resumable channels:

 - client and server are distinct roles, with different flags
 - the client sends a signature of his channel state with each
   commitment_signed or revoke_and_ack message.
 - the server reconstructs the state and verifies those signatures
 - actually, the state is made of two parts: one is signed with cs, one with revack
 - in unit tests, both peers are client and server

electrum/lnhtlc.py

@@ -168,19 +168,20 @@ def send_rev(self) -> None:
         self.log[LOCAL]['ctn'] += 1
         self._set_revack_pending(LOCAL, False)
         self.log[LOCAL]['was_revoke_last'] = True
-        # htlcs
+        # htlcs proposed by remote, update locked_in in remote_ctx
         for htlc_id in self._maybe_active_htlc_ids[REMOTE]:
             ctns = self.log[REMOTE]['locked_in'][htlc_id]
             if ctns[REMOTE] is None and ctns[LOCAL] <= self.ctn_latest(LOCAL):
                 ctns[REMOTE] = self.ctn_latest(REMOTE) + 1
+        # htlcs proposed by local, update settles/fails in remote_ctx
         for log_action in ('settles', 'fails'):
             for htlc_id in self._maybe_active_htlc_ids[LOCAL]:
                 ctns = self.log[LOCAL][log_action].get(htlc_id, None)
                 if ctns is None: continue
                 if ctns[REMOTE] is None and ctns[LOCAL] <= self.ctn_latest(LOCAL):
                     ctns[REMOTE] = self.ctn_latest(REMOTE) + 1
         self._update_maybe_active_htlc_ids()
-        # fee updates
+        # fee updates proposed by remote: update remote_ctn
         for k, fee_update in list(self.log[REMOTE]['fee_updates'].items()):
             if fee_update.ctn_remote is None and fee_update.ctn_local <= self.ctn_latest(LOCAL):
                 fee_update.ctn_remote = self.ctn_latest(REMOTE) + 1
@@ -189,19 +190,20 @@ def send_rev(self) -> None:
     def recv_rev(self) -> None:
         self.log[REMOTE]['ctn'] += 1
         self._set_revack_pending(REMOTE, False)
-        # htlcs
+        # htlcs proposed by local, update locked_in in local_ctx
         for htlc_id in self._maybe_active_htlc_ids[LOCAL]:
             ctns = self.log[LOCAL]['locked_in'][htlc_id]
             if ctns[LOCAL] is None and ctns[REMOTE] <= self.ctn_latest(REMOTE):
                 ctns[LOCAL] = self.ctn_latest(LOCAL) + 1
+        # htlcs proposed by remote, update settles/fails in local_ctx
         for log_action in ('settles', 'fails'):
             for htlc_id in self._maybe_active_htlc_ids[REMOTE]:
                 ctns = self.log[REMOTE][log_action].get(htlc_id, None)
                 if ctns is None: continue
                 if ctns[LOCAL] is None and ctns[REMOTE] <= self.ctn_latest(REMOTE):
                     ctns[LOCAL] = self.ctn_latest(LOCAL) + 1
         self._update_maybe_active_htlc_ids()
-        # fee updates
+        # fee updates proposed by local
         for k, fee_update in list(self.log[LOCAL]['fee_updates'].items()):
             if fee_update.ctn_local is None and fee_update.ctn_remote <= self.ctn_latest(REMOTE):
                 fee_update.ctn_local = self.ctn_latest(LOCAL) + 1

electrum/lnutil.py

@@ -102,6 +102,10 @@ class ChannelConfig(StoredObject):
     upfront_shutdown_script = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
     announcement_node_sig = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
     announcement_bitcoin_sig = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
+    current_commitment_signature = attr.ib(type=bytes, converter=hex_to_bytes)
+    current_htlc_signatures = attr.ib(type=bytes, converter=hex_to_bytes)
+    next_per_commitment_point = attr.ib(type=bytes, converter=hex_to_bytes)
+    current_per_commitment_point = attr.ib(type=bytes, converter=hex_to_bytes)
 
     def validate_params(self, *, funding_sat: int, config: 'SimpleConfig', peer_features: 'LnFeatures') -> None:
         conf_name = type(self).__name__
@@ -205,8 +209,6 @@ def cross_validate_params(
 class LocalConfig(ChannelConfig):
     channel_seed = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)  # type: Optional[bytes]
     funding_locked_received = attr.ib(type=bool)
-    current_commitment_signature = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
-    current_htlc_signatures = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
     per_commitment_secret_seed = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
 
     @classmethod
@@ -240,8 +242,7 @@ def validate_params(self, *, funding_sat: int, config: 'SimpleConfig', peer_feat
 @stored_as('remote_config')
 @attr.s
 class RemoteConfig(ChannelConfig):
-    next_per_commitment_point = attr.ib(type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
-    current_per_commitment_point = attr.ib(default=None, type=bytes, converter=hex_to_bytes, repr=bytes_to_hex)
+    encrypted_seed = attr.ib(type=bytes, converter=hex_to_bytes)  # type: Optional[bytes]
 
 @stored_in('fee_updates')
 @attr.s
@@ -485,6 +486,19 @@ def __init__(self, storage):
         self.storage = storage
         self.buckets = storage['buckets']
 
+    @classmethod
+    def from_seed_and_index(cls, seed, index):
+        # this is vastly inefficient, but ok for now...
+        storage = {
+            'index': RevocationStore.START_INDEX,
+            'buckets': {},
+        }
+        s = RevocationStore(storage)
+        for i in range(index):
+            secret = get_per_commitment_secret_from_seed(seed, RevocationStore.START_INDEX - i)
+            s.add_next_entry(secret)
+        return s
+
     def add_next_entry(self, hsh):
         index = self.storage['index']
         new_element = ShachainElement(index=index, secret=hsh)
@@ -510,6 +524,9 @@ def retrieve_secret(self, index: int) -> bytes:
             return element.secret
         raise UnableToDeriveSecret()
 
+    def serialize(self):
+        pass
+
     def __eq__(self, o):
         return type(o) is RevocationStore and self.serialize() == o.serialize()
 
@@ -559,6 +576,18 @@ def get_per_commitment_secret_from_seed(seed: bytes, i: int, bits: int = 48) ->
     bajts = bytes(per_commitment_secret)
     return bajts
 
+def get_pcp_from_seed(seed: bytes, i: int) -> bytes:
+    per_commitment_secret = get_per_commitment_secret_from_seed(seed, i)
+    return secret_to_pubkey(int.from_bytes(per_commitment_secret, 'big'))
+
+def get_revocations_from_seed(seed:bytes, ctn:int):
+    revocations = b''
+    for i in range(49):
+        ctn_i = pow(2, i)
+        secret = get_per_commitment_secret_from_seed(seed, RevocationStore.START_INDEX - ctn_i)
+        revocations += ctn
+    return revocations
+
 def secret_to_pubkey(secret: int) -> bytes:
     assert type(secret) is int
     return ecc.ECPrivkey.from_secret_scalar(secret).get_public_key_bytes(compressed=True)
@@ -1223,6 +1252,18 @@ class LnFeatures(IntFlag):
     _ln_feature_contexts[OPTION_ZEROCONF_REQ] = (LNFC.INIT | LNFC.NODE_ANN)
     _ln_feature_contexts[OPTION_ZEROCONF_OPT] = (LNFC.INIT | LNFC.NODE_ANN)
 
+    OPTION_ELECTRUM_PEERBACKUP_CLIENT_REQ = 1 << 152
+    OPTION_ELECTRUM_PEERBACKUP_CLIENT_OPT = 1 << 153
+
+    _ln_feature_contexts[OPTION_ELECTRUM_PEERBACKUP_CLIENT_REQ] = (LNFC.INIT)
+    _ln_feature_contexts[OPTION_ELECTRUM_PEERBACKUP_CLIENT_OPT] = (LNFC.INIT)
+
+    OPTION_ELECTRUM_PEERBACKUP_SERVER_REQ = 1 << 154
+    OPTION_ELECTRUM_PEERBACKUP_SERVER_OPT = 1 << 155
+
+    _ln_feature_contexts[OPTION_ELECTRUM_PEERBACKUP_SERVER_REQ] = (LNFC.INIT | LNFC.NODE_ANN)
+    _ln_feature_contexts[OPTION_ELECTRUM_PEERBACKUP_SERVER_OPT] = (LNFC.INIT | LNFC.NODE_ANN)
+
     def validate_transitive_dependencies(self) -> bool:
         # for all even bit set, set corresponding odd bit:
         features = self  # copy
@@ -1384,6 +1425,7 @@ def name_minimal(self):
         | LnFeatures.OPTION_SHUTDOWN_ANYSEGWIT_OPT | LnFeatures.OPTION_SHUTDOWN_ANYSEGWIT_REQ
         | LnFeatures.OPTION_CHANNEL_TYPE_OPT | LnFeatures.OPTION_CHANNEL_TYPE_REQ
         | LnFeatures.OPTION_SCID_ALIAS_OPT | LnFeatures.OPTION_SCID_ALIAS_REQ
+        | LnFeatures.OPTION_ELECTRUM_PEERBACKUP_CLIENT_OPT | LnFeatures.OPTION_ELECTRUM_PEERBACKUP_SERVER_OPT
 )
 
 

electrum/lnwire/peer_wire.csv

@@ -63,6 +63,8 @@ tlvtype,open_channel_tlvs,channel_type,1
 tlvdata,open_channel_tlvs,channel_type,type,byte,...
 tlvtype,open_channel_tlvs,channel_opening_fee,10000
 tlvdata,open_channel_tlvs,channel_opening_fee,channel_opening_fee,u64,
+tlvtype,open_channel_tlvs,encrypted_seed,10001
+tlvdata,open_channel_tlvs,encrypted_seed,seed,byte,...
 msgtype,accept_channel,33
 msgdata,accept_channel,temporary_channel_id,byte,32
 msgdata,accept_channel,dust_limit_satoshis,u64,
@@ -83,6 +85,8 @@ tlvtype,accept_channel_tlvs,upfront_shutdown_script,0
 tlvdata,accept_channel_tlvs,upfront_shutdown_script,shutdown_scriptpubkey,byte,...
 tlvtype,accept_channel_tlvs,channel_type,1
 tlvdata,accept_channel_tlvs,channel_type,type,byte,...
+tlvtype,accept_channel_tlvs,encrypted_seed,2
+tlvdata,accept_channel_tlvs,encrypted_seed,seed,byte,...
 msgtype,funding_created,34
 msgdata,funding_created,temporary_channel_id,byte,32
 msgdata,funding_created,funding_txid,sha256,
@@ -135,10 +139,26 @@ msgdata,commitment_signed,channel_id,channel_id,
 msgdata,commitment_signed,signature,signature,
 msgdata,commitment_signed,num_htlcs,u16,
 msgdata,commitment_signed,htlc_signature,signature,num_htlcs
+msgdata,commitment_signed,tlvs,commitment_signed_tlvs,
+tlvtype,commitment_signed_tlvs,peerbackup,1
+tlvdata,commitment_signed_tlvs,peerbackup,signature,signature,
+tlvtype,commitment_signed_tlvs,time_commitment,2
+tlvdata,commitment_signed_tlvs,time_commitment,timestamp,u64,
+tlvdata,commitment_signed_tlvs,time_commitment,local_ctn,u64,
+tlvdata,commitment_signed_tlvs,time_commitment,remote_ctn,u64,
+tlvdata,commitment_signed_tlvs,time_commitment,signature,signature,
 msgtype,revoke_and_ack,133
 msgdata,revoke_and_ack,channel_id,channel_id,
 msgdata,revoke_and_ack,per_commitment_secret,byte,32
 msgdata,revoke_and_ack,next_per_commitment_point,point,
+msgdata,revoke_and_ack,tlvs,revoke_and_ack_tlvs,
+tlvtype,revoke_and_ack_tlvs,peerbackup,1
+tlvdata,revoke_and_ack_tlvs,peerbackup,signature,signature,
+tlvtype,revoke_and_ack_tlvs,time_commitment,2
+tlvdata,revoke_and_ack_tlvs,time_commitment,timestamp,u64,
+tlvdata,revoke_and_ack_tlvs,time_commitment,local_ctn,u64,
+tlvdata,revoke_and_ack_tlvs,time_commitment,remote_ctn,u64,
+tlvdata,revoke_and_ack_tlvs,time_commitment,signature,signature,
 msgtype,update_fee,134
 msgdata,update_fee,channel_id,channel_id,
 msgdata,update_fee,feerate_per_kw,u32,
@@ -148,6 +168,13 @@ msgdata,channel_reestablish,next_commitment_number,u64,
 msgdata,channel_reestablish,next_revocation_number,u64,
 msgdata,channel_reestablish,your_last_per_commitment_secret,byte,32
 msgdata,channel_reestablish,my_current_per_commitment_point,point,
+msgdata,channel_reestablish,tlvs,channel_reestablish_tlvs,
+tlvtype,channel_reestablish_tlvs,your_local_peerbackup,100
+tlvdata,channel_reestablish_tlvs,your_local_peerbackup,signature,signature,
+tlvdata,channel_reestablish_tlvs,your_local_peerbackup,state,byte,...
+tlvtype,channel_reestablish_tlvs,your_remote_peerbackup,101
+tlvdata,channel_reestablish_tlvs,your_remote_peerbackup,signature,signature,
+tlvdata,channel_reestablish_tlvs,your_remote_peerbackup,state,byte,...
 msgtype,announcement_signatures,259
 msgdata,announcement_signatures,channel_id,channel_id,
 msgdata,announcement_signatures,short_channel_id,short_channel_id,

electrum/lnworker.py

@@ -224,6 +224,7 @@ class ErrorAddingPeer(Exception): pass
     | LnFeatures.OPTION_CHANNEL_TYPE_OPT
     | LnFeatures.OPTION_SCID_ALIAS_OPT
     | LnFeatures.OPTION_SUPPORT_LARGE_CHANNEL_OPT
+    | LnFeatures.OPTION_ELECTRUM_PEERBACKUP_CLIENT_OPT
 )
 
 LNGOSSIP_FEATURES = (
@@ -814,6 +815,8 @@ def __init__(self, wallet: 'Abstract_Wallet', xprv):
         features = LNWALLET_FEATURES
         if self.config.ACCEPT_ZEROCONF_CHANNELS:
             features |= LnFeatures.OPTION_ZEROCONF_OPT
+        if self.config.LIGHTNING_PEERBACKUP_SERVER:
+            features |= LnFeatures.OPTION_ELECTRUM_PEERBACKUP_SERVER_OPT
         LNWorker.__init__(self, self.node_keypair, features, config=self.config)
         self.lnwatcher = None
         self.lnrater: LNRater = None
@@ -858,7 +861,6 @@ def __init__(self, wallet: 'Abstract_Wallet', xprv):
         self.payment_bundles = []                       # lists of hashes. todo:persist
         self.swap_manager = SwapManager(wallet=self.wallet, lnworker=self)
 
-
     def has_deterministic_node_id(self) -> bool:
         return bool(self.db.get('lightning_xprv'))
 
@@ -1040,7 +1042,8 @@ def get_lightning_history(self):
             label = self.wallet.get_label_for_rhash(key)
             if not label and direction == PaymentDirection.FORWARDING:
                 label = _('Forwarding')
-            preimage = self.get_preimage(payment_hash).hex()
+            preimage = self.get_preimage(payment_hash)
+            preimage = preimage.hex() if preimage else None
             item = {
                 'type': 'payment',
                 'label': label,

electrum/simple_config.py

@@ -1195,6 +1195,9 @@ def _default_swapserver_url(self) -> str:
     ZEROCONF_TRUSTED_NODE = ConfigVar('zeroconf_trusted_node', default='', type_=str)
     ZEROCONF_MIN_OPENING_FEE = ConfigVar('zeroconf_min_opening_fee', default=5000, type_=int)
 
+    # serve channel backups
+    LIGHTNING_PEERBACKUP_SERVER = ConfigVar('lightning_peerbackup_server', default=False, type_=bool)
+
     # connect to remote WT
     WATCHTOWER_CLIENT_ENABLED = ConfigVar(
         'use_watchtower', default=False, type_=bool,

electrum/wallet_db.py

@@ -72,7 +72,7 @@ def __init__(self, wallet_db: 'WalletDB'):
 # seed_version is now used for the version of the wallet file
 OLD_SEED_VERSION = 4        # electrum versions < 2.0
 NEW_SEED_VERSION = 11       # electrum versions >= 2.0
-FINAL_SEED_VERSION = 59     # electrum >= 2.7 will set this to prevent
+FINAL_SEED_VERSION = 60     # electrum >= 2.7 will set this to prevent
                             # old versions from overwriting new format
 
 
@@ -234,6 +234,7 @@ def upgrade(self):
         self._convert_version_57()
         self._convert_version_58()
         self._convert_version_59()
+        self._convert_version_60()
         self.put('seed_version', FINAL_SEED_VERSION)  # just to be sure
 
     def _convert_wallet_type(self):
@@ -1146,6 +1147,19 @@ def _convert_version_59(self):
         self.data['channels'] = channels
         self.data['seed_version'] = 59
 
+    def _convert_version_60(self):
+        if not self._is_upgrade_method_needed(59, 59):
+            return
+        channels = self.data.get('channels', {})
+        for _key, chan in channels.items():
+            chan['local_config']['next_per_commitment_point'] = None
+            chan['local_config']['current_per_commitment_point'] = None
+            chan['remote_config']['current_commitment_signature'] = None
+            chan['remote_config']['current_htlc_signatures'] = None
+            chan['remote_config']['encrypted_seed'] = None
+        self.data['channels'] = channels
+        self.data['seed_version'] = 60
+
     def _convert_imported(self):
         if not self._is_upgrade_method_needed(0, 13):
             return

tests/regtest.py

@@ -123,6 +123,7 @@ class TestLightningJIT(TestLightning):
             'lightning_listen': 'localhost:9735',
             'lightning_forward_payments': 'true',
             'accept_zeroconf_channels': 'true',
+            'lightning_peerbackup_server': 'true',
         },
         'carol':{
         }

tests/test_lnchannel.py

@@ -74,6 +74,9 @@ def create_channel_state(funding_txid, funding_index, funding_sat, is_initiator,
                 upfront_shutdown_script=b'',
                 announcement_node_sig=b'',
                 announcement_bitcoin_sig=b'',
+                current_commitment_signature=None,
+                current_htlc_signatures=None,
+                encrypted_seed=None,
             ),
             "local_config":lnpeer.LocalConfig(
                 channel_seed = None,
@@ -96,6 +99,8 @@ def create_channel_state(funding_txid, funding_index, funding_sat, is_initiator,
                 upfront_shutdown_script=b'',
                 announcement_node_sig=b'',
                 announcement_bitcoin_sig=b'',
+                next_per_commitment_point=None,
+                current_per_commitment_point=None,
             ),
             "constraints":lnpeer.ChannelConstraints(
                 flags=0,