From: Matt Corallo <git@bluematt.me>
Date: Mon, 7 Jun 2021 01:41:51 +0000 (+0000)
Subject: Clean up docs on public chan_utils methods to be more useful
X-Git-Tag: v0.0.98~2^2
X-Git-Url: http://git.bitcoin.ninja/index.cgi?a=commitdiff_plain;h=8cdc855f016f4a36fd43c04d75c31569f7eb0c4b;p=rust-lightning

Clean up docs on public chan_utils methods to be more useful
---

diff --git a/lightning/src/ln/chan_utils.rs b/lightning/src/ln/chan_utils.rs
index b698558e1..9f98cd03b 100644
--- a/lightning/src/ln/chan_utils.rs
+++ b/lightning/src/ln/chan_utils.rs
@@ -525,12 +525,18 @@ pub fn make_funding_redeemscript(broadcaster: &PublicKey, countersignatory: &Pub
 	}.push_opcode(opcodes::all::OP_PUSHNUM_2).push_opcode(opcodes::all::OP_CHECKMULTISIG).into_script()
 }
 
-/// panics if htlc.transaction_output_index.is_none()!
-pub fn build_htlc_transaction(prev_hash: &Txid, feerate_per_kw: u32, contest_delay: u16, htlc: &HTLCOutputInCommitment, broadcaster_delayed_payment_key: &PublicKey, revocation_key: &PublicKey) -> Transaction {
+/// Builds an unsigned HTLC-Success or HTLC-Timeout transaction from the given channel and HTLC
+/// parameters. This is used by [`TrustedCommitmentTransaction::get_htlc_sigs`] to fetch the
+/// transaction which needs signing, and can be used to construct an HTLC transaction which is
+/// broadcastable given a counterparty HTLC signature.
+///
+/// Panics if htlc.transaction_output_index.is_none() (as such HTLCs do not appear in the
+/// commitment transaction).
+pub fn build_htlc_transaction(commitment_txid: &Txid, feerate_per_kw: u32, contest_delay: u16, htlc: &HTLCOutputInCommitment, broadcaster_delayed_payment_key: &PublicKey, revocation_key: &PublicKey) -> Transaction {
 	let mut txins: Vec<TxIn> = Vec::new();
 	txins.push(TxIn {
 		previous_output: OutPoint {
-			txid: prev_hash.clone(),
+			txid: commitment_txid.clone(),
 			vout: htlc.transaction_output_index.expect("Can't build an HTLC transaction for a dust output"),
 		},
 		script_sig: Script::new(),
@@ -1177,7 +1183,12 @@ impl<'a> TrustedCommitmentTransaction<'a> {
 	}
 }
 
-/// Get the transaction number obscure factor
+/// Commitment transaction numbers which appear in the transactions themselves are XOR'd with a
+/// shared secret first. This prevents on-chain observers from discovering how many commitment
+/// transactions occurred in a channel before it was closed.
+///
+/// This function gets the shared secret from relevant channel public keys and can be used to
+/// "decrypt" the commitment transaction number given a commitment transaction on-chain.
 pub fn get_commitment_transaction_number_obscure_factor(
 	broadcaster_payment_basepoint: &PublicKey,
 	countersignatory_payment_basepoint: &PublicKey,