Add SigstoreSignature for storing Sigstore signatures

Signed-off-by: Mihai Maruseac <mihaimaruseac@google.com>

model_signing/signing/in_toto.py

@@ -132,7 +132,7 @@ def _convert_descriptors_to_hashed_statement(
     *,
     predicate_type: str,
     predicate_top_level_name: str,
-):
+) -> statement.Statement:
     """Converts manifest descriptors to an in-toto statement with payload.
 
     Args:
@@ -359,7 +359,7 @@ def from_manifest(cls, manifest: manifest_module.Manifest) -> Self:
 
 def _convert_descriptors_to_direct_statement(
     manifest: manifest_module.Manifest, predicate_type: str
-):
+) -> statement.Statement:
     """Converts manifest descriptors to an in-toto statement, as subjects.
 
     Args:

model_signing/signing/sigstore.py

@@ -0,0 +1,68 @@
+# Copyright 2024 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Sigstore based signature, signers and verifiers."""
+
+import pathlib
+from typing import Self
+
+from sigstore import models as sigstore_models
+from typing_extensions import override
+
+from model_signing.signing import signing
+
+
+class SigstoreSignature(signing.Signature):
+    """Sigstore signature support, wrapping around `sigstore_models.Bundle`."""
+
+    def __init__(self, bundle: sigstore_models.Bundle):
+        """Builds an instance of this signature.
+
+        Args:
+            bundle: the Sigstore `Bundle` to wrap around.
+        """
+        self.bundle = bundle
+
+    @override
+    def write(self, path: pathlib.Path) -> None:
+        """Writes the signature to disk, to the given path.
+
+        The Sigstore `Bundle` is written in JSON format, per the
+        canonicalization defined by the `sigstore-python` library.
+
+        Args:
+            path: the path to write the signature to.
+        """
+        path.write_text(self.bundle.to_json())
+
+    @classmethod
+    @override
+    def read(cls, path: pathlib.Path) -> Self:
+        """Reads the signature from disk.
+
+        Does not perform any signature verification, except what is needed to
+        parse the signature file.
+
+        Args:
+            path: the path to read the signature from.
+
+        Returns:
+            A `SigstoreSignature` object wrapping a Sigstore `Bundle`.
+
+        Raises:
+            ValueError: If the Sigstore `Bundle` could not be deserialized from
+              the contents of the file pointed to by `path`.
+        """
+        content = path.read_text()
+        return cls(sigstore_models.Bundle.from_json(content))