Add holder anchor signing support to InMemorySigner

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;
@@ -645,6 +646,25 @@ impl InMemorySigner {
 		witness.push(witness_script.clone().into_bytes());
 		Ok(witness)
 	}
+
+	/// Computes the fully-signed witness for a commitment transaction's anchor output used as an
+	/// input within `anchor_tx`, which spends the commitment transaction, at index `input`.
+	///
+	/// Panics if `anchor_tx`'s sighash cannot be computed or index `input` is not valid within
+	/// `anchor_tx`.
+	pub fn sign_holder_anchor_output(
+		&self, anchor_tx: &mut Transaction, input: usize, secp_ctx: &Secp256k1<secp256k1::All>,
+	) {
+		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();
+		let msg = hash_to_message!(&sighash[..]);
+		let sig = sign(secp_ctx, &msg, &self.funding_key);
+		let mut funding_sig = sig.serialize_der().to_vec();
+		funding_sig.push(EcdsaSighashType::All as u8);
+		anchor_tx.input[input].witness = Witness::from_vec(vec![funding_sig, witness_script.to_bytes()]);
+	}
 }
 
 impl BaseSign for InMemorySigner {