only require libsecp256k1 if lightning is enabled

related: #5606

electrum/ecc_fast.py

@@ -186,8 +186,10 @@ def _prepare_monkey_patching_of_python_ecdsa_internals_with_libsecp256k1():
 
 def do_monkey_patching_of_python_ecdsa_internals_with_libsecp256k1():
     if not _libsecp256k1:
-        raise Exception('libsecp256k1 library not available. '
-                        'Verifying Lightning channels is too computationally expensive without libsecp256k1, aborting.')
+        # FIXME logging 'verbosity' is not yet initialised
+        _logger.info('libsecp256k1 library not available, falling back to python-ecdsa. '
+                     'This means signing operations will be slower.')
+        return
     if not _patched_functions.prepared_to_patch:
         raise Exception("can't patch python-ecdsa without preparations")
     ecdsa.ecdsa.Private_key.sign      = _patched_functions.fast_sign

electrum/lnworker.py

@@ -36,6 +36,7 @@ from .lntransport import LNTransport, LNResponderTransport
 from .lnpeer import Peer, LN_P2P_NETWORK_TIMEOUT
 from .lnaddr import lnencode, LnAddr, lndecode
 from .ecc import der_sig_from_sig_string
+from .ecc_fast import is_using_fast_ecc
 from .lnchannel import Channel, ChannelJsonEncoder
 from . import lnutil
 from .lnutil import (Outpoint, calc_short_channel_id, LNPeerAddr,
@@ -251,6 +252,7 @@ class LNGossip(LNWorker):
         self.localfeatures |= LnLocalFeatures.GOSSIP_QUERIES_OPT
         self.localfeatures |= LnLocalFeatures.GOSSIP_QUERIES_REQ
         self.unknown_ids = set()
+        assert is_using_fast_ecc(), "verifying LN gossip msgs without libsecp256k1 is hopeless"
 
     def start_network(self, network: 'Network'):
         super().start_network(network)