Envelope API: add dsse equivalent for Metadata API

Add Envelope class with basic de/serialization methods, currently
hardcoded to JSON.

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

Author: Lukas Puehringer

tuf/api/dsse.py

@@ -0,0 +1,96 @@
+"""Low-level TUF Envelope API.
+
+"""
+import json
+from typing import Generic, Type, cast
+
+from securesystemslib.dsse import Envelope as BaseEnvelope
+
+from tuf.api._payload import (  # noqa: F401
+    _ROOT,
+    _SNAPSHOT,
+    _TARGETS,
+    _TIMESTAMP,
+    SPECIFICATION_VERSION,
+    TOP_LEVEL_ROLE_NAMES,
+    BaseFile,
+    DelegatedRole,
+    Delegations,
+    MetaFile,
+    Role,
+    Root,
+    RootVerificationResult,
+    Signed,
+    Snapshot,
+    SuccinctRoles,
+    T,
+    TargetFile,
+    Targets,
+    Timestamp,
+    VerificationResult,
+)
+from tuf.api.serialization import DeserializationError, SerializationError
+
+
+class Envelope(Generic[T], BaseEnvelope):
+    """TODO: doc"""
+
+    _DEFAULT_PAYLOAD_TYPE = "application/vnd.tuf+json"
+
+    @classmethod
+    def from_bytes(cls, data: bytes) -> "Envelope[T]":
+        """TODO: doc"""
+        try:
+            envelope_dict = json.loads(data.decode())
+            envelope = Envelope.from_dict(envelope_dict)
+
+        except Exception as e:
+            raise SerializationError from e
+
+        return envelope
+
+    def to_bytes(self) -> bytes:
+        """TODO: doc"""
+        try:
+            envelope_dict = self.to_dict()
+            json_bytes = json.dumps(envelope_dict).encode()
+
+        except Exception as e:
+            raise SerializationError from e
+
+        return json_bytes
+
+    @classmethod
+    def from_signed(cls, signed: T) -> "Envelope[T]":
+        """TODO: doc"""
+        try:
+            signed_dict = signed.to_dict()
+            json_bytes = json.dumps(signed_dict).encode()
+
+        except Exception as e:
+            raise SerializationError from e
+
+        return cls(json_bytes, cls._DEFAULT_PAYLOAD_TYPE, [])
+
+    def get_signed(self) -> T:
+        """TODO: doc"""
+        try:
+            payload_dict = json.loads(self.payload.decode())
+
+            # TODO: can we move this to tuf.api._payload?
+            _type = payload_dict["_type"]
+            if _type == _TARGETS:
+                inner_cls: Type[Signed] = Targets
+            elif _type == _SNAPSHOT:
+                inner_cls = Snapshot
+            elif _type == _TIMESTAMP:
+                inner_cls = Timestamp
+            elif _type == _ROOT:
+                inner_cls = Root
+            else:
+                raise ValueError(f'unrecognized metadata type "{_type}"')
+
+        except Exception as e:
+            raise DeserializationError from e
+
+        return cast(T, inner_cls.from_dict(payload_dict))