Raise on bad signature count in Metadata.verify

Change Metadata.verify(key) behavior to raise an exception if
none or multiple signatures for the passed key are found on the
Metadata object.

Signed-off-by: Lukas Puehringer <[email protected]>

tests/test_api.py

@@ -28,6 +28,7 @@ def setUpModule():
 
 # Since setUpModule is called after imports we need to import conditionally.
 if IS_PY_VERSION_SUPPORTED:
+    import tuf.exceptions
     from tuf.api.metadata import (
         Metadata,
         Snapshot,
@@ -152,12 +153,9 @@ def test_sign_verify(self):
 
         # ... it has a single existing signature,
         self.assertTrue(len(metadata_obj.signatures) == 1)
-        # ... valid for the correct key, but
+        # ... which is valid for the correct key.
         self.assertTrue(metadata_obj.verify(
                 self.keystore['targets']['public']))
-        # ... invalid for an unrelated key.
-        self.assertFalse(metadata_obj.verify(
-                self.keystore['snapshot']['public']))
 
         # Append a new signature with the unrelated key and assert that ...
         metadata_obj.sign(self.keystore['snapshot']['private'], append=True)
@@ -177,15 +175,20 @@ def test_sign_verify(self):
         self.assertTrue(metadata_obj.verify(
                 self.keystore['timestamp']['public']))
 
-
-        # Update the metadata, invalidating the existing signature, append
-        # a new signature with the same key, and assert that ...
-        metadata_obj.signed.bump_version()
+        # Assert exception if there are more than one signatures for a key
         metadata_obj.sign(self.keystore['timestamp']['private'], append=True)
-        # ... verify returns False, because all signatures identified by a
-        # keyid must be valid
-        self.assertFalse(metadata_obj.verify(
-                self.keystore['timestamp']['public']))
+        with self.assertRaises(tuf.exceptions.Error) as ctx:
+            metadata_obj.verify(self.keystore['timestamp']['public'])
+        self.assertTrue(
+                '2 signatures for key' in str(ctx.exception),
+                str(ctx.exception))
+
+        # Assert exception if there is no signature for a key
+        with self.assertRaises(tuf.exceptions.Error) as ctx:
+            metadata_obj.verify(self.keystore['targets']['public'])
+        self.assertTrue(
+                'no signature for' in str(ctx.exception),
+                str(ctx.exception))
 
 
     def test_metadata_base(self):

tuf/api/metadata.py

@@ -45,6 +45,7 @@
 
 import iso8601
 import tuf.formats
+import tuf.exceptions
 
 
 # Types
@@ -242,33 +243,33 @@ def verify(self, key: JsonDict) -> bool:
             key: A securesystemslib-style public key object.
 
         Raises:
+            # TODO: Revise exception taxonomy
+            tuf.exceptions.Error: None or multiple signatures found for key.
             securesystemslib.exceptions.FormatError: Key argument is malformed.
             securesystemslib.exceptions.CryptoError, \
                     securesystemslib.exceptions.UnsupportedAlgorithmError:
                 Signing errors.
 
         Returns:
-            A boolean indicating if all identified signatures are valid. False
-            if no signature was found for the keyid or any of the found
-            signatures is invalid.
-
-            FIXME: Is this behavior expected? An alternative approach would be
-            to raise an exception if no signature is found for the keyid,
-            and/or if more than one sigantures are found for the keyid.
+            A boolean indicating if the signature is valid for the passed key.
 
         """
         signatures_for_keyid = list(filter(
                 lambda sig: sig['keyid'] == key['keyid'], self.signatures))
 
         if not signatures_for_keyid:
-            return False
+            raise tuf.exceptions.Error(
+                    f'no signature for key {key["keyid"]}.')
 
-        for signature in signatures_for_keyid:
-            if not verify_signature(
-                    key, signature, self.signed.to_canonical_bytes()):
-                return False
+        elif len(signatures_for_keyid) > 1:
+            raise tuf.exceptions.Error(
+                    f'{len(signatures_for_keyid)} signatures for key '
+                    f'{key["keyid"]}, not sure which one to verify.')
+        else:
+            return verify_signature(
+                    key, signatures_for_keyid[0],
+                    self.signed.to_canonical_bytes())
 
-        return True
 
 
 class Signed: