Metadata API: Make sure Signed.expires is UTC

* Most importantly use strftime() to serialize the datetime
* Force the timezone as UTC when deserializing

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

tests/generated_data/generate_md.py

@@ -5,7 +5,7 @@
 
 import os
 import sys
-from datetime import datetime
+from datetime import UTC, datetime
 from typing import Dict, List, Optional
 
 from securesystemslib.signer import SSlibKey, SSlibSigner
@@ -48,8 +48,7 @@
         }
     )
 
-expires_str = "2050-01-01T00:00:00Z"
-EXPIRY = datetime.strptime(expires_str, "%Y-%m-%dT%H:%M:%SZ")
+EXPIRY = datetime(2050, 1, 1, tzinfo=UTC)
 OUT_DIR = "generated_data/ed25519_metadata"
 if not os.path.exists(OUT_DIR):
     os.mkdir(OUT_DIR)

tests/test_api.py

@@ -313,7 +313,8 @@ def test_metadata_signed_is_expired(self) -> None:
         snapshot_path = os.path.join(self.repo_dir, "metadata", "snapshot.json")
         md = Metadata.from_file(snapshot_path)
 
-        self.assertEqual(md.signed.expires, datetime(2030, 1, 1, 0, 0))
+        expected_expiry = datetime(2030, 1, 1, 0, 0, tzinfo=timezone.utc)
+        self.assertEqual(md.signed.expires, expected_expiry)
 
         # Test is_expired with reference_time provided
         is_expired = md.signed.is_expired(md.signed.expires)

tests/test_trusted_metadata_set.py

@@ -4,7 +4,7 @@
 import os
 import sys
 import unittest
-from datetime import datetime
+from datetime import UTC, datetime
 from typing import Callable, ClassVar, Dict, List, Optional, Tuple
 
 from securesystemslib.interface import (
@@ -279,7 +279,7 @@ def test_update_root_new_root_ver_same_as_trusted_root_ver(self) -> None:
 
     def test_root_expired_final_root(self) -> None:
         def root_expired_modifier(root: Root) -> None:
-            root.expires = datetime(1970, 1, 1)
+            root.expires = datetime(1970, 1, 1, tzinfo=UTC)
 
         # intermediate root can be expired
         root = self.modify_metadata(Root.type, root_expired_modifier)
@@ -329,7 +329,7 @@ def bump_snapshot_version(timestamp: Timestamp) -> None:
     def test_update_timestamp_expired(self) -> None:
         # new_timestamp has expired
         def timestamp_expired_modifier(timestamp: Timestamp) -> None:
-            timestamp.expires = datetime(1970, 1, 1)
+            timestamp.expires = datetime(1970, 1, 1, tzinfo=UTC)
 
         # expired intermediate timestamp is loaded but raises
         timestamp = self.modify_metadata(
@@ -406,7 +406,7 @@ def test_update_snapshot_expired_new_snapshot(self) -> None:
         self.trusted_set.update_timestamp(self.metadata[Timestamp.type])
 
         def snapshot_expired_modifier(snapshot: Snapshot) -> None:
-            snapshot.expires = datetime(1970, 1, 1)
+            snapshot.expires = datetime(1970, 1, 1, tzinfo=UTC)
 
         # expired intermediate snapshot is loaded but will raise
         snapshot = self.modify_metadata(
@@ -486,7 +486,7 @@ def test_update_targets_expired_new_target(self) -> None:
 
         # new_delegated_target has expired
         def target_expired_modifier(target: Targets) -> None:
-            target.expires = datetime(1970, 1, 1)
+            target.expires = datetime(1970, 1, 1, tzinfo=UTC)
 
         targets = self.modify_metadata(Targets.type, target_expired_modifier)
         with self.assertRaises(exceptions.ExpiredMetadataError):

tuf/api/_payload.py

@@ -176,6 +176,7 @@ def _common_fields_from_dict(
         # what the constructor expects and what we store. The inverse operation
         # is implemented in '_common_fields_to_dict'.
         expires = datetime.strptime(expires_str, "%Y-%m-%dT%H:%M:%SZ")
+        expires = expires.replace(tzinfo=timezone.utc)
 
         return version, spec_version, expires
 
@@ -190,7 +191,7 @@ def _common_fields_to_dict(self) -> Dict[str, Any]:
             "_type": self._type,
             "version": self.version,
             "spec_version": self.spec_version,
-            "expires": self.expires.isoformat() + "Z",
+            "expires": self.expires.strftime("%Y-%m-%dT%H:%M:%SZ"),
             **self.unrecognized_fields,
         }