Introduce constants for derivation path indices

This commit replaces magic numbers with descriptive constant names for the indices used in key derivation paths
within the `new` function.

- Added constants:
  - `NODE_SECRET_INDEX`
  - `DESTINATION_SCRIPT_INDEX`
  - `SHUTDOWN_PUBKEY_INDEX`
  - `CHANNEL_MASTER_KEY_INDEX`
  - `INBOUND_PAYMENT_KEY_INDEX`
  - `PEER_STORAGE_KEY_INDEX`

Author: adi2011

lightning/src/sign/mod.rs

@@ -1818,18 +1818,30 @@ impl KeysManager {
 	///
 	/// [`ChannelMonitor`]: crate::chain::channelmonitor::ChannelMonitor
 	pub fn new(seed: &[u8; 32], starting_time_secs: u64, starting_time_nanos: u32) -> Self {
+		// Constants for key derivation path indices used in this function.
+		const NODE_SECRET_INDEX: u32 = 0;
+		const DESTINATION_SCRIPT_INDEX: u32 = 1;
+		const SHUTDOWN_PUBKEY_INDEX: u32 = 2;
+		const CHANNEL_MASTER_KEY_INDEX: u32 = 3;
+		const INBOUND_PAYMENT_KEY_INDEX: u32 = 5;
+		const PEER_STORAGE_KEY_INDEX: u32 = 6;
+
 		let secp_ctx = Secp256k1::new();
 		// Note that when we aren't serializing the key, network doesn't matter
 		match Xpriv::new_master(Network::Testnet, seed) {
 			Ok(master_key) => {
 				let node_secret = master_key
-					.derive_priv(&secp_ctx, &ChildNumber::from_hardened_idx(0).unwrap())
+					.derive_priv(
+						&secp_ctx,
+						&ChildNumber::from_hardened_idx(NODE_SECRET_INDEX).unwrap(),
+					)
 					.expect("Your RNG is busted")
 					.private_key;
 				let node_id = PublicKey::from_secret_key(&secp_ctx, &node_secret);
-				let destination_script = match master_key
-					.derive_priv(&secp_ctx, &ChildNumber::from_hardened_idx(1).unwrap())
-				{
+				let destination_script = match master_key.derive_priv(
+					&secp_ctx,
+					&ChildNumber::from_hardened_idx(DESTINATION_SCRIPT_INDEX).unwrap(),
+				) {
 					Ok(destination_key) => {
 						let wpubkey_hash = WPubkeyHash::hash(
 							&Xpub::from_priv(&secp_ctx, &destination_key).to_pub().to_bytes(),
@@ -1841,23 +1853,33 @@ impl KeysManager {
 					},
 					Err(_) => panic!("Your RNG is busted"),
 				};
-				let shutdown_pubkey = match master_key
-					.derive_priv(&secp_ctx, &ChildNumber::from_hardened_idx(2).unwrap())
-				{
+				let shutdown_pubkey = match master_key.derive_priv(
+					&secp_ctx,
+					&ChildNumber::from_hardened_idx(SHUTDOWN_PUBKEY_INDEX).unwrap(),
+				) {
 					Ok(shutdown_key) => Xpub::from_priv(&secp_ctx, &shutdown_key).public_key,
 					Err(_) => panic!("Your RNG is busted"),
 				};
 				let channel_master_key = master_key
-					.derive_priv(&secp_ctx, &ChildNumber::from_hardened_idx(3).unwrap())
+					.derive_priv(
+						&secp_ctx,
+						&ChildNumber::from_hardened_idx(CHANNEL_MASTER_KEY_INDEX).unwrap(),
+					)
 					.expect("Your RNG is busted");
 				let inbound_payment_key: SecretKey = master_key
-					.derive_priv(&secp_ctx, &ChildNumber::from_hardened_idx(5).unwrap())
+					.derive_priv(
+						&secp_ctx,
+						&ChildNumber::from_hardened_idx(INBOUND_PAYMENT_KEY_INDEX).unwrap(),
+					)
 					.expect("Your RNG is busted")
 					.private_key;
 				let mut inbound_pmt_key_bytes = [0; 32];
 				inbound_pmt_key_bytes.copy_from_slice(&inbound_payment_key[..]);
-				let peer_storage_key: SecretKey = master_key
-					.derive_priv(&secp_ctx, &ChildNumber::from_hardened_idx(6).unwrap())
+				let peer_storage_key = master_key
+					.derive_priv(
+						&secp_ctx,
+						&ChildNumber::from_hardened_idx(PEER_STORAGE_KEY_INDEX).unwrap(),
+					)
 					.expect("Your RNG is busted")
 					.private_key;