cli: allow DSSE verification (#1015)

sigstore · May 16, 2024 · dbab104 · dbab104
1 parent 09da22b
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -28,6 +28,11 @@ All versions prior to 0.9.0 are untracked.
   for representing in-toto statements and DSSE envelopes
   ([#930](https://github.com/sigstore/sigstore-python/pull/930))
 
+* CLI: The `sigstore verify` subcommands can now verify bundles containing
+  DSSE entries, such as those produced by
+  [GitHub Artifact Attestations](https://docs.github.com/en/actions/security-guides/using-artifact-attestations-to-establish-provenance-for-builds)
+  ([#1015](https://github.com/sigstore/sigstore-python/pull/1015))
+
 ### Removed
 
 * **BREAKING API CHANGE**: `SigningResult` has been removed.
@@ -87,6 +92,13 @@ All versions prior to 0.9.0 are untracked.
   contains. No functional changes have been made to it
   ([#1016](https://github.com/sigstore/sigstore-python/pull/1016))
 
+* API: `policy.Identity` now takes an **optional** OIDC issuer, rather than a
+  required one ([#1015](https://github.com/sigstore/sigstore-python/pull/1015))
+
+* CLI: `sigstore verify github` now requires `--cert-identity` **or**
+  `--repository`, not just `--cert-identity`
+  ([#1015](https://github.com/sigstore/sigstore-python/pull/1015))
+
 ## [2.1.5]
 
 ## Fixed

diff --git a/README.md b/README.md
@@ -205,10 +205,9 @@ to by a particular OIDC provider (like `https://github.com/login/oauth`).
 <!-- @begin-sigstore-verify-identity-help@ -->
 ```
 usage: sigstore verify identity [-h] [-v] [--certificate FILE]
-                                [--signature FILE] [--bundle FILE]
-                                --cert-identity IDENTITY [--offline]
-                                --cert-oidc-issuer URL [--staging]
-                                [--rekor-url URL]
+                                [--signature FILE] [--bundle FILE] [--offline]
+                                --cert-identity IDENTITY --cert-oidc-issuer
+                                URL [--staging] [--rekor-url URL]
                                 FILE [FILE ...]
 
 optional arguments:
@@ -227,11 +226,11 @@ Verification inputs:
   FILE                  The file to verify
 
 Verification options:
+  --offline             Perform offline verification; requires a Sigstore
+                        bundle (default: False)
   --cert-identity IDENTITY
                         The identity to check for in the certificate's Subject
                         Alternative Name (default: None)
-  --offline             Perform offline verification; requires a Sigstore
-                        bundle (default: False)
   --cert-oidc-issuer URL
                         The OIDC issuer URL to check for in the certificate's
                         OIDC issuer extension (default: None)
@@ -258,11 +257,10 @@ claims more precisely than `sigstore verify identity` allows:
 <!-- @begin-sigstore-verify-github-help@ -->
 ```
 usage: sigstore verify github [-h] [-v] [--certificate FILE]
-                              [--signature FILE] [--bundle FILE]
-                              --cert-identity IDENTITY [--offline]
-                              [--trigger EVENT] [--sha SHA] [--name NAME]
-                              [--repository REPO] [--ref REF] [--staging]
-                              [--rekor-url URL]
+                              [--signature FILE] [--bundle FILE] [--offline]
+                              [--cert-identity IDENTITY] [--trigger EVENT]
+                              [--sha SHA] [--name NAME] [--repository REPO]
+                              [--ref REF] [--staging] [--rekor-url URL]
                               FILE [FILE ...]
 
 optional arguments:
@@ -281,11 +279,11 @@ Verification inputs:
   FILE                  The file to verify
 
 Verification options:
+  --offline             Perform offline verification; requires a Sigstore
+                        bundle (default: False)
   --cert-identity IDENTITY
                         The identity to check for in the certificate's Subject
                         Alternative Name (default: None)
-  --offline             Perform offline verification; requires a Sigstore
-                        bundle (default: False)
   --trigger EVENT       The GitHub Actions event name that triggered the
                         workflow (default: None)
   --sha SHA             The `git` commit SHA that the workflow run was invoked

diff --git a/sigstore/_cli.py b/sigstore/_cli.py
@@ -26,7 +26,7 @@
 from cryptography.x509 import load_pem_x509_certificate
 from rich.logging import RichHandler
 
-from sigstore import __version__
+from sigstore import __version__, dsse
 from sigstore._internal.fulcio.client import (
     DEFAULT_FULCIO_URL,
     ExpiredCertificate,
@@ -160,14 +160,6 @@ def _add_shared_verify_input_options(group: argparse._ArgumentGroup) -> None:
 
 
 def _add_shared_verification_options(group: argparse._ArgumentGroup) -> None:
-    group.add_argument(
-        "--cert-identity",
-        metavar="IDENTITY",
-        type=str,
-        default=os.getenv("SIGSTORE_CERT_IDENTITY"),
-        help="The identity to check for in the certificate's Subject Alternative Name",
-        required=True,
-    )
     group.add_argument(
         "--offline",
         action="store_true",
@@ -376,6 +368,14 @@ def _parser() -> argparse.ArgumentParser:
 
     verification_options = verify_identity.add_argument_group("Verification options")
     _add_shared_verification_options(verification_options)
+    verification_options.add_argument(
+        "--cert-identity",
+        metavar="IDENTITY",
+        type=str,
+        default=os.getenv("SIGSTORE_CERT_IDENTITY"),
+        help="The identity to check for in the certificate's Subject Alternative Name",
+        required=True,
+    )
     verification_options.add_argument(
         "--cert-oidc-issuer",
         metavar="URL",
@@ -401,6 +401,13 @@ def _parser() -> argparse.ArgumentParser:
 
     verification_options = verify_github.add_argument_group("Verification options")
     _add_shared_verification_options(verification_options)
+    verification_options.add_argument(
+        "--cert-identity",
+        metavar="IDENTITY",
+        type=str,
+        default=os.getenv("SIGSTORE_CERT_IDENTITY"),
+        help="The identity to check for in the certificate's Subject Alternative Name",
+    )
     verification_options.add_argument(
         "--trigger",
         dest="workflow_trigger",
@@ -813,28 +820,36 @@ def _verify_identity(args: argparse.Namespace) -> None:
         )
 
         try:
-            verifier.verify_artifact(
-                input_=hashed,
-                bundle=bundle,
-                policy=policy_,
-            )
+            _verify_common(verifier, hashed, bundle, policy_)
             print(f"OK: {file}")
-        except VerificationError as exc:
+        except Error as exc:
             _logger.error(f"FAIL: {file}")
             exc.log_and_exit(_logger, args.verbose >= 1)
 
 
 def _verify_github(args: argparse.Namespace) -> None:
-    # Every GitHub verification begins with an identity policy,
-    # for which we know the issuer URL ahead of time.
-    # We then add more policies, as configured by the user's passed-in options.
-    inner_policies: list[policy.VerificationPolicy] = [
-        policy.Identity(
-            identity=args.cert_identity,
-            issuer="https://token.actions.githubusercontent.com",
+    inner_policies: list[policy.VerificationPolicy] = []
+
+    # We require at least one of `--cert-identity` or `--repository`,
+    # to minimize the risk of user confusion about what's being verified.
+    if not (args.cert_identity or args.workflow_repository):
+        _die(args, "--cert-identity or --repository is required")
+
+    # No matter what the user configures above, we require the OIDC issuer to
+    # be GitHub Actions.
+    inner_policies.append(
+        policy.OIDCIssuer("https://token.actions.githubusercontent.com")
+    )
+
+    if args.cert_identity:
+        inner_policies.append(
+            policy.Identity(
+                identity=args.cert_identity,
+                # We always explicitly check the issuer below, so configuring
+                # it here is unnecessary.
+                issuer=None,
+            )
         )
-    ]
-
     if args.workflow_trigger:
         inner_policies.append(policy.GitHubWorkflowTrigger(args.workflow_trigger))
     if args.workflow_sha:
@@ -851,13 +866,47 @@ def _verify_github(args: argparse.Namespace) -> None:
     verifier, materials = _collect_verification_state(args)
     for file, hashed, bundle in materials:
         try:
-            verifier.verify_artifact(input_=hashed, bundle=bundle, policy=policy_)
+            _verify_common(verifier, hashed, bundle, policy_)
             print(f"OK: {file}")
-        except VerificationError as exc:
+        except Error as exc:
             _logger.error(f"FAIL: {file}")
             exc.log_and_exit(_logger, args.verbose >= 1)
 
 
+def _verify_common(
+    verifier: Verifier,
+    hashed: Hashed,
+    bundle: Bundle,
+    policy_: policy.VerificationPolicy,
+) -> None:
+    """
+    Common verification handling.
+
+    This dispatches to either artifact or DSSE verification, depending on
+    `bundle`'s inner type.
+    """
+
+    # If the bundle specifies a DSSE envelope, perform DSSE verification
+    # and assert that the inner payload is an in-toto statement bound
+    # to a subject matching the input's digest.
+    if bundle._dsse_envelope:
+        type_, payload = verifier.verify_dsse(bundle=bundle, policy=policy_)
+        if type_ != dsse.Envelope._TYPE:
+            raise VerificationError(f"expected JSON payload for DSSE, got {type_}")
+
+        stmt = dsse.Statement(payload)
+        if not stmt._matches_digest(hashed):
+            raise VerificationError(
+                f"in-toto statement has no subject for digest {hashed.digest.hex()}"
+            )
+    else:
+        verifier.verify_artifact(
+            input_=hashed,
+            bundle=bundle,
+            policy=policy_,
+        )
+
+
 def _get_identity(args: argparse.Namespace) -> Optional[IdentityToken]:
     token = None
     if not args.oidc_disable_ambient_providers:

diff --git a/sigstore/dsse.py b/sigstore/dsse.py
@@ -25,10 +25,12 @@
 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.dev.sigstore.common.v1 import HashAlgorithm
 from sigstore_protobuf_specs.io.intoto import Envelope as _Envelope
 from sigstore_protobuf_specs.io.intoto import Signature
 
-from sigstore.errors import VerificationError
+from sigstore.errors import Error, VerificationError
+from sigstore.hashes import Hashed
 
 _logger = logging.getLogger(__name__)
 
@@ -97,9 +99,28 @@ def __init__(self, contents: bytes) -> None:
         """
         self._contents = contents
         try:
-            self._statement = _Statement.model_validate_json(contents)
+            self._inner = _Statement.model_validate_json(contents)
         except ValidationError:
-            raise ValueError("malformed in-toto statement")
+            raise Error("malformed in-toto statement")
+
+    def _matches_digest(self, digest: Hashed) -> bool:
+        """
+        Returns a boolean indicating whether this in-toto Statement contains a subject
+        matching the given digest. The subject's name is **not** checked.
+
+        No digests other than SHA256 are currently supported.
+        """
+        if digest.algorithm != HashAlgorithm.SHA2_256:
+            raise VerificationError(f"unexpected digest algorithm: {digest.algorithm}")
+
+        for sub in self._inner.subjects:
+            sub_digest = sub.digest.root.get("sha256")
+            if sub_digest is None:
+                continue
+            if sub_digest == digest.digest.hex():
+                return True
+
+        return False
 
     def _pae(self) -> bytes:
         """
@@ -160,7 +181,7 @@ def build(self) -> Statement:
                 predicate=self._predicate,
             )
         except ValidationError as e:
-            raise ValueError(f"invalid statement: {e}")
+            raise Error(f"invalid statement: {e}")
 
         return Statement(stmt.model_dump_json(by_alias=True).encode())
 

diff --git a/sigstore/hashes.py b/sigstore/hashes.py
@@ -22,6 +22,8 @@
 from pydantic import BaseModel
 from sigstore_protobuf_specs.dev.sigstore.common.v1 import HashAlgorithm
 
+from sigstore.errors import Error
+
 
 class Hashed(BaseModel):
     """
@@ -44,12 +46,12 @@ def _as_hashedrekord_algorithm(self) -> rekor_types.hashedrekord.Algorithm:
         """
         if self.algorithm == HashAlgorithm.SHA2_256:
             return rekor_types.hashedrekord.Algorithm.SHA256
-        raise ValueError(f"unknown hash algorithm: {self.algorithm}")
+        raise Error(f"unknown hash algorithm: {self.algorithm}")
 
     def _as_prehashed(self) -> Prehashed:
         """
         Returns an appropriate Cryptography `Prehashed` for this `Hashed`.
         """
         if self.algorithm == HashAlgorithm.SHA2_256:
             return Prehashed(hashes.SHA256())
-        raise ValueError(f"unknown hash algorithm: {self.algorithm}")
+        raise Error(f"unknown hash algorithm: {self.algorithm}")
diff --git a/sigstore/verify/policy.py b/sigstore/verify/policy.py
@@ -107,7 +107,7 @@ def verify(self, cert: Certificate) -> None:
             raise VerificationError(
                 (
                     f"Certificate's {self.__class__.__name__} does not match "
-                    f"(got {ext_value}, expected {self._value})"
+                    f"(got '{ext_value}', expected '{self._value}')"
                 )
             )
 
@@ -441,26 +441,33 @@ def verify(self, cert: Certificate) -> None:
 class Identity:
     """
     Verifies the certificate's "identity", corresponding to the X.509v3 SAN.
-    Identities are verified modulo an OIDC issuer, so the issuer's URI
-    is also required.
+
+    Identities can be verified modulo an OIDC issuer, to prevent an unexpected
+    issuer from offering a particular identity.
 
     Supported SAN types include emails, URIs, and Sigstore-specific "other names".
     """
 
-    def __init__(self, *, identity: str, issuer: str):
+    _issuer: OIDCIssuer | None
+
+    def __init__(self, *, identity: str, issuer: str | None = None):
         """
         Create a new `Identity`, with the given expected identity and issuer values.
         """
 
         self._identity = identity
-        self._issuer = OIDCIssuer(issuer)
+        if issuer:
+            self._issuer = OIDCIssuer(issuer)
+        else:
+            self._issuer = None
 
     def verify(self, cert: Certificate) -> None:
         """
         Verify `cert` against the policy.
         """
 
-        self._issuer.verify(cert)
+        if self._issuer:
+            self._issuer.verify(cert)
 
         # Build a set of all valid identities.
         san_ext = cert.extensions.get_extension_for_class(SubjectAlternativeName).value