Add functionality to sign DSSE envelopes with arbitrary payloads

sigstore/dsse.py

@@ -1,4 +1,5 @@
 # Copyright 2022 The Sigstore Authors
+# Copyright (c) 2024, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -19,6 +20,7 @@
 from __future__ import annotations
 
 import logging
+from dataclasses import dataclass
 from typing import Any, Dict, List, Literal, Optional, Union
 
 from cryptography.exceptions import InvalidSignature
@@ -186,6 +188,17 @@ def build(self) -> Statement:
         return Statement(stmt.model_dump_json(by_alias=True).encode())
 
 
+@dataclass
+class RawPayload:
+    """
+    Represents a raw payload.
+
+    This type can be signed and wrapped into a `Envelope`.
+    """
+    type: str
+    data: bytes
+
+
 class Envelope:
     """
     Represents a DSSE envelope.
@@ -210,6 +223,26 @@ def _from_json(cls, contents: bytes | str) -> Envelope:
         inner = _Envelope().from_json(contents)
         return cls(inner)
 
+    @classmethod
+    def _from_payload(
+        cls, payload: RawPayload, sigs: list[Signature]
+    ) -> Envelope:
+        """Return an unsigned DSSE envelope.
+
+        Args:
+            payload_type (str): The envelope's payload type
+            payload (bytes): The envelope's payload
+
+        Returns:
+            Envelope: An unsigned DSSE envelope
+        """
+        inner = _Envelope(
+            payload=payload.data,
+            payload_type=payload.type,
+            signatures=sigs,
+        )
+        return cls(inner)
+
     def to_json(self) -> str:
         """
         Return a JSON string with this DSSE envelope's contents.
@@ -256,6 +289,19 @@ def _sign(key: ec.EllipticCurvePrivateKey, stmt: Statement) -> Envelope:
     )
 
 
+def _sign_payload(
+        key: ec.EllipticCurvePrivateKey, payload: RawPayload) -> Envelope:
+    """
+    Sign the given envelope's payload and set the signature field
+    with the generated signature.
+    """
+    pae = _pae(payload.payload_type, payload.payload)
+    signature = key.sign(pae, ec.ECDSA(hashes.SHA256()))
+    return Envelope._from_payload(
+        payload_=payload,
+        sigs=[Signature(sig=signature)])
+
+
 def _verify(key: ec.EllipticCurvePublicKey, evp: Envelope) -> bytes:
     """
     Verify the given in-toto `Envelope`, returning the verified inner payload.

sigstore/sign.py

@@ -1,4 +1,5 @@
 # Copyright 2022 The Sigstore Authors
+# Copyright (c) 2024, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -194,10 +195,10 @@ def _finalize_sign(
 
     def sign_dsse(
         self,
-        input_: dsse.Statement,
+        input_: dsse.Statement | dsse.RawPayload,
     ) -> Bundle:
         """
-        Sign the given in-toto statement as a DSSE envelope, and return a
+        Sign the given in-toto statement or DSSE envelope, and return a
         `Bundle` containing the signed result.
 
         This API is **only** for in-toto statements; to sign arbitrary artifacts,
@@ -211,22 +212,26 @@ def sign_dsse(
         )
 
         # Sign the statement, producing a DSSE envelope
-        content = dsse._sign(self._private_key, input_)
-
-        # Create the proposed DSSE log entry
-        proposed_entry = rekor_types.Dsse(
-            spec=rekor_types.dsse.DsseSchema(
-                # NOTE: mypy can't see that this kwarg is correct due to two interacting
-                # behaviors/bugs (one pydantic, one datamodel-codegen):
-                # See: <https://github.com/pydantic/pydantic/discussions/7418#discussioncomment-9024927>
-                # See: <https://github.com/koxudaxi/datamodel-code-generator/issues/1903>
-                proposed_content=rekor_types.dsse.ProposedContent(  # type: ignore[call-arg]
-                    envelope=content.to_json(),
-                    verifiers=[b64_cert.decode()],
+        content: dsse.Envelope = None
+        proposed_entry: rekor_types.ProposedEntry = None
+        if type(input_) is dsse.Statement:
+            content = dsse._sign(self._private_key, input_)
+            # Create the proposed DSSE log entry
+            proposed_entry = rekor_types.Dsse(
+                spec=rekor_types.dsse.DsseSchema(
+                    # NOTE: mypy can't see that this kwarg is correct due to two interacting
+                    # behaviors/bugs (one pydantic, one datamodel-codegen):
+                    # See: <https://github.com/pydantic/pydantic/discussions/7418#discussioncomment-9024927>
+                    # See: <https://github.com/koxudaxi/datamodel-code-generator/issues/1903>
+                    proposed_content=rekor_types.dsse.ProposedContent(  # type: ignore[call-arg]
+                        envelope=content.to_json(),
+                        verifiers=[b64_cert.decode()],
+                    ),
                 ),
-            ),
-        )
-
+            )
+        elif type(input_) is dsse.RawPayload:
+            content = dsse._sign_payload(self._private_key, input_)
+            # TODO: figure out an entry that works.
         return self._finalize_sign(cert, content, proposed_entry)
 
     def sign_artifact(

test/unit/test_sign.py

@@ -1,4 +1,5 @@
 # Copyright 2022 The Sigstore Authors
+# Copyright (c) 2024, NVIDIA CORPORATION.  All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -20,7 +21,7 @@
 from sigstore_protobuf_specs.dev.sigstore.common.v1 import HashAlgorithm
 
 import sigstore.oidc
-from sigstore.dsse import _StatementBuilder, _Subject
+from sigstore.dsse import _StatementBuilder, _Subject, RawPayload
 from sigstore.errors import VerificationError
 from sigstore.hashes import Hashed
 from sigstore.sign import SigningContext
@@ -169,3 +170,19 @@ def test_sign_dsse(staging):
         bundle = signer.sign_dsse(stmt)
         # Ensures that all of our inner types serialize as expected.
         bundle.to_json()
+
+
+@pytest.mark.staging
+@pytest.mark.ambient_oidc
+def test_sign_dsse_envelope(staging):
+    sign_ctx, _, identity = staging
+
+    ctx = sign_ctx()
+    payload = RawPayload(
+        payload_type="type/custom/my_payload",
+        payload=b"Hello World!")
+
+    with ctx.signer(identity) as signer:
+        bundle = signer.sign_dsse(payload)
+
+        bundle.to_json()