bitcoin: stricter check on WIF for compressed pubkeys

fixes #5290

electrum/bitcoin.py

@@ -578,7 +578,13 @@ def deserialize_privkey(key: str) -> Tuple[str, bytes, bool]:
 
     if len(vch) not in [33, 34]:
         raise BitcoinException('invalid vch len for WIF key: {}'.format(len(vch)))
-    compressed = len(vch) == 34
+    compressed = False
+    if len(vch) == 34:
+        if vch[33] == 0x01:
+            compressed = True
+        else:
+            raise BitcoinException(f'invalid WIF key. length suggests compressed pubkey, '
+                                   f'but last byte is {vch[33]} != 0x01')
 
     if is_segwit_script_type(txin_type) and not compressed:
         raise BitcoinException('only compressed public keys can be used in segwit scripts')

electrum/tests/test_bitcoin.py

@@ -742,6 +742,12 @@ def test_segwit_uncompressed_pubkey(self):
             is_private_key("p2wpkh-p2sh:5JKXxT3wAZHcybJ9YNkuHur9vou6uuAnorBV9A8vVxGNFH5wvTW",
                            raise_on_error=True)
 
+    @needs_test_with_all_ecc_implementations
+    def test_wif_with_invalid_magic_byte_for_compressed_pubkey(self):
+        with self.assertRaises(BitcoinException):
+            is_private_key("KwFAa6AumokBD2dVqQLPou42jHiVsvThY1n25HJ8Ji8REf1wxAQb",
+                           raise_on_error=True)
+
 
 class TestBaseEncode(SequentialTestCase):