaction(&self, closing_tx: &ClosingTransaction, secp_ctx: &Secp256k1<secp256k1::All>) -> Result<Signature, ()>;
 
+	/// 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: &mut Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()>;
+
 	/// Signs a channel announcement message with our funding key and our node secret key (aka
 	/// node_id or network_key), proving it comes from one of the channel participants.
 	///
@@ -645,6 +652,7 @@ impl InMemorySigner {
 		witness.push(witness_script.clone().into_bytes());
 		Ok(witness)
 	}
+
 }
 
 impl BaseSign for InMemorySigner {
@@ -762,6 +770,16 @@ impl BaseSign for InMemorySigner {
 		Ok(closing_tx.trust().sign(&self.funding_key, &channel_funding_redeemscript, self.channel_value_satoshis, secp_ctx))
 	}
 
+	fn sign_holder_anchor_input(
+		&self, anchor_tx: &mut Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()> {
+		let witness_script = chan_utils::get_anchor_redeemscript(&self.holder_channel_pubkeys.funding_pubkey);
+		let sighash = sighash::SighashCache::new(&*anchor_tx).segwit_signature_hash(
+			input, &witness_script, ANCHOR_OUTPUT_VALUE_SATOSHI, EcdsaSighashType::All,
+		).unwrap();
+		Ok(sign(secp_ctx, &hash_to_message!(&sighash[..]), &self.funding_key))
+	}
+
 	fn sign_channel_announcement(&self, msg: &UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>)
 	-> Result<(Signature, Signature), ()> {
 		let msghash = hash_to_message!(&Sha256dHash::hash(&msg.encode()[..])[..]);

lightning/src/util/enforcing_trait_impls.rs

@@ -7,6 +7,7 @@
 // You may not use this file except in accordance with one or both of these
 // licenses.
 
+use ln::channel::{ANCHOR_OUTPUT_VALUE_SATOSHI, MIN_CHAN_DUST_LIMIT_SATOSHIS};
 use ln::chan_utils::{HTLCOutputInCommitment, ChannelPublicKeys, HolderCommitmentTransaction, CommitmentTransaction, ChannelTransactionParameters, TrustedCommitmentTransaction, ClosingTransaction};
 use ln::{chan_utils, msgs, PaymentPreimage};
 use chain::keysinterface::{Sign, InMemorySigner, BaseSign};
@@ -199,6 +200,16 @@ impl BaseSign for EnforcingSigner {
 		Ok(self.inner.sign_closing_transaction(closing_tx, secp_ctx).unwrap())
 	}
 
+	fn sign_holder_anchor_input(
+		&self, anchor_tx: &mut Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+	) -> Result<Signature, ()> {
+		debug_assert!(MIN_CHAN_DUST_LIMIT_SATOSHIS > ANCHOR_OUTPUT_VALUE_SATOSHI);
+		// As long as our minimum dust limit is enforced and is greater than our anchor output
+		// value, an anchor output can only have an index within [0, 1].
+		assert!(anchor_tx.input[input].previous_output.vout == 0 || anchor_tx.input[input].previous_output.vout == 1);
+		self.inner.sign_holder_anchor_input(anchor_tx, input, secp_ctx)
+	}
+
 	fn sign_channel_announcement(&self, msg: &msgs::UnsignedChannelAnnouncement, secp_ctx: &Secp256k1<secp256k1::All>)
 	-> Result<(Signature, Signature), ()> {
 		self.inner.sign_channel_announcement(msg, secp_ctx)