Enforce that revocation can only occur after we validated a new commitment

Author: devrandom

lightning/src/chain/keysinterface.rs

@@ -212,6 +212,11 @@ pub trait BaseSign {
 	/// Note that the commitment number starts at (1 << 48) - 1 and counts backwards.
 	// TODO: return a Result so we can signal a validation error
 	fn release_commitment_secret(&self, idx: u64) -> [u8; 32];
+	/// Validate the counterparty's signatures on the holder commitment transaction and HTLCs.
+	///
+	/// This is required in order for the signer to make sure that releasing a commitment
+	/// secret won't leave us without a broadcastable holder transaction.
+	fn validate_holder_commitment(&self, holder_tx: &HolderCommitmentTransaction);
 	/// Gets the holder's channel public keys and basepoints
 	fn pubkeys(&self) -> &ChannelPublicKeys;
 	/// Gets an arbitrary identifier describing the set of keys which are provided back to you in
@@ -558,6 +563,9 @@ impl BaseSign for InMemorySigner {
 		chan_utils::build_commitment_secret(&self.commitment_seed, idx)
 	}
 
+	fn validate_holder_commitment(&self, _holder_tx: &HolderCommitmentTransaction) {
+	}
+
 	fn pubkeys(&self) -> &ChannelPublicKeys { &self.holder_channel_pubkeys }
 	fn channel_keys_id(&self) -> [u8; 32] { self.channel_keys_id }
 

lightning/src/ln/channel.rs

@@ -1791,6 +1791,8 @@ impl<Signer: Sign> Channel<Signer> {
 			self.counterparty_funding_pubkey()
 		);
 
+		self.holder_signer.validate_holder_commitment(&holder_commitment_tx);
+
 		// Now that we're past error-generating stuff, update our local state:
 
 		let funding_redeemscript = self.get_funding_redeemscript();
@@ -1865,6 +1867,7 @@ impl<Signer: Sign> Channel<Signer> {
 			self.counterparty_funding_pubkey()
 		);
 
+		self.holder_signer.validate_holder_commitment(&holder_commitment_tx);
 
 		let funding_redeemscript = self.get_funding_redeemscript();
 		let funding_txo = self.get_funding_txo().unwrap();
@@ -2502,6 +2505,7 @@ impl<Signer: Sign> Channel<Signer> {
 		);
 
 		let next_per_commitment_point = self.holder_signer.get_per_commitment_point(self.cur_holder_commitment_transaction_number - 1, &self.secp_ctx);
+		self.holder_signer.validate_holder_commitment(&holder_commitment_tx);
 		let per_commitment_secret = self.holder_signer.release_commitment_secret(self.cur_holder_commitment_transaction_number + 1);
 
 		// Update state now that we've passed all the can-fail calls...

lightning/src/ln/functional_tests.rs

@@ -1301,6 +1301,9 @@ fn test_fee_spike_violation_fails_htlc() {
 		let chan_lock = nodes[0].node.channel_state.lock().unwrap();
 		let local_chan = chan_lock.by_id.get(&chan.2).unwrap();
 		let chan_signer = local_chan.get_signer();
+		// Make the signer believe we validated another commitment, so we can release the secret
+		chan_signer.get_enforcement_state().last_holder_commitment -= 1;
+
 		let pubkeys = chan_signer.pubkeys();
 		(pubkeys.revocation_basepoint, pubkeys.htlc_basepoint,
 		 chan_signer.release_commitment_secret(INITIAL_COMMITMENT_NUMBER),
@@ -7981,7 +7984,7 @@ fn test_counterparty_raa_skip_no_crash() {
 	// commitment transaction, we would have happily carried on and provided them the next
 	// commitment transaction based on one RAA forward. This would probably eventually have led to
 	// channel closure, but it would not have resulted in funds loss. Still, our
-	// EnforcingSigner would have paniced as it doesn't like jumps into the future. Here, we
+	// EnforcingSigner would have panicked as it doesn't like jumps into the future. Here, we
 	// check simply that the channel is closed in response to such an RAA, but don't check whether
 	// we decide to punish our counterparty for revoking their funds (as we don't currently
 	// implement that).
@@ -7992,11 +7995,19 @@ fn test_counterparty_raa_skip_no_crash() {
 	let channel_id = create_announced_chan_between_nodes(&nodes, 0, 1, InitFeatures::known(), InitFeatures::known()).2;
 
 	let mut guard = nodes[0].node.channel_state.lock().unwrap();
-	let keys = &guard.by_id.get_mut(&channel_id).unwrap().get_signer();
+	let keys = guard.by_id.get_mut(&channel_id).unwrap().get_signer();
+
 	const INITIAL_COMMITMENT_NUMBER: u64 = (1 << 48) - 1;
+
+	// Make signer believe we got a counterparty signature, so that it allows the revocation
+	keys.get_enforcement_state().last_holder_commitment -= 1;
 	let per_commitment_secret = keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER);
+
 	// Must revoke without gaps
+	keys.get_enforcement_state().last_holder_commitment -= 1;
 	keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER - 1);
+
+	keys.get_enforcement_state().last_holder_commitment -= 1;
 	let next_per_commitment_point = PublicKey::from_secret_key(&Secp256k1::new(),
 		&SecretKey::from_slice(&keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER - 2)).unwrap());
 

lightning/src/ln/msgs.rs

@@ -194,7 +194,7 @@ pub struct FundingCreated {
 	pub funding_txid: Txid,
 	/// The specific output index funding this channel
 	pub funding_output_index: u16,
-	/// The signature of the channel initiator (funder) on the funding transaction
+	/// The signature of the channel initiator (funder) on the initial commitment transaction
 	pub signature: Signature,
 }
 
@@ -203,7 +203,7 @@ pub struct FundingCreated {
 pub struct FundingSigned {
 	/// The channel ID
 	pub channel_id: [u8; 32],
-	/// The signature of the channel acceptor (fundee) on the funding transaction
+	/// The signature of the channel acceptor (fundee) on the initial commitment transaction
 	pub signature: Signature,
 }
 

lightning/src/util/enforcing_trait_impls.rs

@@ -15,6 +15,7 @@ use io;
 use prelude::*;
 use core::cmp;
 use sync::{Mutex, Arc};
+#[cfg(test)] use sync::MutexGuard;
 
 use bitcoin::blockdata::transaction::{Transaction, SigHashType};
 use bitcoin::util::bip143;
@@ -35,12 +36,17 @@ pub const INITIAL_REVOKED_COMMITMENT_NUMBER: u64 = 1 << 48;
 /// - When signing, the holder transaction has not been revoked
 /// - When revoking, the holder transaction has not been signed
 /// - The holder commitment number is monotonic and without gaps
+/// - The revoked holder commitment number is monotonic and without gaps
+/// - There is at least one unrevoked holder transaction at all times
 /// - The counterparty commitment number is monotonic and without gaps
 /// - The pre-derived keys and pre-built transaction in CommitmentTransaction were correctly built
 ///
 /// Eventually we will probably want to expose a variant of this which would essentially
 /// be what you'd want to run on a hardware wallet.
 ///
+/// Note that counterparty signatures on the holder transaction are not checked, but it should
+/// be in a complete implementation.
+///
 /// Note that before we do so we should ensure its serialization format has backwards- and
 /// forwards-compatibility prefix/suffixes!
 #[derive(Clone)]
@@ -74,6 +80,11 @@ impl EnforcingSigner {
 			disable_revocation_policy_check
 		}
 	}
+
+	#[cfg(test)]
+	pub fn get_enforcement_state(&self) -> MutexGuard<EnforcementState> {
+		self.state.lock().unwrap()
+	}
 }
 
 impl BaseSign for EnforcingSigner {
@@ -84,12 +95,20 @@ impl BaseSign for EnforcingSigner {
 	fn release_commitment_secret(&self, idx: u64) -> [u8; 32] {
 		{
 			let mut state = self.state.lock().unwrap();
-			assert!(idx == state.revoked_commitment || idx == state.revoked_commitment - 1, "can only revoke the current or next unrevoked commitment - trying {}, revoked {}", idx, state.revoked_commitment);
-			state.revoked_commitment = idx;
+			assert!(idx == state.last_holder_revoked_commitment || idx == state.last_holder_revoked_commitment - 1, "can only revoke the current or next unrevoked commitment - trying {}, last revoked {}", idx, state.last_holder_revoked_commitment);
+			assert!(idx > state.last_holder_commitment, "cannot revoke the last holder commitment - attempted to revoke {} last commitment {}", idx, state.last_holder_commitment);
+			state.last_holder_revoked_commitment = idx;
 		}
 		self.inner.release_commitment_secret(idx)
 	}
 
+	fn validate_holder_commitment(&self, holder_tx: &HolderCommitmentTransaction) {
+		let mut state = self.state.lock().unwrap();
+		let idx = holder_tx.commitment_number();
+		assert!(idx == state.last_holder_commitment || idx == state.last_holder_commitment - 1, "expecting to validate the current or next holder commitment - trying {}, current {}", idx, state.last_holder_commitment);
+		state.last_holder_commitment = idx;
+	}
+
 	fn pubkeys(&self) -> &ChannelPublicKeys { self.inner.pubkeys() }
 	fn channel_keys_id(&self) -> [u8; 32] { self.inner.channel_keys_id() }
 
@@ -116,10 +135,10 @@ impl BaseSign for EnforcingSigner {
 
 		let state = self.state.lock().unwrap();
 		let commitment_number = trusted_tx.commitment_number();
-		if state.revoked_commitment - 1 != commitment_number && state.revoked_commitment - 2 != commitment_number {
+		if state.last_holder_revoked_commitment - 1 != commitment_number && state.last_holder_revoked_commitment - 2 != commitment_number {
 			if !self.disable_revocation_policy_check {
 				panic!("can only sign the next two unrevoked commitment numbers, revoked={} vs requested={} for {}",
-				       state.revoked_commitment, commitment_number, self.inner.commitment_seed[0])
+				       state.last_holder_revoked_commitment, commitment_number, self.inner.commitment_seed[0])
 			}
 		}
 
@@ -212,16 +231,18 @@ pub struct EnforcementState {
 	/// The last counterparty commitment number we signed, backwards counting
 	pub last_counterparty_commitment: u64,
 	/// The last holder commitment number we revoked, backwards counting
-	pub revoked_commitment: u64,
-
+	pub last_holder_revoked_commitment: u64,
+	/// The last validated holder commitment number, backwards counting
+	pub last_holder_commitment: u64,
 }
 
 impl EnforcementState {
 	/// Enforcement state for a new channel
 	pub fn new() -> Self {
 		EnforcementState {
 			last_counterparty_commitment: INITIAL_REVOKED_COMMITMENT_NUMBER,
-			revoked_commitment: INITIAL_REVOKED_COMMITMENT_NUMBER,
+			last_holder_revoked_commitment: INITIAL_REVOKED_COMMITMENT_NUMBER,
+			last_holder_commitment: INITIAL_REVOKED_COMMITMENT_NUMBER,
 		}
 	}
 }