protect against getting robbed through routing fees

electrum/lnonion.py

@@ -33,11 +33,10 @@ from cryptography.hazmat.backends import default_backend
 from . import ecc
 from .crypto import sha256, hmac_oneshot
 from .util import bh2u, profiler, xor_bytes, bfh
-from .lnutil import get_ecdh
+from .lnutil import get_ecdh, PaymentFailure, NUM_MAX_HOPS_IN_PAYMENT_PATH
 from .lnrouter import RouteEdge
 
 
-NUM_MAX_HOPS_IN_PATH = 20
 HOPS_DATA_SIZE = 1300      # also sometimes called routingInfoSize in bolt-04
 PER_HOP_FULL_SIZE = 65     # HOPS_DATA_SIZE / 20
 NUM_STREAM_BYTES = HOPS_DATA_SIZE + PER_HOP_FULL_SIZE
@@ -192,6 +191,9 @@ def calc_hops_data_for_payment(route: List[RouteEdge], amount_msat: int, final_c
     """Returns the hops_data to be used for constructing an onion packet,
     and the amount_msat and cltv to be used on our immediate channel.
     """
+    if len(route) > NUM_MAX_HOPS_IN_PAYMENT_PATH:
+        raise PaymentFailure(f"too long route ({len(route)} hops)")
+
     amt = amount_msat
     cltv = final_cltv
     hops_data = [OnionHopsDataSingle(OnionPerHop(b"\x00" * 8,
@@ -209,7 +211,7 @@ def calc_hops_data_for_payment(route: List[RouteEdge], amount_msat: int, final_c
 
 def generate_filler(key_type: bytes, num_hops: int, hop_size: int,
                     shared_secrets: Sequence[bytes]) -> bytes:
-    filler_size = (NUM_MAX_HOPS_IN_PATH + 1) * hop_size
+    filler_size = (NUM_MAX_HOPS_IN_PAYMENT_PATH + 1) * hop_size
     filler = bytearray(filler_size)
 
     for i in range(0, num_hops-1):  # -1, as last hop does not obfuscate
@@ -219,7 +221,7 @@ def generate_filler(key_type: bytes, num_hops: int, hop_size: int,
         stream_bytes = generate_cipher_stream(stream_key, filler_size)
         filler = xor_bytes(filler, stream_bytes)
 
-    return filler[(NUM_MAX_HOPS_IN_PATH-num_hops+2)*hop_size:]
+    return filler[(NUM_MAX_HOPS_IN_PAYMENT_PATH-num_hops+2)*hop_size:]
 
 
 def generate_cipher_stream(stream_key: bytes, num_bytes: int) -> bytes:

electrum/lnrouter.py

@@ -39,7 +39,7 @@ from .storage import JsonDB
 from .lnchannelverifier import LNChannelVerifier, verify_sig_for_channel_update
 from .crypto import Hash
 from . import ecc
-from .lnutil import LN_GLOBAL_FEATURES_KNOWN_SET, LNPeerAddr
+from .lnutil import LN_GLOBAL_FEATURES_KNOWN_SET, LNPeerAddr, NUM_MAX_HOPS_IN_PAYMENT_PATH
 
 
 class UnknownEvenFeatureBits(Exception): pass
@@ -502,10 +502,61 @@ class RouteEdge(NamedTuple("RouteEdge", [('node_id', bytes),
                                          ('cltv_expiry_delta', int)])):
     """if you travel through short_channel_id, you will reach node_id"""
 
-    def fee_for_edge(self, amount_msat):
+    def fee_for_edge(self, amount_msat: int) -> int:
         return self.fee_base_msat \
                + (amount_msat * self.fee_proportional_millionths // 1_000_000)
 
+    @classmethod
+    def from_channel_policy(cls, channel_policy: ChannelInfoDirectedPolicy,
+                            short_channel_id: bytes, end_node: bytes) -> 'RouteEdge':
+        return RouteEdge(end_node,
+                         short_channel_id,
+                         channel_policy.fee_base_msat,
+                         channel_policy.fee_proportional_millionths,
+                         channel_policy.cltv_expiry_delta)
+
+    def is_sane_to_use(self, amount_msat: int) -> bool:
+        # TODO revise ad-hoc heuristics
+        # cltv cannot be more than 2 weeks
+        if self.cltv_expiry_delta > 14 * 144: return False
+        total_fee = self.fee_for_edge(amount_msat)
+        # fees below 50 sat are fine
+        if total_fee > 50_000:
+            # fee cannot be higher than amt
+            if total_fee > amount_msat: return False
+            # fee cannot be higher than 5000 sat
+            if total_fee > 5_000_000: return False
+            # unless amt is tiny, fee cannot be more than 10%
+            if amount_msat > 1_000_000 and total_fee > amount_msat/10: return False
+        return True
+
+
+def is_route_sane_to_use(route: List[RouteEdge], invoice_amount_msat: int, min_final_cltv_expiry: int) -> bool:
+    """Run some sanity checks on the whole route, before attempting to use it.
+    called when we are paying; so e.g. lower cltv is better
+    """
+    if len(route) > NUM_MAX_HOPS_IN_PAYMENT_PATH:
+        return False
+    amt = invoice_amount_msat
+    cltv = min_final_cltv_expiry
+    for route_edge in reversed(route[1:]):
+        if not route_edge.is_sane_to_use(amt): return False
+        amt += route_edge.fee_for_edge(amt)
+        cltv += route_edge.cltv_expiry_delta
+    total_fee = amt - invoice_amount_msat
+    # TODO revise ad-hoc heuristics
+    # cltv cannot be more than 2 months
+    if cltv > 60 * 144: return False
+    # fees below 50 sat are fine
+    if total_fee > 50_000:
+        # fee cannot be higher than amt
+        if total_fee > invoice_amount_msat: return False
+        # fee cannot be higher than 5000 sat
+        if total_fee > 5_000_000: return False
+        # unless amt is tiny, fee cannot be more than 10%
+        if invoice_amount_msat > 1_000_000 and total_fee > invoice_amount_msat/10: return False
+    return True
+
 
 class LNPathFinder(PrintError):
 
@@ -513,11 +564,9 @@ class LNPathFinder(PrintError):
         self.channel_db = channel_db
         self.blacklist = set()
 
-    def _edge_cost(self, short_channel_id: bytes, start_node: bytes, payment_amt_msat: int,
-                   ignore_cltv=False) -> float:
-        """Heuristic cost of going through a channel.
-        direction: 0 or 1. --- 0 means node_id_1 -> node_id_2
-        """
+    def _edge_cost(self, short_channel_id: bytes, start_node: bytes, end_node: bytes,
+                   payment_amt_msat: int, ignore_cltv=False) -> float:
+        """Heuristic cost of going through a channel."""
         channel_info = self.channel_db.get_channel_info(short_channel_id)  # type: ChannelInfo
         if channel_info is None:
             return float('inf')
@@ -525,41 +574,39 @@ class LNPathFinder(PrintError):
         channel_policy = channel_info.get_policy_for_node(start_node)
         if channel_policy is None: return float('inf')
         if channel_policy.disabled: return float('inf')
-        cltv_expiry_delta           = channel_policy.cltv_expiry_delta
-        htlc_minimum_msat           = channel_policy.htlc_minimum_msat
-        fee_base_msat               = channel_policy.fee_base_msat
-        fee_proportional_millionths = channel_policy.fee_proportional_millionths
-        if payment_amt_msat is not None:
-            if payment_amt_msat < htlc_minimum_msat:
-                return float('inf')  # payment amount too little
-            if channel_info.capacity_sat is not None and \
-                    payment_amt_msat // 1000 > channel_info.capacity_sat:
-                return float('inf')  # payment amount too large
-            if channel_policy.htlc_maximum_msat is not None and \
-                    payment_amt_msat > channel_policy.htlc_maximum_msat:
-                return float('inf')  # payment amount too large
-        amt = payment_amt_msat or 50000 * 1000  # guess for typical payment amount
-        fee_msat = fee_base_msat + amt * fee_proportional_millionths / 1_000_000
+        route_edge = RouteEdge.from_channel_policy(channel_policy, short_channel_id, end_node)
+        if payment_amt_msat < channel_policy.htlc_minimum_msat:
+            return float('inf')  # payment amount too little
+        if channel_info.capacity_sat is not None and \
+                payment_amt_msat // 1000 > channel_info.capacity_sat:
+            return float('inf')  # payment amount too large
+        if channel_policy.htlc_maximum_msat is not None and \
+                payment_amt_msat > channel_policy.htlc_maximum_msat:
+            return float('inf')  # payment amount too large
+        if not route_edge.is_sane_to_use(payment_amt_msat):
+            return float('inf')  # thanks but no thanks
+        fee_msat = route_edge.fee_for_edge(payment_amt_msat)
         # TODO revise
         # paying 10 more satoshis ~ waiting one more block
         fee_cost = fee_msat / 1000 / 10
-        cltv_cost = cltv_expiry_delta if not ignore_cltv else 0
+        cltv_cost = route_edge.cltv_expiry_delta if not ignore_cltv else 0
         return cltv_cost + fee_cost + 1
 
     @profiler
     def find_path_for_payment(self, from_node_id: bytes, to_node_id: bytes,
-                              amount_msat: int=None, my_channels: List=None) -> Sequence[Tuple[bytes, bytes]]:
+                              amount_msat: int, my_channels: List=None) -> Sequence[Tuple[bytes, bytes]]:
         """Return a path between from_node_id and to_node_id.
 
         Returns a list of (node_id, short_channel_id) representing a path.
         To get from node ret[n][0] to ret[n+1][0], use channel ret[n+1][1];
         i.e. an element reads as, "to get to node_id, travel through short_channel_id"
         """
-        if amount_msat is not None: assert type(amount_msat) is int
+        assert type(amount_msat) is int
         if my_channels is None: my_channels = []
         unable_channels = set(map(lambda x: x.short_channel_id, filter(lambda x: not x.can_pay(amount_msat), my_channels)))
 
         # TODO find multiple paths??
+        # FIXME paths cannot be longer than 20 (onion packet)...
 
         # run Dijkstra
         distance_from_start = defaultdict(lambda: float('inf'))
@@ -584,7 +631,7 @@ class LNPathFinder(PrintError):
                 node1, node2 = channel_info.node_id_1, channel_info.node_id_2
                 neighbour = node2 if node1 == cur_node else node1
                 ignore_cltv_delta_in_edge_cost = cur_node == from_node_id
-                edge_cost = self._edge_cost(edge_channel_id, cur_node, amount_msat,
+                edge_cost = self._edge_cost(edge_channel_id, cur_node, neighbour, amount_msat,
                                             ignore_cltv=ignore_cltv_delta_in_edge_cost)
                 alt_dist_to_neighbour = distance_from_start[cur_node] + edge_cost
                 if alt_dist_to_neighbour < distance_from_start[neighbour]:
@@ -614,10 +661,6 @@ class LNPathFinder(PrintError):
             channel_policy = self.channel_db.get_routing_policy_for_channel(prev_node_id, short_channel_id)
             if channel_policy is None:
                 raise Exception(f'cannot find channel policy for short_channel_id: {bh2u(short_channel_id)}')
-            route.append(RouteEdge(node_id,
-                                   short_channel_id,
-                                   channel_policy.fee_base_msat,
-                                   channel_policy.fee_proportional_millionths,
-                                   channel_policy.cltv_expiry_delta))
+            route.append(RouteEdge.from_channel_policy(channel_policy, short_channel_id, node_id))
             prev_node_id = node_id
         return route

electrum/lnutil.py

@@ -1,7 +1,7 @@
 from enum import IntFlag, IntEnum
 import json
 from collections import namedtuple
-from typing import NamedTuple, List, Tuple, Mapping
+from typing import NamedTuple, List, Tuple, Mapping, Optional
 import re
 
 from .util import bfh, bh2u, inv_dict
@@ -16,6 +16,7 @@ from .i18n import _
 from .lnaddr import lndecode
 from .keystore import BIP32_KeyStore
 
+
 HTLC_TIMEOUT_WEIGHT = 663
 HTLC_SUCCESS_WEIGHT = 703
 
@@ -597,8 +598,6 @@ def generate_keypair(ln_keystore: BIP32_KeyStore, key_family: LnKeyFamily, index
     return Keypair(*ln_keystore.get_keypair([key_family, 0, index], None))
 
 
-from typing import Optional
-
 class EncumberedTransaction(NamedTuple("EncumberedTransaction", [('tx', Transaction),
                                                                  ('csv_delay', Optional[int])])):
     def to_json(self) -> dict:
@@ -612,3 +611,7 @@ class EncumberedTransaction(NamedTuple("EncumberedTransaction", [('tx', Transact
         d2 = dict(d)
         d2['tx'] = Transaction(d['tx'])
         return EncumberedTransaction(**d2)
+
+
+NUM_MAX_HOPS_IN_PAYMENT_PATH = 20
+

electrum/lnworker.py

@@ -25,10 +25,11 @@ from .lnutil import (Outpoint, calc_short_channel_id, LNPeerAddr,
                      get_compressed_pubkey_from_bech32, extract_nodeid,
                      PaymentFailure, split_host_port, ConnStringFormatError,
                      generate_keypair, LnKeyFamily, LOCAL, REMOTE,
-                     UnknownPaymentHash, MIN_FINAL_CLTV_EXPIRY_FOR_INVOICE)
+                     UnknownPaymentHash, MIN_FINAL_CLTV_EXPIRY_FOR_INVOICE,
+                     NUM_MAX_HOPS_IN_PAYMENT_PATH)
 from .lnaddr import lndecode
 from .i18n import _
-from .lnrouter import RouteEdge
+from .lnrouter import RouteEdge, is_route_sane_to_use
 
 NUM_PEERS_TARGET = 4
 PEER_RETRY_INTERVAL = 600  # seconds
@@ -253,6 +254,10 @@ class LNWorker(PrintError):
         if amount_sat is None:
             raise InvoiceError(_("Missing amount"))
         amount_msat = int(amount_sat * 1000)
+        if addr.get_min_final_cltv_expiry() > 60 * 144:
+            raise InvoiceError("{}\n{}".format(
+                _("Invoice wants us to risk locking funds for unreasonably long."),
+                f"min_final_cltv_expiry: {addr.get_min_final_cltv_expiry()}"))
         route = self._create_route_from_invoice(decoded_invoice=addr, amount_msat=amount_msat)
         node_id, short_channel_id = route[0].node_id, route[0].short_channel_id
         peer = self.peers[node_id]
@@ -281,6 +286,7 @@ class LNWorker(PrintError):
             channels = list(self.channels.values())
         for private_route in r_tags:
             if len(private_route) == 0: continue
+            if len(private_route) > NUM_MAX_HOPS_IN_PAYMENT_PATH: continue
             border_node_pubkey = private_route[0][0]
             path = self.network.path_finder.find_path_for_payment(self.node_keypair.pubkey, border_node_pubkey, amount_msat, channels)
             if not path: continue
@@ -301,6 +307,11 @@ class LNWorker(PrintError):
                 route.append(RouteEdge(node_pubkey, short_channel_id, fee_base_msat, fee_proportional_millionths,
                                        cltv_expiry_delta))
                 prev_node_id = node_pubkey
+            # test sanity
+            if not is_route_sane_to_use(route, amount_msat, decoded_invoice.get_min_final_cltv_expiry()):
+                self.print_error(f"rejecting insane route {route}")
+                route = None
+                continue
             break
         # if could not find route using any hint; try without hint now
         if route is None:
@@ -308,6 +319,9 @@ class LNWorker(PrintError):
             if not path:
                 raise PaymentFailure(_("No path found"))
             route = self.network.path_finder.create_route_from_path(path, self.node_keypair.pubkey)
+            if not is_route_sane_to_use(route, amount_msat, decoded_invoice.get_min_final_cltv_expiry()):
+                self.print_error(f"rejecting insane route {route}")
+                raise PaymentFailure(_("No path found"))
         return route
 
     def add_invoice(self, amount_sat, message):