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.

2 files changed, 16 insertions, 0 deletions

diff --git a/asn1crypto/core.py b/asn1crypto/core.py
index 7133367..364c6b5 100644
--- a/asn1crypto/core.py
+++ b/asn1crypto/core.py
@@ -4113,6 +4113,10 @@ class Sequence(Asn1Value):
         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)
 
diff --git a/tests/test_core.py b/tests/test_core.py
index 9821c63..7ac9196 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -978,6 +978,18 @@ class CoreTests(unittest.TestCase):
         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())