ChannelKeys - separate commitment revocation from getting the per-commitment point

The commitment secret is sensitive - it can be used by an attacker to
steal funds if the node also signs the same transaction. Therefore,
only release the secret from ChannelKeys when we are revoking a
transaction.

diff --git a/lightning/src/chain/keysinterface.rs b/lightning/src/chain/keysinterface.rs
index 9e5d362bb0af65897db621dd1d65883d976398bc..55bf7c65d62b71e16c5ce82a15f87db5f969e0f1 100644 (file)
--- a/lightning/src/chain/keysinterface.rs
+++ b/lightning/src/chain/keysinterface.rs
@@ -195,9 +195,20 @@ impl Readable for SpendableOutputDescriptor {
 // TODO: We should remove Clone by instead requesting a new ChannelKeys copy when we create
 // ChannelMonitors instead of expecting to clone the one out of the Channel into the monitors.
 pub trait ChannelKeys : Send+Clone {
-       /// Gets the commitment seed for a specific commitment number
-       /// Note that the commitment number starts at (1 << 48) - 1 and counts backwards
-       fn commitment_secret(&self, idx: u64) -> [u8; 32];
+       /// Gets the per-commitment point for a specific commitment number
+       ///
+       /// Note that the commitment number starts at (1 << 48) - 1 and counts backwards.
+       fn get_per_commitment_point<T: secp256k1::Signing + secp256k1::Verification>(&self, idx: u64, secp_ctx: &Secp256k1<T>) -> PublicKey;
+       /// Gets the commitment secret for a specific commitment number as part of the revocation process
+       ///
+       /// An external signer implementation should error here if the commitment was already signed
+       /// and should refuse to sign it in the future.
+       ///
+       /// May be called more than once for the same index.
+       ///
+       /// 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];
        /// Gets the local channel public keys and basepoints
        fn pubkeys(&self) -> &ChannelPublicKeys;
        /// Gets arbitrary identifiers describing the set of keys which are provided back to you in
@@ -217,6 +228,7 @@ pub trait ChannelKeys : Send+Clone {
        /// Create a signature for a local commitment transaction. This will only ever be called with
        /// the same local_commitment_tx (or a copy thereof), though there are currently no guarantees
        /// that it will not be called multiple times.
+       /// An external signer implementation should check that the commitment has not been revoked.
        //
        // TODO: Document the things someone using this interface should enforce before signing.
        // TODO: Add more input vars to enable better checking (preferably removing commitment_tx and
@@ -405,7 +417,12 @@ impl InMemoryChannelKeys {
 }
 
 impl ChannelKeys for InMemoryChannelKeys {
-       fn commitment_secret(&self, idx: u64) -> [u8; 32] {
+       fn get_per_commitment_point<T: secp256k1::Signing + secp256k1::Verification>(&self, idx: u64, secp_ctx: &Secp256k1<T>) -> PublicKey {
+               let commitment_secret = SecretKey::from_slice(&chan_utils::build_commitment_secret(&self.commitment_seed, idx)).unwrap();
+               PublicKey::from_secret_key(secp_ctx, &commitment_secret)
+       }
+
+       fn release_commitment_secret(&self, idx: u64) -> [u8; 32] {
                chan_utils::build_commitment_secret(&self.commitment_seed, idx)
        }
 

diff --git a/lightning/src/ln/chan_utils.rs b/lightning/src/ln/chan_utils.rs
index d780c33c1462b28f322356e28a27788d1fa99c10..ba175733abeb632065d0cf16fe44ca13b6560b66 100644 (file)
--- a/lightning/src/ln/chan_utils.rs
+++ b/lightning/src/ln/chan_utils.rs
@@ -497,8 +497,8 @@ pub fn build_htlc_transaction(prev_hash: &Txid, feerate_per_kw: u32, to_self_del
 
 #[derive(Clone)]
 /// We use this to track local commitment transactions and put off signing them until we are ready
-/// to broadcast. Eventually this will require a signer which is possibly external, but for now we
-/// just pass in the SecretKeys required.
+/// to broadcast. This class can be used inside a signer implementation to generate a signature
+/// given the relevant secret key.
 pub struct LocalCommitmentTransaction {
        // TODO: We should migrate away from providing the transaction, instead providing enough to
        // allow the ChannelKeys to construct it from scratch. Luckily we already have HTLC data here,

diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index a0879d4e7a554895a5636d57a4e6485bcc1bb086..fc3b4fd81746f7dc878a0bf138735ee805026c01 100644 (file)
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -785,13 +785,6 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                Ok(chan)
        }
 
-       // Utilities to derive keys:
-
-       fn build_local_commitment_secret(&self, idx: u64) -> SecretKey {
-               let res = self.local_keys.commitment_secret(idx);
-               SecretKey::from_slice(&res).unwrap()
-       }
-
        // Utilities to build transactions:
 
        fn get_commitment_transaction_number_obscure_factor(&self) -> u64 {
@@ -1123,7 +1116,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
        /// The result is a transaction which we can revoke ownership of (ie a "local" transaction)
        /// TODO Some magic rust shit to compile-time check this?
        fn build_local_transaction_keys(&self, commitment_number: u64) -> Result<TxCreationKeys, ChannelError> {
-               let per_commitment_point = PublicKey::from_secret_key(&self.secp_ctx, &self.build_local_commitment_secret(commitment_number));
+               let per_commitment_point = self.local_keys.get_per_commitment_point(commitment_number, &self.secp_ctx);
                let delayed_payment_base = &self.local_keys.pubkeys().delayed_payment_basepoint;
                let htlc_basepoint = &self.local_keys.pubkeys().htlc_basepoint;
                let their_pubkeys = self.their_pubkeys.as_ref().unwrap();
@@ -2028,8 +2021,8 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                        }
                }
 
-               let next_per_commitment_point = PublicKey::from_secret_key(&self.secp_ctx, &self.build_local_commitment_secret(self.cur_local_commitment_transaction_number - 1));
-               let per_commitment_secret = self.local_keys.commitment_secret(self.cur_local_commitment_transaction_number + 1);
+               let next_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number - 1, &self.secp_ctx);
+               let per_commitment_secret = self.local_keys.release_commitment_secret(self.cur_local_commitment_transaction_number + 1);
 
                // Update state now that we've passed all the can-fail calls...
                let mut need_our_commitment = false;
@@ -2614,8 +2607,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                let funding_locked = if self.monitor_pending_funding_locked {
                        assert!(!self.channel_outbound, "Funding transaction broadcast without FundingBroadcastSafe!");
                        self.monitor_pending_funding_locked = false;
-                       let next_per_commitment_secret = self.build_local_commitment_secret(self.cur_local_commitment_transaction_number);
-                       let next_per_commitment_point = PublicKey::from_secret_key(&self.secp_ctx, &next_per_commitment_secret);
+                       let next_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number, &self.secp_ctx);
                        Some(msgs::FundingLocked {
                                channel_id: self.channel_id(),
                                next_per_commitment_point: next_per_commitment_point,
@@ -2667,8 +2659,8 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
        }
 
        fn get_last_revoke_and_ack(&self) -> msgs::RevokeAndACK {
-               let next_per_commitment_point = PublicKey::from_secret_key(&self.secp_ctx, &self.build_local_commitment_secret(self.cur_local_commitment_transaction_number));
-               let per_commitment_secret = self.local_keys.commitment_secret(self.cur_local_commitment_transaction_number + 2);
+               let next_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number, &self.secp_ctx);
+               let per_commitment_secret = self.local_keys.release_commitment_secret(self.cur_local_commitment_transaction_number + 2);
                msgs::RevokeAndACK {
                        channel_id: self.channel_id,
                        per_commitment_secret,
@@ -2751,7 +2743,10 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                if msg.next_remote_commitment_number > 0 {
                        match msg.data_loss_protect {
                                OptionalField::Present(ref data_loss) => {
-                                       if self.local_keys.commitment_secret(INITIAL_COMMITMENT_NUMBER - msg.next_remote_commitment_number + 1) != data_loss.your_last_per_commitment_secret {
+                                       let expected_point = self.local_keys.get_per_commitment_point(INITIAL_COMMITMENT_NUMBER - msg.next_remote_commitment_number + 1, &self.secp_ctx);
+                                       let given_secret = SecretKey::from_slice(&data_loss.your_last_per_commitment_secret)
+                                               .map_err(|_| ChannelError::Close("Peer sent a garbage channel_reestablish with unparseable secret key".to_owned()))?;
+                                       if expected_point != PublicKey::from_secret_key(&self.secp_ctx, &given_secret) {
                                                return Err(ChannelError::Close("Peer sent a garbage channel_reestablish with secret key not matching the commitment height provided".to_owned()));
                                        }
                                        if msg.next_remote_commitment_number > INITIAL_COMMITMENT_NUMBER - self.cur_local_commitment_transaction_number {
@@ -2787,8 +2782,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                        }
 
                        // We have OurFundingLocked set!
-                       let next_per_commitment_secret = self.build_local_commitment_secret(self.cur_local_commitment_transaction_number);
-                       let next_per_commitment_point = PublicKey::from_secret_key(&self.secp_ctx, &next_per_commitment_secret);
+                       let next_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number, &self.secp_ctx);
                        return Ok((Some(msgs::FundingLocked {
                                channel_id: self.channel_id(),
                                next_per_commitment_point: next_per_commitment_point,
@@ -2818,8 +2812,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
 
                let resend_funding_locked = if msg.next_local_commitment_number == 1 && INITIAL_COMMITMENT_NUMBER - self.cur_local_commitment_transaction_number == 1 {
                        // We should never have to worry about MonitorUpdateFailed resending FundingLocked
-                       let next_per_commitment_secret = self.build_local_commitment_secret(self.cur_local_commitment_transaction_number);
-                       let next_per_commitment_point = PublicKey::from_secret_key(&self.secp_ctx, &next_per_commitment_secret);
+                       let next_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number, &self.secp_ctx);
                        Some(msgs::FundingLocked {
                                channel_id: self.channel_id(),
                                next_per_commitment_point: next_per_commitment_point,
@@ -3405,8 +3398,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                                        //a protocol oversight, but I assume I'm just missing something.
                                        if need_commitment_update {
                                                if self.channel_state & (ChannelState::MonitorUpdateFailed as u32) == 0 {
-                                                       let next_per_commitment_secret = self.build_local_commitment_secret(self.cur_local_commitment_transaction_number);
-                                                       let next_per_commitment_point = PublicKey::from_secret_key(&self.secp_ctx, &next_per_commitment_secret);
+                                                       let next_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number, &self.secp_ctx);
                                                        return Ok((Some(msgs::FundingLocked {
                                                                channel_id: self.channel_id,
                                                                next_per_commitment_point: next_per_commitment_point,
@@ -3457,7 +3449,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                        panic!("Tried to send an open_channel for a channel that has already advanced");
                }
 
-               let local_commitment_secret = self.build_local_commitment_secret(self.cur_local_commitment_transaction_number);
+               let first_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number, &self.secp_ctx);
                let local_keys = self.local_keys.pubkeys();
 
                msgs::OpenChannel {
@@ -3477,7 +3469,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                        payment_point: local_keys.payment_point,
                        delayed_payment_basepoint: local_keys.delayed_payment_basepoint,
                        htlc_basepoint: local_keys.htlc_basepoint,
-                       first_per_commitment_point: PublicKey::from_secret_key(&self.secp_ctx, &local_commitment_secret),
+                       first_per_commitment_point,
                        channel_flags: if self.config.announced_channel {1} else {0},
                        shutdown_scriptpubkey: OptionalField::Present(if self.config.commit_upfront_shutdown_pubkey { self.get_closing_scriptpubkey() } else { Builder::new().into_script() })
                }
@@ -3494,7 +3486,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                        panic!("Tried to send an accept_channel for a channel that has already advanced");
                }
 
-               let local_commitment_secret = self.build_local_commitment_secret(self.cur_local_commitment_transaction_number);
+               let first_per_commitment_point = self.local_keys.get_per_commitment_point(self.cur_local_commitment_transaction_number, &self.secp_ctx);
                let local_keys = self.local_keys.pubkeys();
 
                msgs::AcceptChannel {
@@ -3511,7 +3503,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
                        payment_point: local_keys.payment_point,
                        delayed_payment_basepoint: local_keys.delayed_payment_basepoint,
                        htlc_basepoint: local_keys.htlc_basepoint,
-                       first_per_commitment_point: PublicKey::from_secret_key(&self.secp_ctx, &local_commitment_secret),
+                       first_per_commitment_point,
                        shutdown_scriptpubkey: OptionalField::Present(if self.config.commit_upfront_shutdown_pubkey { self.get_closing_scriptpubkey() } else { Builder::new().into_script() })
                }
        }

diff --git a/lightning/src/ln/functional_tests.rs b/lightning/src/ln/functional_tests.rs
index 4d2b155ce30b1ff4e1085e62df26e0debc1e6b72..ce9ec1d5853adc9851fc4475de83b7f312570877 100644 (file)
--- a/lightning/src/ln/functional_tests.rs
+++ b/lightning/src/ln/functional_tests.rs
@@ -1612,7 +1612,7 @@ fn test_fee_spike_violation_fails_htlc() {
                let chan_keys = local_chan.get_local_keys();
                let pubkeys = chan_keys.pubkeys();
                (pubkeys.revocation_basepoint, pubkeys.htlc_basepoint, pubkeys.payment_point,
-                chan_keys.commitment_secret(INITIAL_COMMITMENT_NUMBER), chan_keys.commitment_secret(INITIAL_COMMITMENT_NUMBER - 2))
+                chan_keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER), chan_keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER - 2))
        };
        let (remote_delayed_payment_basepoint, remote_htlc_basepoint, remote_payment_point, remote_secret1) = {
                let chan_lock = nodes[1].node.channel_state.lock().unwrap();
@@ -1620,7 +1620,7 @@ fn test_fee_spike_violation_fails_htlc() {
                let chan_keys = remote_chan.get_local_keys();
                let pubkeys = chan_keys.pubkeys();
                (pubkeys.delayed_payment_basepoint, pubkeys.htlc_basepoint, pubkeys.payment_point,
-                chan_keys.commitment_secret(INITIAL_COMMITMENT_NUMBER - 1))
+                chan_keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER - 1))
        };
 
        // Assemble the set of keys we can use for signatures for our commitment_signed message.
@@ -8132,8 +8132,8 @@ fn test_counterparty_raa_skip_no_crash() {
        let local_keys = &guard.by_id.get_mut(&channel_id).unwrap().local_keys;
        const INITIAL_COMMITMENT_NUMBER: u64 = (1 << 48) - 1;
        let next_per_commitment_point = PublicKey::from_secret_key(&Secp256k1::new(),
-               &SecretKey::from_slice(&local_keys.commitment_secret(INITIAL_COMMITMENT_NUMBER - 2)).unwrap());
-       let per_commitment_secret = local_keys.commitment_secret(INITIAL_COMMITMENT_NUMBER);
+               &SecretKey::from_slice(&local_keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER - 2)).unwrap());
+       let per_commitment_secret = local_keys.release_commitment_secret(INITIAL_COMMITMENT_NUMBER);
 
        nodes[1].node.handle_revoke_and_ack(&nodes[0].node.get_our_node_id(),
                &msgs::RevokeAndACK { channel_id, per_commitment_secret, next_per_commitment_point });

diff --git a/lightning/src/util/enforcing_trait_impls.rs b/lightning/src/util/enforcing_trait_impls.rs
index 88cdd94afb8de227c2475fe16de4d5b1b130818c..c1d3d490262b84d41e08bda7509f6cff8e1b8b69 100644 (file)
--- a/lightning/src/util/enforcing_trait_impls.rs
+++ b/lightning/src/util/enforcing_trait_impls.rs
@@ -48,7 +48,15 @@ impl EnforcingChannelKeys {
 }
 
 impl ChannelKeys for EnforcingChannelKeys {
-       fn commitment_secret(&self, idx: u64) -> [u8; 32] { self.inner.commitment_secret(idx) }
+       fn get_per_commitment_point<T: secp256k1::Signing + secp256k1::Verification>(&self, idx: u64, secp_ctx: &Secp256k1<T>) -> PublicKey {
+               self.inner.get_per_commitment_point(idx, secp_ctx)
+       }
+
+       fn release_commitment_secret(&self, idx: u64) -> [u8; 32] {
+               // TODO: enforce the ChannelKeys contract - error here if we already signed this commitment
+               self.inner.release_commitment_secret(idx)
+       }
+
        fn pubkeys(&self) -> &ChannelPublicKeys { self.inner.pubkeys() }
        fn key_derivation_params(&self) -> (u64, u64) { self.inner.key_derivation_params() }
 
@@ -71,6 +79,8 @@ impl ChannelKeys for EnforcingChannelKeys {
        }
 
        fn sign_local_commitment<T: secp256k1::Signing + secp256k1::Verification>(&self, local_commitment_tx: &LocalCommitmentTransaction, secp_ctx: &Secp256k1<T>) -> Result<Signature, ()> {
+               // TODO: enforce the ChannelKeys contract - error if this commitment was already revoked
+               // TODO: need the commitment number
                Ok(self.inner.sign_local_commitment(local_commitment_tx, secp_ctx).unwrap())
        }