implement data_loss_protect

so that we can spend their_ctx_to_remote even when we lost our state
but have an old backup

electrum/lnchannel.py

@@ -65,6 +65,8 @@ class ChannelJsonEncoder(json.JSONEncoder):
 RevokeAndAck = namedtuple("RevokeAndAck", ["per_commitment_secret", "next_per_commitment_point"])
 
 
+class RemoteCtnTooFarInFuture(Exception): pass
+
 
 def decodeAll(d, local):
     for k, v in d.items():
@@ -85,10 +87,10 @@ def htlcsum(htlcs):
 
 # following two functions are used because json
 # doesn't store int keys and byte string values
-def str_bytes_dict_from_save(x):
+def str_bytes_dict_from_save(x) -> Dict[int, bytes]:
     return {int(k): bfh(v) for k,v in x.items()}
 
-def str_bytes_dict_to_save(x):
+def str_bytes_dict_to_save(x) -> Dict[str, str]:
     return {str(k): bh2u(v) for k, v in x.items()}
 
 
@@ -132,6 +134,7 @@ class Channel(Logger):
         self.short_channel_id_predicted = self.short_channel_id
         self.onion_keys = str_bytes_dict_from_save(state.get('onion_keys', {}))
         self.force_closed = state.get('force_closed')
+        self.data_loss_protect_remote_pcp = str_bytes_dict_from_save(state.get('data_loss_protect_remote_pcp', {}))
 
         log = state.get('log')
         self.hm = HTLCManager(local_ctn=self.config[LOCAL].ctn,
@@ -507,11 +510,12 @@ class Channel(Logger):
         fee_for_htlc = lambda htlc: htlc.amount_msat // 1000 - (weight * feerate // 1000)
         return list(filter(lambda htlc: fee_for_htlc(htlc) >= conf.dust_limit_sat, htlcs))
 
-    def get_secret_and_point(self, subject, ctn):
+    def get_secret_and_point(self, subject, ctn) -> Tuple[Optional[bytes], bytes]:
         assert type(subject) is HTLCOwner
         offset = ctn - self.get_current_ctn(subject)
         if subject == REMOTE:
-            assert offset <= 1, offset
+            if offset > 1:
+                raise RemoteCtnTooFarInFuture(f"offset: {offset}")
             conf = self.config[REMOTE]
             if offset == 1:
                 secret = None
@@ -621,6 +625,7 @@ class Channel(Logger):
                 "log": self.hm.to_save(),
                 "onion_keys": str_bytes_dict_to_save(self.onion_keys),
                 "force_closed": self.force_closed,
+                "data_loss_protect_remote_pcp": str_bytes_dict_to_save(self.data_loss_protect_remote_pcp),
         }
         return to_save
 

electrum/lnpeer.py

@@ -30,7 +30,7 @@ from .logging import Logger
 from .lnonion import (new_onion_packet, decode_onion_error, OnionFailureCode, calc_hops_data_for_payment,
                       process_onion_packet, OnionPacket, construct_onion_error, OnionRoutingFailureMessage,
                       ProcessedOnionPacket)
-from .lnchannel import Channel, RevokeAndAck, htlcsum
+from .lnchannel import Channel, RevokeAndAck, htlcsum, RemoteCtnTooFarInFuture
 from .lnutil import (Outpoint, LocalConfig, RECEIVED, UpdateAddHtlc,
                      RemoteConfig, OnlyPubkeyKeypair, ChannelConstraints, RevocationStore,
                      funding_output_script, get_per_commitment_secret_from_seed,
@@ -720,7 +720,8 @@ class Peer(Logger):
         latest_remote_ctn = chan.hm.ctn_latest(REMOTE)
         next_remote_ctn = latest_remote_ctn + 1
         # send message
-        if self.localfeatures & LnLocalFeatures.OPTION_DATA_LOSS_PROTECT_OPT:
+        dlp_enabled = self.localfeatures & LnLocalFeatures.OPTION_DATA_LOSS_PROTECT_OPT
+        if dlp_enabled:
             if oldest_unrevoked_remote_ctn == 0:
                 last_rev_secret = 0
             else:
@@ -746,6 +747,8 @@ class Peer(Logger):
         chan.set_state('OPENING')
         their_next_local_ctn = int.from_bytes(channel_reestablish_msg["next_local_commitment_number"], 'big')
         their_oldest_unrevoked_remote_ctn = int.from_bytes(channel_reestablish_msg["next_remote_revocation_number"], 'big')
+        their_local_pcp = channel_reestablish_msg.get("my_current_per_commitment_point")
+        their_claim_of_our_last_per_commitment_secret = channel_reestablish_msg.get("your_last_per_commitment_secret")
 
         should_close_we_are_ahead = False
         should_close_they_are_ahead = False
@@ -786,9 +789,34 @@ class Peer(Logger):
                     should_close_we_are_ahead = True
                 else:
                     should_close_they_are_ahead = True
-
-        # TODO option_data_loss_protect
-
+        # option_data_loss_protect
+        def are_datalossprotect_fields_valid() -> bool:
+            if their_local_pcp is None or their_claim_of_our_last_per_commitment_secret is None:
+                # if DLP was enabled, absence of fields is not OK
+                return not dlp_enabled
+            our_pcs, __ = chan.get_secret_and_point(LOCAL, their_oldest_unrevoked_remote_ctn - 1)
+            if our_pcs != their_claim_of_our_last_per_commitment_secret:
+                self.logger.error(f"channel_reestablish: (DLP) local PCS mismatch: {bh2u(our_pcs)} != {bh2u(their_claim_of_our_last_per_commitment_secret)}")
+                return False
+            try:
+                __, our_remote_pcp = chan.get_secret_and_point(REMOTE, their_next_local_ctn - 1)
+            except RemoteCtnTooFarInFuture:
+                pass
+            else:
+                if our_remote_pcp != their_local_pcp:
+                    self.logger.error(f"channel_reestablish: (DLP) remote PCP mismatch: {bh2u(our_remote_pcp)} != {bh2u(their_local_pcp)}")
+                    return False
+            return True
+
+        if not are_datalossprotect_fields_valid():
+            self.logger.error(f"channel_reestablish: data loss protect fields invalid.")
+            # TODO should we force-close?
+            return
+        else:
+            if dlp_enabled and should_close_they_are_ahead:
+                self.logger.warning(f"channel_reestablish: remote is ahead of us! luckily DLP is enabled. remote PCP: {bh2u(their_local_pcp)}")
+                chan.data_loss_protect_remote_pcp[their_next_local_ctn - 1] = their_local_pcp
+                self.lnworker.save_channel(chan)
         if should_close_they_are_ahead:
             self.logger.warning(f"channel_reestablish: remote is ahead of us! trying to get them to force-close.")
             self.try_to_get_remote_to_force_close_with_their_latest(chan_id)

electrum/lnsweep.py

@@ -245,7 +245,7 @@ def create_sweeptxs_for_our_ctx(chan: 'Channel', ctx: Transaction, ctn: int,
         create_txns_for_htlc(htlc, is_received_htlc=True)
     return txs
 
-def analyze_ctx(chan, ctx):
+def analyze_ctx(chan: 'Channel', ctx: Transaction):
     # note: the remote sometimes has two valid non-revoked commitment transactions,
     # either of which could be broadcast (their_conf.ctn, their_conf.ctn+1)
     our_conf, their_conf = get_ordered_channel_configs(chan=chan, for_us=True)
@@ -265,6 +265,9 @@ def analyze_ctx(chan, ctx):
         their_pcp = ecc.ECPrivkey(per_commitment_secret).get_public_key_bytes(compressed=True)
         is_revocation = True
         #_logger.info(f'tx for revoked: {list(txs.keys())}')
+    elif ctn in chan.data_loss_protect_remote_pcp:
+        their_pcp = chan.data_loss_protect_remote_pcp[ctn]
+        is_revocation = False
     else:
         return
     return ctn, their_pcp, is_revocation, per_commitment_secret