Initial DSSE verify APIs

sigstore/_cli.py

@@ -813,7 +813,7 @@ def _verify_identity(args: argparse.Namespace) -> None:
         )
 
         try:
-            verifier.verify(
+            verifier.verify_artifact(
                 input_=hashed,
                 bundle=bundle,
                 policy=policy_,
@@ -851,7 +851,7 @@ def _verify_github(args: argparse.Namespace) -> None:
     verifier, materials = _collect_verification_state(args)
     for file, hashed, bundle in materials:
         try:
-            verifier.verify(input_=hashed, bundle=bundle, policy=policy_)
+            verifier.verify_artifact(input_=hashed, bundle=bundle, policy=policy_)
             print(f"OK: {file}")
         except VerificationError as exc:
             _logger.error(f"FAIL: {file}")

sigstore/_internal/rekor/__init__.py

@@ -21,6 +21,7 @@
 import rekor_types
 from cryptography.x509 import Certificate
 
+from sigstore import dsse
 from sigstore._utils import base64_encode_pem_cert
 from sigstore.hashes import Hashed
 
@@ -30,6 +31,26 @@
 __all__ = ["RekorClient", "SignedCheckpoint"]
 
 
+def _dsse_from_parts(cert: Certificate, evp: dsse.Envelope) -> rekor_types.Dsse:
+    signature = rekor_types.dsse.Signature(
+        signature=evp._inner.signatures[0].sig,
+        verifier=base64_encode_pem_cert(cert),
+    )
+    return rekor_types.Dsse(
+        spec=rekor_types.dsse.DsseV001Schema(
+            signatures=[signature],
+            envelope_hash=rekor_types.dsse.EnvelopeHash(
+                algorithm=rekor_types.dsse.Algorithm.SHA256,
+                value=None,
+            ),
+            payload_hash=rekor_types.dsse.PayloadHash(
+                algorithm=rekor_types.dsse.Algorithm.SHA256,
+                value=None,
+            ),
+        )
+    )
+
+
 # TODO: This should probably live somewhere better.
 def _hashedrekord_from_parts(
     cert: Certificate, sig: bytes, hashed: Hashed

sigstore/dsse.py

@@ -21,12 +21,15 @@
 import logging
 from typing import Any, Dict, List, Literal, Optional, Union
 
+from cryptography.exceptions import InvalidSignature
 from cryptography.hazmat.primitives import hashes
 from cryptography.hazmat.primitives.asymmetric import ec
 from pydantic import BaseModel, ConfigDict, Field, RootModel, StrictStr, ValidationError
 from sigstore_protobuf_specs.io.intoto import Envelope as _Envelope
 from sigstore_protobuf_specs.io.intoto import Signature
 
+from sigstore.errors import VerificationError
+
 _logger = logging.getLogger(__name__)
 
 _Digest = Union[
@@ -103,12 +106,7 @@ def _pae(self) -> bytes:
         Construct the PAE encoding for this statement.
         """
 
-        # See:
-        # https://github.com/secure-systems-lab/dsse/blob/v1.0.0/envelope.md
-        # https://github.com/in-toto/attestation/blob/v1.0/spec/v1.0/envelope.md
-        pae = f"DSSEv1 {len(Envelope._TYPE)} {Envelope._TYPE} ".encode()
-        pae += b" ".join([str(len(self._contents)).encode(), self._contents])
-        return pae
+        return _pae(Envelope._TYPE, self._contents)
 
 
 class _StatementBuilder:
@@ -193,6 +191,19 @@ def to_json(self) -> str:
         return self._inner.to_json()  # type: ignore[no-any-return]
 
 
+def _pae(type_: str, body: bytes) -> bytes:
+    """
+    Compute the PAE encoding for the given `type_` and `body`.
+    """
+
+    # See:
+    # https://github.com/secure-systems-lab/dsse/blob/v1.0.0/envelope.md
+    # https://github.com/in-toto/attestation/blob/v1.0/spec/v1.0/envelope.md
+    pae = f"DSSEv1 {len(type_)} {type_} ".encode()
+    pae += b" ".join([str(len(body)).encode(), body])
+    return pae
+
+
 def _sign(key: ec.EllipticCurvePrivateKey, stmt: Statement) -> Envelope:
     """
     Sign for the given in-toto `Statement`, and encapsulate the resulting
@@ -209,3 +220,26 @@ def _sign(key: ec.EllipticCurvePrivateKey, stmt: Statement) -> Envelope:
             signatures=[Signature(sig=signature, keyid=None)],
         )
     )
+
+
+def _verify(key: ec.EllipticCurvePublicKey, evp: Envelope) -> bytes:
+    """
+    Verify the given in-toto `Envelope`, returning the verified inner payload.
+
+    This function does **not** check the envelope's payload type. The caller
+    is responsible for performing this check.
+
+    Assumes that the envelope only has a single signature.
+    """
+
+    pae = _pae(evp._inner.payload_type, evp._inner.payload)
+
+    try:
+        # NB: Assumes only one signature in the DSSE envelope.
+        key.verify(evp._inner.signatures[0].sig, pae, ec.ECDSA(hashes.SHA256()))
+    except IndexError:
+        raise VerificationError("DSSE: no signature to verify")
+    except InvalidSignature:
+        raise VerificationError("DSSE: invalid signature")
+
+    return evp._inner.payload

sigstore/verify/models.py

@@ -245,6 +245,17 @@ def log_entry(self) -> LogEntry:
         """
         return self._log_entry
 
+    @property
+    def _dsse_envelope(self) -> dsse.Envelope | None:
+        """
+        Returns the DSSE envelope within this Bundle as a `dsse.Envelope`.
+
+        @private
+        """
+        if self._inner.dsse_envelope:
+            return dsse.Envelope(self._inner.dsse_envelope)
+        return None
+
     @classmethod
     def from_json(cls, raw: bytes | str) -> Bundle:
         """