Fix calling .dump(force=True) on a core.Sequence() with no field info

For instance, when parsing a CMS structure, we may not know the various
OIDs and the Sequence values they imply. Before this change, this could
result in the unknown Sequence values being encoded as an empty value.

asn1crypto/core.py

@@ -4113,6 +4113,10 @@ def dump(self, force=False):
         if self._header is not None and self._header[-1:] == b'\x80':
             force = True
 
+        # We can't force encoding if we don't have a spec
+        if force and self._fields == [] and self.__class__ is Sequence:
+            force = False
+
         if force:
             self._set_contents(force=force)
 

tests/test_core.py

@@ -978,6 +978,18 @@ def test_dump_set(self):
         st = SetTest({'two': 2, 'one': 1})
         self.assertEqual(b'1\x06\x81\x01\x01\x82\x01\x02', st.dump())
 
+    def test_force_dump_unknown_sequence(self):
+        seq = Seq({
+            'id': '1.2.3',
+            'value': 1
+        })
+        der = seq.dump(force=True)
+        # Ensure we don't erase the contents of a sequence we don't know
+        # the fields for when force re-encoding
+        unknown_seq = core.Sequence.load(der)
+        unknown_der = unknown_seq.dump(force=True)
+        self.assertEqual(der, unknown_der)
+
     def test_dump_set_of(self):
         st = SetOfTest([3, 2, 1])
         self.assertEqual(b'1\x09\x02\x01\x01\x02\x01\x02\x02\x01\x03', st.dump())