trust: Fail less hard when unsupported keys are seen

CHANGELOG.md

@@ -43,6 +43,10 @@ All versions prior to 0.9.0 are untracked.
   still required.
   [#1381](https://github.com/sigstore/sigstore-python/pull/1381)
 
+* Verify: Avoid hard failure if trusted root contains unsupported keytypes (as verification
+  may succeed without that key).
+  [#1424](https://github.com/sigstore/sigstore-python/pull/1424)
+
 * CI: Timestamp Authority tests use latest release, not latest tag, of
   [sigstore/timestamp-authority](https://github.com/sigstore/timestamp-authority)
   [#1377](https://github.com/sigstore/sigstore-python/pull/1377)

sigstore/_internal/trust.py

@@ -18,6 +18,7 @@
 
 from __future__ import annotations
 
+import logging
 from collections import defaultdict
 from collections.abc import Iterable
 from dataclasses import dataclass
@@ -77,6 +78,8 @@
 FULCIO_VERSIONS = [1]
 OIDC_VERSIONS = [1]
 
+_logger = logging.getLogger(__name__)
+
 
 def _is_timerange_valid(period: TimeRange | None, *, allow_expired: bool) -> bool:
     """
@@ -200,8 +203,11 @@ def __init__(self, public_keys: list[_PublicKey] = []):
         self._keyring: dict[KeyID, Key] = {}
 
         for public_key in public_keys:
-            key = Key(public_key)
-            self._keyring[key.key_id] = key
+            try:
+                key = Key(public_key)
+                self._keyring[key.key_id] = key
+            except VerificationError as e:
+                _logger.warning(f"Failed to load a trusted root key: {e}")
 
     def verify(self, *, key_id: KeyID, signature: bytes, data: bytes) -> None:
         """

test/assets/trusted_root/trustedroot.v1.json

@@ -14,6 +14,20 @@
             "logId": {
                 "keyId": "wNI9atQGlz+VWfO6LRygH4QUfY/8W4RFwiT5i5WRgB0="
             }
+        },
+        {
+            "baseUrl": "https://example.com/unsupported_key",
+            "hashAlgorithm": "SHA2_256",
+            "publicKey": {
+                "rawBytes": "",
+                "keyDetails": "UNSPECIFIED",
+                "validFor": {
+                    "start": "2021-01-12T11:53:27.000Z"
+                }
+            },
+            "logId": {
+                "keyId": "xNI9atQGlz+VWfO6LRygH4QUfY/8W4RFwiT5i5WRgB0="
+            }
         }
     ],
     "certificateAuthorities": [

test/assets/trusted_root/trustedroot.v1.local_tlog_ed25519_rekor-tiles.json

@@ -14,6 +14,20 @@
             "logId": {
                 "keyId": "tAlACZWkUrif9Z9sOIrpk1ak1I8loRNufk79N6l1SNg="
             }
+        },
+        {
+            "baseUrl": "https://example.com/unsupported_key",
+            "hashAlgorithm": "SHA2_256",
+            "publicKey": {
+                "rawBytes": "",
+                "keyDetails": "UNSPECIFIED",
+                "validFor": {
+                    "start": "2021-01-12T11:53:27.000Z"
+                }
+            },
+            "logId": {
+                "keyId": "xNI9atQGlz+VWfO6LRygH4QUfY/8W4RFwiT5i5WRgB0="
+            }
         }
     ],
     "certificateAuthorities": [

test/unit/internal/test_trust.py

@@ -208,20 +208,22 @@ class TestTrustedRoot:
     )
     def test_good(self, asset, file):
         """
-        Ensures that the trusted_roots are well-formed and that the embedded keys are supported.
+        Ensures that the trusted_roots are well-formed and that the expected embedded keys are supported.
         """
         path = asset(file)
         root = TrustedRoot.from_file(path)
 
         assert (
             root._inner.media_type == TrustedRoot.TrustedRootType.TRUSTED_ROOT_0_1.value
         )
-        assert len(root._inner.tlogs) == 1
+        assert len(root._inner.tlogs) == 2
         assert len(root._inner.certificate_authorities) == 2
         assert len(root._inner.ctlogs) == 2
         assert len(root._inner.timestamp_authorities) == 1
-        assert root.rekor_keyring(KeyringPurpose.VERIFY) is not None
-        assert root.ct_keyring(KeyringPurpose.VERIFY) is not None
+
+        # only one of the two rekor keys is actually supported
+        assert len(root.rekor_keyring(KeyringPurpose.VERIFY)._keyring) == 1
+        assert len(root.ct_keyring(KeyringPurpose.VERIFY)._keyring) == 2
         assert root.get_fulcio_certs() is not None
         assert root.get_timestamp_authorities() is not None