Clean verified time handling

Try to handle TSA timestamps and rekor v1 integrated time in a sensible
manner:
* no special cases for when TSA timestamps are present
* require one verified time by default
* Only allow integrated time to be a verified time if entry is from rekor v1
* VERIFY_TIMESTAMP_THRESHOLD now refers to "number of verified times",
  not just TSA timestamps
* Tests use a rekor v1 bundle but expect it to be invalid if the
  timestamp is invalid -- but the integrated time is
  enough. Fix this by monkeypatching VERIFY_TIMESTAMP_THRESHOLD

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

sigstore/verify/verifier.py

@@ -215,13 +215,6 @@ def _establish_time(self, bundle: Bundle) -> list[TimestampVerificationResult]:
                 raise VerificationError(msg)
 
             timestamp_from_tsa = self._verify_timestamp_authority(bundle)
-            if len(timestamp_from_tsa) < VERIFY_TIMESTAMP_THRESHOLD:
-                msg = (
-                    f"not enough timestamps validated to meet the validation "
-                    f"threshold ({len(timestamp_from_tsa)}/{VERIFY_TIMESTAMP_THRESHOLD})"
-                )
-                raise VerificationError(msg)
-
             verified_timestamps.extend(timestamp_from_tsa)
 
         # If a timestamp from the Transparency Service is available, the Verifier MUST
@@ -232,6 +225,12 @@ def _establish_time(self, bundle: Bundle) -> list[TimestampVerificationResult]:
         if (
             timestamp := bundle.log_entry.integrated_time
         ) and bundle.log_entry.inclusion_promise:
+            kv = bundle.log_entry._kind_version
+            if not (kv.kind in ["dsse", "hashedrekord"] and kv.version == "0.0.1"):
+                raise VerificationError(
+                    "Integrated time only supported for dsse/hashedrekord 0.0.1 types"
+                )
+
             verified_timestamps.append(
                 TimestampVerificationResult(
                     source=TimestampSource.TRANSPARENCY_SERVICE,
@@ -320,13 +319,12 @@ def _verify_common_signing_cert(
             store.add_cert(parent_cert_ossl)
 
         # (0): Establishing a Time for the Signature
-        # First, establish a time for the signature. This timestamp is required to
+        # First, establish a time for the signature. This is required to
         # validate the certificate chain, so this step comes first.
-        # While this step is optional and only performed if timestamp data has been
-        # provided within the bundle, providing a signed timestamp without a TSA to
-        # verify it result in a VerificationError.
+        # These include TSA timestamps and (in the case of rekor v1 entries)
+        # rekor log integrated time.
         verified_timestamps = self._establish_time(bundle)
-        if not verified_timestamps:
+        if len(verified_timestamps) < VERIFY_TIMESTAMP_THRESHOLD:
             raise VerificationError("not enough sources of verified time")
 
         # (1): verify that the signing certificate is signed by the root

test/unit/verify/test_verifier.py

@@ -219,7 +219,11 @@ def verifier(self, asset) -> Verifier:
         verifier._trusted_root._inner.timestamp_authorities = [authority._inner]
         return verifier
 
-    def test_verifier_verify_timestamp(self, verifier, asset, null_policy):
+    def test_verifier_verify_timestamp(self, verifier, asset, null_policy, monkeypatch):
+        # asset is a rekor v1 bundle: set threshold to 2 so both integrated time and the
+        # TSA timestamp are required
+        monkeypatch.setattr("sigstore.verify.verifier.VERIFY_TIMESTAMP_THRESHOLD", 2)
+
         verifier.verify_artifact(
             asset("tsa/bundle.txt").read_bytes(),
             Bundle.from_json(asset("tsa/bundle.txt.sigstore").read_bytes()),
@@ -296,15 +300,21 @@ def test_verifier_duplicate_timestamp(self, verifier, asset, null_policy):
             )
 
     def test_verifier_outside_validity_range(
-        self, caplog, verifier, asset, null_policy
+        self, caplog, verifier, asset, null_policy, monkeypatch
     ):
+        # asset is a rekor v1 bundle: set threshold to 2 so both integrated time and the
+        # TSA timestamp are required
+        monkeypatch.setattr("sigstore.verify.verifier.VERIFY_TIMESTAMP_THRESHOLD", 2)
+
         # Set a date before the timestamp range
         verifier._trusted_root.get_timestamp_authorities()[
             0
         ]._inner.valid_for.end = datetime(2024, 10, 31, tzinfo=timezone.utc)
 
         with caplog.at_level(logging.DEBUG, logger="sigstore.verify.verifier"):
-            with pytest.raises(VerificationError, match="not enough timestamps"):
+            with pytest.raises(
+                VerificationError, match="not enough sources of verified time"
+            ):
                 verifier.verify_artifact(
                     asset("tsa/bundle.txt").read_bytes(),
                     Bundle.from_json(asset("tsa/bundle.txt.sigstore").read_bytes()),
@@ -319,13 +329,19 @@ def test_verifier_outside_validity_range(
     def test_verifier_rfc3161_error(
         self, verifier, asset, null_policy, caplog, monkeypatch
     ):
+        # asset is a rekor v1 bundle: set threshold to 2 so both integrated time and the
+        # TSA timestamp are required
+        monkeypatch.setattr("sigstore.verify.verifier.VERIFY_TIMESTAMP_THRESHOLD", 2)
+
         def verify_function(*args):
             raise rfc3161_client.VerificationError()
 
         monkeypatch.setattr(rfc3161_client.verify._Verifier, "verify", verify_function)
 
         with caplog.at_level(logging.DEBUG, logger="sigstore.verify.verifier"):
-            with pytest.raises(VerificationError, match="not enough timestamps"):
+            with pytest.raises(
+                VerificationError, match="not enough sources of verified time"
+            ):
                 verifier.verify_artifact(
                     asset("tsa/bundle.txt").read_bytes(),
                     Bundle.from_json(asset("tsa/bundle.txt.sigstore").read_bytes()),
@@ -345,15 +361,21 @@ def test_verifier_no_authorities(self, asset, null_policy):
                 null_policy,
             )
 
-    def test_late_timestamp(self, caplog, verifier, asset, null_policy):
+    def test_late_timestamp(self, caplog, verifier, asset, null_policy, monkeypatch):
         """
         Ensures that verifying the signing certificate fails because the timestamp
         is outside the certificate's validity window. The sample bundle
         "tsa/bundle.txt.late_timestamp.sigstore" was generated by adding `time.sleep(12*60)`
         into `sigstore.sign.Signer._finalize_sign()`, just after the entry is posted to Rekor
         but before the timestamp is requested.
         """
-        with pytest.raises(VerificationError, match="not enough timestamps"):
+        # asset is a rekor v1 bundle: set threshold to 2 so both integrated time and the
+        # TSA timestamp are required
+        monkeypatch.setattr("sigstore.verify.verifier.VERIFY_TIMESTAMP_THRESHOLD", 2)
+
+        with pytest.raises(
+            VerificationError, match="not enough sources of verified time"
+        ):
             verifier.verify_artifact(
                 asset("tsa/bundle.txt").read_bytes(),
                 Bundle.from_json(
@@ -370,8 +392,12 @@ def test_late_timestamp(self, caplog, verifier, asset, null_policy):
     def test_verifier_not_enough_timestamp(
         self, verifier, asset, null_policy, monkeypatch
     ):
-        monkeypatch.setattr("sigstore.verify.verifier.VERIFY_TIMESTAMP_THRESHOLD", 2)
-        with pytest.raises(VerificationError, match="not enough timestamps"):
+        # asset is a rekor v1 bundle: set threshold to 3 so integrated time and one
+        # TSA timestamp are not enough
+        monkeypatch.setattr("sigstore.verify.verifier.VERIFY_TIMESTAMP_THRESHOLD", 3)
+        with pytest.raises(
+            VerificationError, match="not enough sources of verified time"
+        ):
             verifier.verify_artifact(
                 asset("tsa/bundle.txt").read_bytes(),
                 Bundle.from_json(asset("tsa/bundle.txt.sigstore").read_bytes()),