Metadata API: add Metadata.signed_bytes property

- Add shortcut to canonical json representation of self.signed
- Use in tests and Metadata.sign
- Do not use in _Delegator.verify_delegate (will be updated in subsequent
  commit).

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

tests/test_api.py

@@ -43,7 +43,7 @@
     Timestamp,
 )
 from tuf.api.serialization import DeserializationError, SerializationError
-from tuf.api.serialization.json import CanonicalJSONSerializer, JSONSerializer
+from tuf.api.serialization.json import JSONSerializer
 
 logger = logging.getLogger(__name__)
 
@@ -202,7 +202,7 @@ def test_sign_verify(self) -> None:
         # Load sample metadata (targets) and assert ...
         md_obj = Metadata.from_file(os.path.join(path, "targets.json"))
         sig = md_obj.signatures[targets_keyid]
-        data = CanonicalJSONSerializer().serialize(md_obj.signed)
+        data = md_obj.signed_bytes
 
         # ... it has a single existing signature,
         self.assertEqual(len(md_obj.signatures), 1)
@@ -257,7 +257,7 @@ def test_key_verify_failures(self) -> None:
         path = os.path.join(self.repo_dir, "metadata", "timestamp.json")
         md_obj = Metadata.from_file(path)
         sig = md_obj.signatures[timestamp_keyid]
-        data = CanonicalJSONSerializer().serialize(md_obj.signed)
+        data = md_obj.signed_bytes
 
         # Test failure on unknown scheme (securesystemslib
         # UnsupportedAlgorithmError)

tuf/api/metadata.py

@@ -150,6 +150,16 @@ def __eq__(self, other: Any) -> bool:
             and self.unrecognized_fields == other.unrecognized_fields
         )
 
+    @property
+    def signed_bytes(self) -> bytes:
+        """Default canonical json byte representation of ``self.signed``."""
+
+        # Use local scope import to avoid circular import errors
+        # pylint: disable=import-outside-toplevel
+        from tuf.api.serialization.json import CanonicalJSONSerializer
+
+        return CanonicalJSONSerializer().serialize(self.signed)
+
     @classmethod
     def from_dict(cls, metadata: Dict[str, Any]) -> "Metadata[T]":
         """Create ``Metadata`` object from its json/dict representation.
@@ -366,13 +376,9 @@ def sign(
         """
 
         if signed_serializer is None:
-            # Use local scope import to avoid circular import errors
-            # pylint: disable=import-outside-toplevel
-            from tuf.api.serialization.json import CanonicalJSONSerializer
-
-            signed_serializer = CanonicalJSONSerializer()
-
-        bytes_data = signed_serializer.serialize(self.signed)
+            bytes_data = self.signed_bytes
+        else:
+            bytes_data = signed_serializer.serialize(self.signed)
 
         try:
             signature = signer.sign(bytes_data)