backups: restore from closing tx, sweep to_remote

* add a method for backups to sweep to_remote
* to_remote sweeping needs the payment_basepoint's private key
  to sign the sweep transaction
* we restore the private key from our funding multisig pubkey
  (pubished with the closing transaction) and a static payment key secret

electrum/lnchannel.py

@@ -56,9 +56,10 @@ from .lnutil import (Outpoint, LocalConfig, RemoteConfig, Keypair, OnlyPubkeyKey
                      ShortChannelID, map_htlcs_to_ctx_output_idxs,
                      fee_for_htlc_output, offered_htlc_trim_threshold_sat,
                      received_htlc_trim_threshold_sat, make_commitment_output_to_remote_address, FIXED_ANCHOR_SAT,
-                     ChannelType, LNProtocolWarning)
+                     ChannelType, LNProtocolWarning, ctx_has_anchors)
 from .lnsweep import create_sweeptxs_for_our_ctx, create_sweeptxs_for_their_ctx
 from .lnsweep import create_sweeptx_for_their_revoked_htlc, SweepInfo
+from .lnsweep import create_sweeptx_their_backup_ctx
 from .lnhtlc import HTLCManager
 from .lnmsg import encode_msg, decode_msg
 from .address_synchronizer import TX_HEIGHT_LOCAL
@@ -594,14 +595,19 @@ class ChannelBackup(AbstractChannel):
         return True
 
     def create_sweeptxs_for_their_ctx(self, ctx):
-        return {}
+        return create_sweeptx_their_backup_ctx(chan=self, ctx=ctx, sweep_address=self.get_sweep_address())
 
     def create_sweeptxs_for_our_ctx(self, ctx):
         if self.is_imported:
             return create_sweeptxs_for_our_ctx(chan=self, ctx=ctx, sweep_address=self.get_sweep_address())
         else:
-            # backup from op_return
-            return {}
+            return
+
+    def maybe_sweep_revoked_htlc(self, ctx: Transaction, htlc_tx: Transaction) -> Optional[SweepInfo]:
+        return None
+
+    def extract_preimage_from_htlc_txin(self, txin: TxInput) -> None:
+        return None
 
     def get_funding_address(self):
         return self.cb.funding_address

electrum/lnsweep.py

@@ -18,14 +18,15 @@ from .lnutil import (make_commitment_output_to_remote_address, make_commitment_o
                      LOCAL, REMOTE, make_htlc_output_witness_script,
                      get_ordered_channel_configs, get_per_commitment_secret_from_seed,
                      RevocationStore, extract_ctn_from_tx_and_chan, UnableToDeriveSecret, SENT, RECEIVED,
-                     map_htlcs_to_ctx_output_idxs, Direction, make_commitment_output_to_remote_witness_script)
-from .transaction import (Transaction, TxOutput, PartialTransaction, PartialTxInput,
-                          PartialTxOutput, TxOutpoint)
+                     map_htlcs_to_ctx_output_idxs, Direction, make_commitment_output_to_remote_witness_script,
+                     derive_payment_basepoint, ctx_has_anchors, SCRIPT_TEMPLATE_FUNDING)
+from .transaction import (Transaction, TxInput, PartialTransaction, PartialTxInput,
+                          PartialTxOutput, TxOutpoint, script_GetOp, match_script_against_template)
 from .simple_config import SimpleConfig
 from .logging import get_logger, Logger
 
 if TYPE_CHECKING:
-    from .lnchannel import Channel, AbstractChannel
+    from .lnchannel import Channel, AbstractChannel, ChannelBackup
 
 
 _logger = get_logger(__name__)
@@ -352,6 +353,69 @@ def extract_ctx_secrets(chan: 'Channel', ctx: Transaction):
     return ctn, their_pcp, is_revocation, per_commitment_secret
 
 
+def extract_funding_pubkeys_from_ctx(txin: TxInput) -> Tuple[bytes, bytes]:
+    """Extract the two funding pubkeys from the published commitment transaction.
+
+    We expect to see a witness script of: OP_2 pk1 pk2 OP_2 OP_CHECKMULTISIG"""
+    elements = txin.witness_elements()
+    witness_script = elements[-1]
+    assert match_script_against_template(witness_script, SCRIPT_TEMPLATE_FUNDING)
+    parsed_script = [x for x in script_GetOp(witness_script)]
+    pubkey1 = parsed_script[1][1]
+    pubkey2 = parsed_script[2][1]
+    return (pubkey1, pubkey2)
+
+
+def create_sweeptx_their_backup_ctx(
+            *, chan: 'ChannelBackup',
+            ctx: Transaction,
+            sweep_address: str) -> Optional[Dict[str, SweepInfo]]:
+    txs = {}  # type: Dict[str, SweepInfo]
+    """If we only have a backup, and the remote force-closed with their ctx,
+    and anchors are enabled, we need to sweep to_remote."""
+
+    if ctx_has_anchors(ctx):
+        # for anchors we need to sweep to_remote
+        funding_pubkeys = extract_funding_pubkeys_from_ctx(ctx.inputs()[0])
+        _logger.debug(f'checking their ctx for funding pubkeys: {[pk.hex() for pk in funding_pubkeys]}')
+        # check which of the pubkey was ours
+        for pubkey in funding_pubkeys:
+            candidate_basepoint = derive_payment_basepoint(chan.lnworker.static_payment_key.privkey, funding_pubkey=pubkey)
+            candidate_to_remote_address = make_commitment_output_to_remote_address(candidate_basepoint.pubkey, has_anchors=True)
+            if ctx.get_output_idxs_from_address(candidate_to_remote_address):
+                our_payment_pubkey = candidate_basepoint
+                to_remote_address = candidate_to_remote_address
+                _logger.debug(f'found funding pubkey')
+                break
+        else:
+            return
+    else:
+        # we are dealing with static_remotekey which is locked to a wallet address
+        return {}
+
+    # to_remote
+    csv_delay = 1
+    our_payment_privkey = ecc.ECPrivkey(our_payment_pubkey.privkey)
+    output_idxs = ctx.get_output_idxs_from_address(to_remote_address)
+    if output_idxs:
+        output_idx = output_idxs.pop()
+        prevout = ctx.txid() + ':%d' % output_idx
+        sweep_tx = lambda: create_sweeptx_their_ctx_to_remote(
+            sweep_address=sweep_address,
+            ctx=ctx,
+            output_idx=output_idx,
+            our_payment_privkey=our_payment_privkey,
+            config=chan.lnworker.config,
+            has_anchors=True
+        )
+        txs[prevout] = SweepInfo(
+            name='their_ctx_to_remote_backup',
+            csv_delay=csv_delay,
+            cltv_abs=0,
+            gen_tx=sweep_tx)
+    return txs
+
+
 def create_sweeptxs_for_their_ctx(
         *, chan: 'Channel',
         ctx: Transaction,

electrum/lnutil.py

@@ -21,7 +21,7 @@ from .util import format_short_id as format_short_channel_id
 
 from .crypto import sha256, pw_decode_with_version_and_mac
 from .transaction import (Transaction, PartialTransaction, PartialTxInput, TxOutpoint,
-                          PartialTxOutput, opcodes, TxOutput)
+                          PartialTxOutput, opcodes, TxOutput, OPPushDataPubkey)
 from . import bitcoin, crypto, transaction
 from . import descriptor
 from .bitcoin import (redeem_script_to_address, address_to_script,
@@ -57,6 +57,8 @@ FIXED_ANCHOR_SAT = 330
 LN_MAX_FUNDING_SAT_LEGACY = pow(2, 24) - 1
 DUST_LIMIT_MAX = 1000
 
+SCRIPT_TEMPLATE_FUNDING = [opcodes.OP_2, OPPushDataPubkey, OPPushDataPubkey, opcodes.OP_2, opcodes.OP_CHECKMULTISIG]
+
 
 from .json_db import StoredObject, stored_in, stored_as
 
@@ -1264,6 +1266,14 @@ def extract_ctn_from_tx_and_chan(tx: Transaction, chan: 'AbstractChannel') -> in
                                funder_payment_basepoint=funder_conf.payment_basepoint.pubkey,
                                fundee_payment_basepoint=fundee_conf.payment_basepoint.pubkey)
 
+def ctx_has_anchors(tx: Transaction):
+    output_values = [output.value for output in tx.outputs()]
+    if FIXED_ANCHOR_SAT in output_values:
+        return True
+    else:
+        return False
+
+
 
 class LnFeatureContexts(enum.Flag):
     INIT = enum.auto()