Allow KeysInterface to re-derive previously generated signers

With this change, we'll be able to re-derive signers rather than persist
them. There's no point in storing data that can be easily re-derived.

Author: wpaulino

lightning/src/chain/keysinterface.rs

@@ -450,6 +450,12 @@ pub trait KeysInterface {
 	///
 	/// This method must return a different value each time it is called.
 	fn get_channel_signer(&self, inbound: bool, channel_value_satoshis: u64) -> Self::Signer;
+	/// Re-derives a `Signer` previously generated by [`get_channel_signer`]. A `Signer`'s
+	/// `channel_keys_id` can be obtained through it's trait method [`BaseSign::channel_keys_id`].
+	///
+	/// [`get_channel_signer`]: KeysInterface::get_channel_signer
+	/// [`BaseSign::channel_keys_id`]: BaseSign::channel_keys_id
+	fn derive_channel_signer(&self, channel_value_satoshis: u64, channel_keys_id: [u8; 32]) -> Self::Signer;
 	/// Gets a unique, cryptographically-secure, random 32 byte value. This is used for encrypting
 	/// onion packets and for temporary channel IDs. There is no requirement that these be
 	/// persisted anywhere, though they must be unique across restarts.
@@ -1217,6 +1223,10 @@ impl KeysInterface for KeysManager {
 		self.derive_channel_keys(channel_value_satoshis, &id)
 	}
 
+	fn derive_channel_signer(&self, channel_value_satoshis: u64, channel_keys_id: [u8; 32]) -> Self::Signer {
+		self.derive_channel_keys(channel_value_satoshis, &channel_keys_id)
+    }
+
 	fn get_secure_random_bytes(&self) -> [u8; 32] {
 		let mut sha = self.rand_bytes_unique_start.clone();
 
@@ -1311,6 +1321,10 @@ impl KeysInterface for PhantomKeysManager {
 		self.inner.get_channel_signer(inbound, channel_value_satoshis)
 	}
 
+	fn derive_channel_signer(&self, channel_value_satoshis: u64, channel_keys_id: [u8; 32]) -> Self::Signer {
+		self.inner.derive_channel_keys(channel_value_satoshis, &channel_keys_id)
+    }
+
 	fn get_secure_random_bytes(&self) -> [u8; 32] {
 		self.inner.get_secure_random_bytes()
 	}

lightning/src/ln/channel.rs

@@ -6835,6 +6835,9 @@ mod tests {
 		fn get_channel_signer(&self, _inbound: bool, _channel_value_satoshis: u64) -> InMemorySigner {
 			self.signer.clone()
 		}
+		fn derive_channel_signer(&self, _channel_value_satoshis: u64, _channel_keys_id: [u8; 32]) -> Self::Signer {
+			self.signer.clone()
+		}
 		fn get_secure_random_bytes(&self) -> [u8; 32] { [0; 32] }
 		fn read_chan_signer(&self, _data: &[u8]) -> Result<Self::Signer, DecodeError> { panic!(); }
 		fn sign_invoice(&self, _hrp_bytes: &[u8], _invoice_data: &[u5], _recipient: Recipient) -> Result<RecoverableSignature, ()> { panic!(); }

lightning/src/util/test_utils.rs

@@ -81,6 +81,7 @@ impl keysinterface::KeysInterface for OnlyReadsKeysInterface {
 	fn get_destination_script(&self) -> Script { unreachable!(); }
 	fn get_shutdown_scriptpubkey(&self) -> ShutdownScript { unreachable!(); }
 	fn get_channel_signer(&self, _inbound: bool, _channel_value_satoshis: u64) -> EnforcingSigner { unreachable!(); }
+	fn derive_channel_signer(&self, _channel_value_satoshis: u64, _channel_keys_id: [u8; 32]) -> Self::Signer { unreachable!(); }
 	fn get_secure_random_bytes(&self) -> [u8; 32] { [0; 32] }
 
 	fn read_chan_signer(&self, mut reader: &[u8]) -> Result<Self::Signer, msgs::DecodeError> {
@@ -635,6 +636,12 @@ impl keysinterface::KeysInterface for TestKeysInterface {
 		EnforcingSigner::new_with_revoked(keys, state, self.disable_revocation_policy_check)
 	}
 
+	fn derive_channel_signer(&self, channel_value_satoshis: u64, channel_keys_id: [u8; 32]) -> EnforcingSigner {
+		let keys = self.backing.derive_channel_keys(channel_value_satoshis, &channel_keys_id);
+		let state = self.make_enforcement_state_cell(keys.commitment_seed);
+		EnforcingSigner::new_with_revoked(keys, state, self.disable_revocation_policy_check)
+	}
+
 	fn get_secure_random_bytes(&self) -> [u8; 32] {
 		let override_random_bytes = self.override_random_bytes.lock().unwrap();
 		if let Some(bytes) = &*override_random_bytes {