ecc: make libsecp256k1 "schnorrsig" module only required when used

It would be simple to hard fail at import time if any of the interesting
libsecp modules are missing, as it was done before this commit. However,
some Linux distros (atm current ubuntu lts, 22.04) lack new enough libsecp.
Also, for now, we don't use the schnorr APIs yet anyway. Until we start
to rely on them more, it is feasible to only require them when they are
used.

I am hoping we will be able to revert this commit later though, to keep
things simple.

electrum/ecc.py

@@ -35,7 +35,8 @@ from .util import bfh, assert_bytes, to_bytes, InvalidPassword, profiler, randra
 from .crypto import (sha256, sha256d, aes_encrypt_with_iv, aes_decrypt_with_iv, hmac_oneshot)
 from . import constants
 from .logging import get_logger
-from .ecc_fast import _libsecp256k1, SECP256K1_EC_UNCOMPRESSED
+from . import ecc_fast
+from .ecc_fast import _libsecp256k1, SECP256K1_EC_UNCOMPRESSED, LibModuleMissing
 
 _logger = get_logger(__name__)
 
@@ -251,6 +252,10 @@ class ECPubkey(object):
 
     def _to_libsecp256k1_xonly_pubkey_ptr(self):
         """pointer to `secp256k1_xonly_pubkey` C struct"""
+        if not ecc_fast.HAS_SCHNORR:
+            raise LibModuleMissing(
+                'libsecp256k1 library found but it was built '
+                'without required modules (--enable-module-schnorrsig --enable-module-extrakeys)')
         pubkey = create_string_buffer(64)
         pk_bytes = self.get_public_key_bytes(compressed=True)[1:]
         ret = _libsecp256k1.secp256k1_xonly_pubkey_parse(
@@ -359,6 +364,10 @@ class ECPubkey(object):
         assert len(sig64) == 64, len(sig64)
         assert isinstance(msg32, bytes), type(msg32)
         assert len(msg32) == 32, len(msg32)
+        if not ecc_fast.HAS_SCHNORR:
+            raise LibModuleMissing(
+                'libsecp256k1 library found but it was built '
+                'without required modules (--enable-module-schnorrsig --enable-module-extrakeys)')
         msglen = 32
         pubkey = self._to_libsecp256k1_xonly_pubkey_ptr()
         if 1 != _libsecp256k1.secp256k1_schnorrsig_verify(_libsecp256k1.ctx, sig64, msg32, msglen, pubkey):
@@ -534,6 +543,10 @@ class ECPrivkey(ECPubkey):
             aux_rand32 = bytes(32)
         assert isinstance(aux_rand32, bytes), type(aux_rand32)
         assert len(aux_rand32) == 32, len(aux_rand32)
+        if not ecc_fast.HAS_SCHNORR:
+            raise LibModuleMissing(
+                'libsecp256k1 library found but it was built '
+                'without required modules (--enable-module-schnorrsig --enable-module-extrakeys)')
         # construct "keypair" obj
         privkey_bytes = self.secret_scalar.to_bytes(32, byteorder="big")
         keypair = create_string_buffer(96)

electrum/ecc_fast.py

@@ -42,6 +42,8 @@ class LibModuleMissing(Exception): pass
 
 
 def load_library():
+    global HAS_SCHNORR
+
     # note: for a mapping between bitcoin-core/secp256k1 git tags and .so.V libtool version numbers,
     #       see https://github.com/bitcoin-core/secp256k1/pull/1055#issuecomment-1227505189
     tested_libversions = [2, 1, 0, ]  # try latest version first
@@ -140,8 +142,10 @@ def load_library():
             secp256k1.secp256k1_schnorrsig_verify.argtypes = [c_void_p, c_char_p, c_char_p, c_size_t, c_char_p]
             secp256k1.secp256k1_schnorrsig_verify.restype = c_int
         except (OSError, AttributeError):
-            raise LibModuleMissing('libsecp256k1 library found but it was built '
-                                   'without required module (--enable-module-schnorrsig)')
+            _logger.warning(f"libsecp256k1 library found but it was built without desired module (--enable-module-schnorrsig)")
+            HAS_SCHNORR = False
+            # raise LibModuleMissing('libsecp256k1 library found but it was built '
+            #                        'without required module (--enable-module-schnorrsig)')
 
         # --enable-module-extrakeys
         try:
@@ -154,8 +158,10 @@ def load_library():
             secp256k1.secp256k1_keypair_create.argtypes = [c_void_p, c_char_p, c_char_p]
             secp256k1.secp256k1_keypair_create.restype = c_int
         except (OSError, AttributeError):
-            raise LibModuleMissing('libsecp256k1 library found but it was built '
-                                   'without required module (--enable-module-extrakeys)')
+            _logger.warning(f"libsecp256k1 library found but it was built without desired module (--enable-module-extrakeys)")
+            HAS_SCHNORR = False
+            # raise LibModuleMissing('libsecp256k1 library found but it was built '
+            #                        'without required module (--enable-module-extrakeys)')
 
         secp256k1.ctx = secp256k1.secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY)
         ret = secp256k1.secp256k1_context_randomize(secp256k1.ctx, os.urandom(32))
@@ -170,6 +176,7 @@ def load_library():
 
 
 _libsecp256k1 = None
+HAS_SCHNORR = True
 try:
     _libsecp256k1 = load_library()
 except BaseException as e: