diff --git a/tests/test_api.py b/tests/test_api.py
index 544937ef36..dac4a008ed 100755
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -349,6 +349,37 @@ def test_metadata_targets(self):
         # Verify that data is updated
         self.assertEqual(targets.signed.targets[filename], fileinfo)
 
+    def setup_dict_with_unrecognized_field(self, file_path, field, value):
+        json_dict = {}
+        with open(file_path) as f:
+            json_dict = json.loads(f.read())
+        # We are changing the json dict without changing the signature.
+        # This could be a problem if we want to do verification on this dict.
+        json_dict["signed"][field] = value
+        return json_dict
+
+    def test_support_for_unrecognized_fields(self):
+        for metadata in ["root", "timestamp", "snapshot", "targets"]:
+            path = os.path.join(self.repo_dir, "metadata", metadata + ".json")
+            dict1 = self.setup_dict_with_unrecognized_field(path, "f", "b")
+            # Test that the metadata classes store unrecognized fields when
+            # initializing and passes them when casting the instance to a dict.
+
+            temp_copy = copy.deepcopy(dict1)
+            metadata_obj = Metadata.from_dict(temp_copy)
+
+            self.assertEqual(dict1["signed"], metadata_obj.signed.to_dict())
+
+            # Test that two instances of the same class could have different
+            # unrecognized fields.
+            dict2 = self.setup_dict_with_unrecognized_field(path, "f2", "b2")
+            temp_copy2 = copy.deepcopy(dict2)
+            metadata_obj2 = Metadata.from_dict(temp_copy2)
+            self.assertNotEqual(
+                metadata_obj.signed.to_dict(), metadata_obj2.signed.to_dict()
+            )
+
+
 # Run unit test.
 if __name__ == '__main__':
     utils.configure_test_logging(sys.argv)
diff --git a/tuf/api/metadata.py b/tuf/api/metadata.py
index 86d1b7df9e..e28310ae56 100644
--- a/tuf/api/metadata.py
+++ b/tuf/api/metadata.py
@@ -164,7 +164,7 @@ def from_bytes(
         return deserializer.deserialize(data)
 
     def to_dict(self) -> Dict[str, Any]:
-        """Returns the dict representation of self. """
+        """Returns the dict representation of self."""
 
         signatures = []
         for sig in self.signatures:
@@ -320,6 +320,7 @@ class Signed:
         spec_version: The TUF specification version number (semver) the
             metadata format adheres to.
         expires: The metadata expiration datetime object.
+        unrecognized_fields: Dictionary of all unrecognized fields.
 
     """
 
@@ -327,7 +328,12 @@ class Signed:
     # we keep it to match spec terminology (I often refer to this as "payload",
     # or "inner metadata")
     def __init__(
-        self, _type: str, version: int, spec_version: str, expires: datetime
+        self,
+        _type: str,
+        version: int,
+        spec_version: str,
+        expires: datetime,
+        unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
 
         self._type = _type
@@ -335,9 +341,10 @@ def __init__(
         self.expires = expires
 
         # TODO: Should we separate data validation from constructor?
-        if version < 0:
-            raise ValueError(f"version must be >= 0, got {version}")
+        if version <= 0:
+            raise ValueError(f"version must be > 0, got {version}")
         self.version = version
+        self.unrecognized_fields = unrecognized_fields or {}
 
     @staticmethod
     def _common_fields_from_dict(signed_dict: Mapping[str, Any]) -> list:
@@ -369,6 +376,7 @@ def _common_fields_to_dict(self) -> Dict[str, Any]:
             "version": self.version,
             "spec_version": self.spec_version,
             "expires": self.expires.isoformat() + "Z",
+            **self.unrecognized_fields,
         }
 
     def is_expired(self, reference_time: datetime = None) -> bool:
@@ -389,7 +397,7 @@ def is_expired(self, reference_time: datetime = None) -> bool:
 
     # Modification.
     def bump_expiration(self, delta: timedelta = timedelta(days=1)) -> None:
-        """Increments the expires attribute by the passed timedelta. """
+        """Increments the expires attribute by the passed timedelta."""
         self.expires += delta
 
     def bump_version(self) -> None:
@@ -445,8 +453,11 @@ def __init__(
         consistent_snapshot: bool,
         keys: Mapping[str, Any],
         roles: Mapping[str, Any],
+        unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(_type, version, spec_version, expires)
+        super().__init__(
+            _type, version, spec_version, expires, unrecognized_fields
+        )
         # TODO: Add classes for keys and roles
         self.consistent_snapshot = consistent_snapshot
         self.keys = keys
@@ -454,15 +465,16 @@ def __init__(
 
     @classmethod
     def from_dict(cls, root_dict: Mapping[str, Any]) -> "Root":
-        """Creates Root object from its dict representation. """
+        """Creates Root object from its dict representation."""
         common_args = cls._common_fields_from_dict(root_dict)
         consistent_snapshot = root_dict.pop("consistent_snapshot")
         keys = root_dict.pop("keys")
         roles = root_dict.pop("roles")
-        return cls(*common_args, consistent_snapshot, keys, roles)
+        # All fields left in the root_dict are unrecognized.
+        return cls(*common_args, consistent_snapshot, keys, roles, root_dict)
 
     def to_dict(self) -> Dict[str, Any]:
-        """Returns the dict representation of self. """
+        """Returns the dict representation of self."""
         root_dict = self._common_fields_to_dict()
         root_dict.update(
             {
@@ -477,14 +489,14 @@ def to_dict(self) -> Dict[str, Any]:
     def add_key(
         self, role: str, keyid: str, key_metadata: Mapping[str, Any]
     ) -> None:
-        """Adds new key for 'role' and updates the key store. """
+        """Adds new key for 'role' and updates the key store."""
         if keyid not in self.roles[role]["keyids"]:
             self.roles[role]["keyids"].append(keyid)
             self.keys[keyid] = key_metadata
 
     # Remove key for a role.
     def remove_key(self, role: str, keyid: str) -> None:
-        """Removes key for 'role' and updates the key store. """
+        """Removes key for 'role' and updates the key store."""
         if keyid in self.roles[role]["keyids"]:
             self.roles[role]["keyids"].remove(keyid)
             for keyinfo in self.roles.values():
@@ -521,20 +533,24 @@ def __init__(
         spec_version: str,
         expires: datetime,
         meta: Mapping[str, Any],
+        unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(_type, version, spec_version, expires)
+        super().__init__(
+            _type, version, spec_version, expires, unrecognized_fields
+        )
         # TODO: Add class for meta
         self.meta = meta
 
     @classmethod
     def from_dict(cls, timestamp_dict: Mapping[str, Any]) -> "Timestamp":
-        """Creates Timestamp object from its dict representation. """
+        """Creates Timestamp object from its dict representation."""
         common_args = cls._common_fields_from_dict(timestamp_dict)
         meta = timestamp_dict.pop("meta")
-        return cls(*common_args, meta)
+        # All fields left in the timestamp_dict are unrecognized.
+        return cls(*common_args, meta, timestamp_dict)
 
     def to_dict(self) -> Dict[str, Any]:
-        """Returns the dict representation of self. """
+        """Returns the dict representation of self."""
         timestamp_dict = self._common_fields_to_dict()
         timestamp_dict.update({"meta": self.meta})
         return timestamp_dict
@@ -543,7 +559,7 @@ def to_dict(self) -> Dict[str, Any]:
     def update(
         self, version: int, length: int, hashes: Mapping[str, Any]
     ) -> None:
-        """Assigns passed info about snapshot metadata to meta dict. """
+        """Assigns passed info about snapshot metadata to meta dict."""
         self.meta["snapshot.json"] = {
             "version": version,
             "length": length,
@@ -585,20 +601,24 @@ def __init__(
         spec_version: str,
         expires: datetime,
         meta: Mapping[str, Any],
+        unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(_type, version, spec_version, expires)
+        super().__init__(
+            _type, version, spec_version, expires, unrecognized_fields
+        )
         # TODO: Add class for meta
         self.meta = meta
 
     @classmethod
     def from_dict(cls, snapshot_dict: Mapping[str, Any]) -> "Snapshot":
-        """Creates Snapshot object from its dict representation. """
+        """Creates Snapshot object from its dict representation."""
         common_args = cls._common_fields_from_dict(snapshot_dict)
         meta = snapshot_dict.pop("meta")
-        return cls(*common_args, meta)
+        # All fields left in the snapshot_dict are unrecognized.
+        return cls(*common_args, meta, snapshot_dict)
 
     def to_dict(self) -> Dict[str, Any]:
-        """Returns the dict representation of self. """
+        """Returns the dict representation of self."""
         snapshot_dict = self._common_fields_to_dict()
         snapshot_dict.update({"meta": self.meta})
         return snapshot_dict
@@ -611,7 +631,7 @@ def update(
         length: Optional[int] = None,
         hashes: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        """Assigns passed (delegated) targets role info to meta dict. """
+        """Assigns passed (delegated) targets role info to meta dict."""
         metadata_fn = f"{rolename}.json"
 
         self.meta[metadata_fn] = {"version": version}
@@ -688,22 +708,26 @@ def __init__(
         expires: datetime,
         targets: Mapping[str, Any],
         delegations: Mapping[str, Any],
+        unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(_type, version, spec_version, expires)
+        super().__init__(
+            _type, version, spec_version, expires, unrecognized_fields
+        )
         # TODO: Add class for meta
         self.targets = targets
         self.delegations = delegations
 
     @classmethod
     def from_dict(cls, targets_dict: Mapping[str, Any]) -> "Targets":
-        """Creates Targets object from its dict representation. """
+        """Creates Targets object from its dict representation."""
         common_args = cls._common_fields_from_dict(targets_dict)
         targets = targets_dict.pop("targets")
         delegations = targets_dict.pop("delegations")
-        return cls(*common_args, targets, delegations)
+        # All fields left in the targets_dict are unrecognized.
+        return cls(*common_args, targets, delegations, targets_dict)
 
     def to_dict(self) -> Dict[str, Any]:
-        """Returns the dict representation of self. """
+        """Returns the dict representation of self."""
         targets_dict = self._common_fields_to_dict()
         targets_dict.update(
             {
@@ -715,5 +739,5 @@ def to_dict(self) -> Dict[str, Any]:
 
     # Modification.
     def update(self, filename: str, fileinfo: Mapping[str, Any]) -> None:
-        """Assigns passed target file info to meta dict. """
+        """Assigns passed target file info to meta dict."""
         self.targets[filename] = fileinfo
diff --git a/tuf/api/serialization/__init__.py b/tuf/api/serialization/__init__.py
index ed3191a103..ed3a4843c0 100644
--- a/tuf/api/serialization/__init__.py
+++ b/tuf/api/serialization/__init__.py
@@ -19,35 +19,35 @@
 
 # TODO: Should these be in tuf.exceptions or inherit from tuf.exceptions.Error?
 class SerializationError(Exception):
-    """Error during serialization. """
+    """Error during serialization."""
 
 
 class DeserializationError(Exception):
-    """Error during deserialization. """
+    """Error during deserialization."""
 
 
 class MetadataDeserializer(metaclass=abc.ABCMeta):
-    """Abstract base class for deserialization of Metadata objects. """
+    """Abstract base class for deserialization of Metadata objects."""
 
     @abc.abstractmethod
     def deserialize(self, raw_data: bytes) -> "Metadata":
-        """Deserialize passed bytes to Metadata object. """
+        """Deserialize passed bytes to Metadata object."""
         raise NotImplementedError
 
 
 class MetadataSerializer(metaclass=abc.ABCMeta):
-    """Abstract base class for serialization of Metadata objects. """
+    """Abstract base class for serialization of Metadata objects."""
 
     @abc.abstractmethod
     def serialize(self, metadata_obj: "Metadata") -> bytes:
-        """Serialize passed Metadata object to bytes. """
+        """Serialize passed Metadata object to bytes."""
         raise NotImplementedError
 
 
 class SignedSerializer(metaclass=abc.ABCMeta):
-    """Abstract base class for serialization of Signed objects. """
+    """Abstract base class for serialization of Signed objects."""
 
     @abc.abstractmethod
     def serialize(self, signed_obj: "Signed") -> bytes:
-        """Serialize passed Signed object to bytes. """
+        """Serialize passed Signed object to bytes."""
         raise NotImplementedError
diff --git a/tuf/api/serialization/json.py b/tuf/api/serialization/json.py
index 3c7828ae9f..3c223d0053 100644
--- a/tuf/api/serialization/json.py
+++ b/tuf/api/serialization/json.py
@@ -28,10 +28,10 @@
 
 
 class JSONDeserializer(MetadataDeserializer):
-    """Provides JSON to Metadata deserialize method. """
+    """Provides JSON to Metadata deserialize method."""
 
     def deserialize(self, raw_data: bytes) -> Metadata:
-        """Deserialize utf-8 encoded JSON bytes into Metadata object. """
+        """Deserialize utf-8 encoded JSON bytes into Metadata object."""
         try:
             json_dict = json.loads(raw_data.decode("utf-8"))
             metadata_obj = Metadata.from_dict(json_dict)
@@ -55,7 +55,7 @@ def __init__(self, compact: bool = False) -> None:
         self.compact = compact
 
     def serialize(self, metadata_obj: Metadata) -> bytes:
-        """Serialize Metadata object into utf-8 encoded JSON bytes. """
+        """Serialize Metadata object into utf-8 encoded JSON bytes."""
         try:
             indent = None if self.compact else 1
             separators = (",", ":") if self.compact else (",", ": ")
@@ -73,7 +73,7 @@ def serialize(self, metadata_obj: Metadata) -> bytes:
 
 
 class CanonicalJSONSerializer(SignedSerializer):
-    """Provides Signed to OLPC Canonical JSON serialize method. """
+    """Provides Signed to OLPC Canonical JSON serialize method."""
 
     def serialize(self, signed_obj: Signed) -> bytes:
         """Serialize Signed object into utf-8 encoded OLPC Canonical JSON
diff --git a/tuf/client_rework/download.py b/tuf/client_rework/download.py
index 858355523f..ec7b3e1ec0 100644
--- a/tuf/client_rework/download.py
+++ b/tuf/client_rework/download.py
@@ -90,7 +90,7 @@ def download_file(url, required_length, fetcher, strict_required_length=True):
 
     # This is the temporary file that we will return to contain the contents of
     # the downloaded file.
-    temp_file = tempfile.TemporaryFile()
+    temp_file = tempfile.TemporaryFile()  # pylint: disable=consider-using-with
 
     average_download_speed = 0
     number_of_bytes_received = 0