Re-derive signers upon deserializing Channel

To do so, we introduce a new serialization version that doesn't store a
channel's signer, and instead stores its signer's `channel_keys_id`.
This is a unique identifier that can be provided to our `KeysInterface`
to re-derive all private key material for said channel.

We choose to not upgrade the minimum compatible serialization version
until a later time, which will also remove any signer serialization
logic on implementations of `KeysInterface` and `Sign`.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -34,7 +34,7 @@ use crate::chain::BestBlock;
 use crate::chain::chaininterface::{FeeEstimator, ConfirmationTarget, LowerBoundedFeeEstimator};
 use crate::chain::channelmonitor::{ChannelMonitor, ChannelMonitorUpdate, ChannelMonitorUpdateStep, LATENCY_GRACE_PERIOD_BLOCKS};
 use crate::chain::transaction::{OutPoint, TransactionData};
-use crate::chain::keysinterface::{Sign, KeysInterface};
+use crate::chain::keysinterface::{Sign, KeysInterface, BaseSign};
 use crate::util::events::ClosureReason;
 use crate::util::ser::{Readable, ReadableArgs, Writeable, Writer, VecWriter};
 use crate::util::logger::Logger;
@@ -737,6 +737,10 @@ pub(super) struct Channel<Signer: Sign> {
 
 	// We track whether we already emitted a `ChannelReady` event.
 	channel_ready_event_emitted: bool,
+
+	/// The unique identifier used to re-derive the private key material for the channel through
+	/// [`KeysInterface::derive_channel_signer`].
+	_channel_keys_id: [u8; 32],
 }
 
 #[cfg(any(test, fuzzing))]
@@ -1072,6 +1076,7 @@ impl<Signer: Sign> Channel<Signer> {
 			historical_inbound_htlc_fulfills: HashSet::new(),
 
 			channel_type: Self::get_initial_channel_type(&config),
+			_channel_keys_id: channel_keys_id,
 		})
 	}
 
@@ -1419,6 +1424,7 @@ impl<Signer: Sign> Channel<Signer> {
 			historical_inbound_htlc_fulfills: HashSet::new(),
 
 			channel_type,
+			_channel_keys_id: channel_keys_id,
 		};
 
 		Ok(chan)
@@ -6246,6 +6252,10 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 		// we write the high bytes as an option here.
 		let user_id_high_opt = Some((self.user_id >> 64) as u64);
 
+		// `channel_keys_id` is serialized as an option to remain backwards compatible until we bump
+		// `SERIALIZATION_VERSION` to 3.
+		let channel_keys_id = Some(self._channel_keys_id);
+
 		write_tlv_fields!(writer, {
 			(0, self.announcement_sigs, option),
 			// minimum_depth and counterparty_selected_channel_reserve_satoshis used to have a
@@ -6270,6 +6280,7 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 			(21, self.outbound_scid_alias, required),
 			(23, channel_ready_event_emitted, option),
 			(25, user_id_high_opt, option),
+			(27, channel_keys_id, option),
 		});
 
 		Ok(())
@@ -6306,6 +6317,8 @@ impl<'a, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<<K::Target as KeysInte
 
 		let latest_monitor_update_id = Readable::read(reader)?;
 
+		// Read the serialize signer bytes. We'll choose to deserialize them or not based on whether
+		// the `channel_keys_id` TLV is present below.
 		let keys_len: u32 = Readable::read(reader)?;
 		let mut keys_data = Vec::with_capacity(cmp::min(keys_len as usize, MAX_ALLOC_SIZE));
 		while keys_data.len() != keys_len as usize {
@@ -6315,7 +6328,6 @@ impl<'a, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<<K::Target as KeysInte
 			reader.read_exact(read_slice)?;
 			keys_data.extend_from_slice(read_slice);
 		}
-		let holder_signer = keys_source.read_chan_signer(&keys_data)?;
 
 		// Read the old serialization for shutdown_pubkey, preferring the TLV field later if set.
 		let mut shutdown_scriptpubkey = match <PublicKey as Readable>::read(reader) {
@@ -6533,6 +6545,7 @@ impl<'a, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<<K::Target as KeysInte
 		let mut channel_ready_event_emitted = None;
 
 		let mut user_id_high_opt: Option<u64> = None;
+		let mut channel_keys_id: Option<[u8; 32]> = None;
 
 		read_tlv_fields!(reader, {
 			(0, announcement_sigs, option),
@@ -6552,8 +6565,23 @@ impl<'a, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<<K::Target as KeysInte
 			(21, outbound_scid_alias, option),
 			(23, channel_ready_event_emitted, option),
 			(25, user_id_high_opt, option),
+			(27, channel_keys_id, option),
 		});
 
+		let (channel_keys_id, holder_signer) = if let Some(channel_keys_id) = channel_keys_id {
+			let mut holder_signer = keys_source.derive_channel_signer(channel_value_satoshis, channel_keys_id);
+			// If we've gotten to the funding stage of the channel, populate the signer with its
+			// required channel parameters.
+			let non_shutdown_state = channel_state & (!MULTI_STATE_FLAGS);
+			if non_shutdown_state >= (ChannelState::FundingCreated as u32) {
+				holder_signer.ready_channel(&channel_parameters);
+			}
+			(channel_keys_id, holder_signer)
+		} else {
+			let holder_signer = keys_source.read_chan_signer(&keys_data)?;
+			(holder_signer.channel_keys_id(), holder_signer)
+		};
+
 		if let Some(preimages) = preimages_opt {
 			let mut iter = preimages.into_iter();
 			for htlc in pending_outbound_htlcs.iter_mut() {
@@ -6703,6 +6731,7 @@ impl<'a, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<<K::Target as KeysInte
 			historical_inbound_htlc_fulfills,
 
 			channel_type: channel_type.unwrap(),
+			_channel_keys_id: channel_keys_id,
 		})
 	}
 }