check dust limits

* on channel opening we verify that the peer's dust limit is above 354
  sat, the limit for unknown segwit versions
* we constrain the allowed scriptpubkey types for channel closing
* we check that the remote's output is above the relay dust limit for
  the collaborative close case

electrum/bitcoin.py

@@ -349,8 +349,12 @@ def relayfee(network: 'Network' = None) -> int:
 
 
 # see https://github.com/bitcoin/bitcoin/blob/a62f0ed64f8bbbdfe6467ac5ce92ef5b5222d1bd/src/policy/policy.cpp#L14
-DUST_LIMIT_DEFAULT_SAT_LEGACY = 546
-DUST_LIMIT_DEFAULT_SAT_SEGWIT = 294
+# and https://github.com/lightningnetwork/lightning-rfc/blob/7e3dce42cbe4fa4592320db6a4e06c26bb99122b/03-transactions.md#dust-limits
+DUST_LIMIT_P2PKH = 546
+DUST_LIMIT_P2SH = 540
+DUST_LIMIT_UNKNOWN_SEGWIT = 354
+DUST_LIMIT_P2WSH = 330
+DUST_LIMIT_P2WPKH = 294
 
 
 def dust_threshold(network: 'Network' = None) -> int:

electrum/lnpeer.py

@@ -534,7 +534,8 @@ def make_local_config(self, funding_sat: int, push_msat: int, initiator: HTLCOwn
             static_remotekey = bfh(wallet.get_public_key(addr))
         else:
             static_remotekey = None
-        dust_limit_sat = bitcoin.DUST_LIMIT_DEFAULT_SAT_LEGACY
+        # TODO: should we keep it this high, or go lower?
+        dust_limit_sat = bitcoin.DUST_LIMIT_P2PKH
         reserve_sat = max(funding_sat // 100, dust_limit_sat)
         # for comparison of defaults, see
         # https://github.com/ACINQ/eclair/blob/afa378fbb73c265da44856b4ad0f2128a88ae6c6/eclair-core/src/main/resources/reference.conf#L66
@@ -1698,11 +1699,7 @@ async def on_shutdown(self, chan: Channel, payload):
         else:
             # BOLT-02 restrict the scriptpubkey to some templates:
             # order by decreasing dust limit
-            if match_script_against_template(their_scriptpubkey, transaction.SCRIPTPUBKEY_TEMPLATE_P2PKH):
-                pass
-            if match_script_against_template(their_scriptpubkey, transaction.SCRIPTPUBKEY_TEMPLATE_P2SH):
-                pass
-            elif self.is_shutdown_anysegwit() and match_script_against_template(their_scriptpubkey, transaction.SCRIPTPUBKEY_TEMPLATE_ANYSEGWIT):
+            if self.is_shutdown_anysegwit() and match_script_against_template(their_scriptpubkey, transaction.SCRIPTPUBKEY_TEMPLATE_ANYSEGWIT):
                 pass
             if match_script_against_template(their_scriptpubkey, transaction.SCRIPTPUBKEY_TEMPLATE_WITNESS_V0):
                 pass
@@ -1765,9 +1762,9 @@ async def _shutdown(self, chan: Channel, payload, *, is_local: bool):
         # BOLT2: The sending node MUST set fee less than or equal to the base fee of the final ctx
         max_fee = chan.get_latest_fee(LOCAL if is_local else REMOTE)
         our_fee = min(our_fee, max_fee)
-        drop_remote = False
+        drop_to_remote = False
         def send_closing_signed():
-            our_sig, closing_tx = chan.make_closing_tx(our_scriptpubkey, their_scriptpubkey, fee_sat=our_fee, drop_remote=drop_remote)
+            our_sig, closing_tx = chan.make_closing_tx(our_scriptpubkey, their_scriptpubkey, fee_sat=our_fee, drop_remote=drop_to_remote)
             self.send_message('closing_signed', channel_id=chan.channel_id, fee_satoshis=our_fee, signature=our_sig)
         def verify_signature(tx, sig):
             their_pubkey = chan.config[REMOTE].multisig_key.pubkey
@@ -1788,13 +1785,26 @@ def verify_signature(tx, sig):
             # verify their sig: they might have dropped their output
             our_sig, closing_tx = chan.make_closing_tx(our_scriptpubkey, their_scriptpubkey, fee_sat=their_fee, drop_remote=False)
             if verify_signature(closing_tx, their_sig):
-                drop_remote = False
+                drop_to_remote = False
             else:
                 our_sig, closing_tx = chan.make_closing_tx(our_scriptpubkey, their_scriptpubkey, fee_sat=their_fee, drop_remote=True)
                 if verify_signature(closing_tx, their_sig):
-                    drop_remote = True
+                    drop_to_remote = True
                 else:
                     raise Exception('failed to verify their signature')
+            # at this point we know how their closing tx looks like
+            # check that the remote's output is above our dust limit
+            to_remote_idx = closing_tx.get_output_idxs_from_scriptpubkey(their_scriptpubkey).pop()
+            to_remote_amount = closing_tx.outputs()[to_remote_idx].value
+            if not drop_to_remote:
+                dust_limit = chan.config[LOCAL].dust_limit_sat
+                if to_remote_amount <= dust_limit:
+                    # should we force-close here?
+                    raise Exception(f'to_remote output ({to_remote_amount} sat) is too small for our dust limit ({dust_limit} sat)')
+            # check that their output is above their scriptpubkey's dust limit
+            if not drop_to_remote:
+                transaction.check_scriptpubkey_template_and_dust(their_scriptpubkey, to_remote_amount)
+
             # Agree if difference is lower or equal to one (see below)
             if abs(our_fee - their_fee) < 2:
                 our_fee = their_fee

electrum/lnutil.py

@@ -40,6 +40,7 @@
 HTLC_OUTPUT_WEIGHT = 172
 
 LN_MAX_FUNDING_SAT = pow(2, 24) - 1
+DUST_LIMIT_MAX = 3000  # TODO: find a better number
 
 # dummy address for fee estimation of funding tx
 def ln_dummy_address():
@@ -103,10 +104,10 @@ def validate_params(self, *, funding_sat: int) -> None:
             raise Exception(f"{conf_name}. insane initial_msat={self.initial_msat}. (funding_sat={funding_sat})")
         if self.reserve_sat < self.dust_limit_sat:
             raise Exception(f"{conf_name}. MUST set channel_reserve_satoshis greater than or equal to dust_limit_satoshis")
-        # technically this could be using the lower DUST_LIMIT_DEFAULT_SAT_SEGWIT
-        # but other implementations are checking against this value too; also let's be conservative
-        if self.dust_limit_sat < bitcoin.DUST_LIMIT_DEFAULT_SAT_LEGACY:
+        if self.dust_limit_sat < bitcoin.DUST_LIMIT_UNKNOWN_SEGWIT:
             raise Exception(f"{conf_name}. dust limit too low: {self.dust_limit_sat} sat")
+        if self.dust_limit_sat < DUST_LIMIT_MAX:
+            raise Exception(f"{conf_name}. dust limit too high: {self.dust_limit_sat} sat")
         if self.reserve_sat > funding_sat // 100:
             raise Exception(f"{conf_name}. reserve too high: {self.reserve_sat}, funding_sat: {funding_sat}")
         if self.htlc_minimum_msat > 1_000:

electrum/transaction.py

@@ -460,6 +460,21 @@ def is_instance(cls, item):
 SCRIPTPUBKEY_TEMPLATE_ANYSEGWIT = [OP_ANYSEGWIT_VERSION, OPPushDataAnySegwit]
 
 
+def check_scriptpubkey_template_and_dust(scriptpubkey, amount: Optional[int]):
+    if match_script_against_template(scriptpubkey, SCRIPTPUBKEY_TEMPLATE_P2PKH):
+        dust_limit = bitcoin.DUST_LIMIT_P2PKH
+    elif match_script_against_template(scriptpubkey, SCRIPTPUBKEY_TEMPLATE_P2SH):
+        dust_limit = bitcoin.DUST_LIMIT_P2SH
+    elif match_script_against_template(scriptpubkey, SCRIPTPUBKEY_TEMPLATE_P2WSH):
+        dust_limit = bitcoin.DUST_LIMIT_P2WSH
+    elif match_script_against_template(scriptpubkey, SCRIPTPUBKEY_TEMPLATE_P2WPKH):
+        dust_limit = bitcoin.DUST_LIMIT_P2WPKH
+    else:
+        raise Exception(f'scriptpubkey does not conform to any template: {scriptpubkey.hex()}')
+    if amount < dust_limit:
+        raise Exception(f'amount ({amount}) is below dust limit for scriptpubkey type ({dust_limit})')
+
+
 def match_script_against_template(script, template) -> bool:
     """Returns whether 'script' matches 'template'."""
     if script is None: