Add a method to get session secret for onion packet to KeysInterface

Author: yuntai

src/chain/keysinterface.rs

@@ -79,6 +79,8 @@ pub trait KeysInterface: Send + Sync {
 	/// Get a new set of ChannelKeys for per-channel secrets. These MUST be unique even if you
 	/// restarted with some stale data!
 	fn get_channel_keys(&self, inbound: bool) -> ChannelKeys;
+	/// Get a secret for construting an onion packet
+	fn get_session_key(&self) -> SecretKey;
 }
 
 /// Set of lightning keys needed to operate a channel as described in BOLT 3
@@ -158,6 +160,8 @@ pub struct KeysManager {
 	shutdown_pubkey: PublicKey,
 	channel_master_key: ExtendedPrivKey,
 	channel_child_index: AtomicUsize,
+	session_master_key: ExtendedPrivKey,
+	session_child_index: AtomicUsize,
 
 	logger: Arc<Logger>,
 }
@@ -184,13 +188,16 @@ impl KeysManager {
 					Err(_) => panic!("Your RNG is busted"),
 				};
 				let channel_master_key = master_key.ckd_priv(&secp_ctx, ChildNumber::from_hardened_idx(3)).expect("Your RNG is busted");
+				let session_master_key = master_key.ckd_priv(&secp_ctx, ChildNumber::from_hardened_idx(4)).expect("Your RNG is busted");
 				KeysManager {
 					secp_ctx,
 					node_secret,
 					destination_script,
 					shutdown_pubkey,
 					channel_master_key,
 					channel_child_index: AtomicUsize::new(0),
+					session_master_key,
+					session_child_index: AtomicUsize::new(0),
 
 					logger,
 				}
@@ -235,4 +242,9 @@ impl KeysInterface for KeysManager {
 		sha.result(&mut seed);
 		ChannelKeys::new_from_seed(&seed)
 	}
+
+	fn get_session_key(&self) -> SecretKey {
+		let child_ix = self.session_child_index.fetch_add(1, Ordering::AcqRel);
+		self.session_master_key.ckd_priv(&self.secp_ctx, ChildNumber::from_hardened_idx(child_ix as u32)).expect("Your RNG is busted").secret_key
+	}
 }

src/ln/channel.rs

@@ -3902,6 +3902,7 @@ mod tests {
 		}
 
 		fn get_channel_keys(&self, _inbound: bool) -> ChannelKeys { self.chan_keys.clone() }
+		fn get_session_key(&self) -> SecretKey { panic!(); }
 	}
 
 	#[test]

src/ln/channelmanager.rs

@@ -1199,11 +1199,7 @@ impl ChannelManager {
 			}
 		}
 
-		let session_priv = SecretKey::from_slice(&self.secp_ctx, &{
-			let mut session_key = [0; 32];
-			rng::fill_bytes(&mut session_key);
-			session_key
-		}).expect("RNG is bad!");
+		let session_priv = self.keys_manager.get_session_key();
 
 		let cur_height = self.latest_block_height.load(Ordering::Acquire) as u32 + 1;