fix(content-marking): require record.alg during verification

src/agentrust_trace/content_marking.py

@@ -145,7 +145,7 @@ def verify_assertion(assertion: dict[str, Any], record_bytes: bytes) -> dict[str
     ref = data.get("record")
     if not isinstance(ref, dict) or not ref.get("url"):
         raise ContentMarkingError("assertion carries no record reference")
-    alg = ref.get("alg", "sha256")
+    alg = ref.get("alg")
     expected = ref.get("hash")
     if not _DIGEST_RE.match(str(expected or "")):
         raise ContentMarkingError(f"record.hash {expected!r} is not a sha256:/sha384: digest")

tests/test_content_marking_required_alg.py

@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from agentrust_trace.content_marking import ContentMarkingError, build_assertion, verify_assertion
+
+URL = "https://registry.example/records/abc123.json"
+
+
+def _record_bytes() -> bytes:
+    return json.dumps(
+        {
+            "eat_profile": "tag:agentrust-io.com,2026:trace-v0.2",
+            "iat": 1760000000,
+            "subject": "spiffe://example.org/agent/image-bot",
+            "data_class": "public",
+        }
+    ).encode()
+
+
+def test_missing_record_alg_is_refused_instead_of_defaulting_to_sha256() -> None:
+    raw = _record_bytes()
+    assertion = build_assertion(raw, url=URL)
+    del assertion["data"]["record"]["alg"]
+
+    with pytest.raises(ContentMarkingError, match="unsupported digest algorithm None"):
+        verify_assertion(assertion, raw)
+
+
+def test_null_record_alg_is_refused() -> None:
+    raw = _record_bytes()
+    assertion = build_assertion(raw, url=URL)
+    assertion["data"]["record"]["alg"] = None
+
+    with pytest.raises(ContentMarkingError, match="unsupported digest algorithm None"):
+        verify_assertion(assertion, raw)
+
+
+def test_explicit_sha256_and_sha384_still_verify() -> None:
+    raw = _record_bytes()
+    verify_assertion(build_assertion(raw, url=URL, alg="sha256"), raw)
+    verify_assertion(build_assertion(raw, url=URL, alg="sha384"), raw)