fix: handle malformed signature keys

CHANGELOG.md

@@ -15,6 +15,8 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixed
 
+- Verification now reports malformed embedded Ed25519 public keys as a failed
+  signature check instead of raising a decoding exception.
 - `verify` and `inspect` now report unreadable or invalid receipt files as
   one-line CLI errors instead of raising tracebacks.
 

src/answerproof/verifier.py

@@ -57,7 +57,10 @@ def failures(self) -> list[CheckResult]:
 
 
 def verify_signature(receipt: Receipt) -> CheckResult:
-    vk = VerifyKey.from_base64(receipt.signature.public_key)
+    try:
+        vk = VerifyKey.from_base64(receipt.signature.public_key)
+    except ValueError:
+        return CheckResult("signature", False, "invalid Ed25519 public key")
     ok = vk.verify(receipt.payload.canonical_bytes(), receipt.signature.signature)
     return CheckResult("signature", ok, "" if ok else "Ed25519 signature does not match payload")
 

tests/test_verifier.py

@@ -32,6 +32,27 @@ def test_signer_pinning_failure(receipt, sources):
     assert any(c.name == "signer_pin" and not c.passed for c in verdict.checks)
 
 
+@pytest.mark.parametrize(
+    "malformed_key",
+    [
+        "!!!not-base64!!!",
+        "c2hvcnQ",
+        "not-ascii-🔑",
+    ],
+)
+def test_malformed_public_key_returns_failed_signature_check(receipt, malformed_key):
+    signature = receipt.signature.model_copy(update={"public_key": malformed_key})
+    corrupted = receipt.model_copy(update={"signature": signature})
+
+    verdict = verify_receipt(corrupted)
+
+    assert not verdict.valid
+    assert len(verdict.failures()) == 1
+    failure = verdict.failures()[0]
+    assert failure.name == "signature"
+    assert failure.detail == "invalid Ed25519 public key"
+
+
 def test_wrong_source_content_fails(receipt):
     bad = {"s1": "This is not the original content."}
     verdict = verify_receipt(receipt, source_contents=bad)