fix: reject underscores in derivation indexes

src/cryptoadvance/specter/helpers.py

@@ -232,6 +232,8 @@ def der_to_bytes(derivation):
         if item[-1] == "h" or item[-1] == "'":
             index += 0x80000000
             item = item[:-1]
+        if not item.isdigit():
+            raise ValueError(f"Invalid derivation index: {item}")
         index += int(item)
         res += index.to_bytes(4, "little")
     return res

tests/test_der_to_bytes.py

@@ -0,0 +1,21 @@
+import unittest
+
+from cryptoadvance.specter.helpers import der_to_bytes
+
+
+class TestDerToBytes(unittest.TestCase):
+    def test_underscore_is_rejected(self):
+        with self.assertRaises(ValueError):
+            der_to_bytes("m/1_0")
+
+    def test_plain_index(self):
+        self.assertEqual(der_to_bytes("m/10"), (10).to_bytes(4, "little"))
+
+    def test_hardened_index(self):
+        self.assertEqual(
+            der_to_bytes("m/1h"), (0x80000001).to_bytes(4, "little")
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()