Use Metadata.verify_delegate

Replace the helper verify_with_threshold() with
the newly implemented method Metadata.verify_delegate.

Add an aditional type annotation helping the IDE.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

tuf/ngclient/_internal/trusted_metadata_set.py

@@ -71,47 +71,11 @@
 from typing import Dict, Iterator, Optional
 
 from tuf import exceptions
-from tuf.api.metadata import Metadata, Root, Targets
+from tuf.api.metadata import Metadata
 from tuf.api.serialization import DeserializationError
 
 logger = logging.getLogger(__name__)
 
-# This is a placeholder until ...
-# TODO issue 1306: implement this in Metadata API
-def verify_with_threshold(
-    delegator: Metadata, role_name: str, unverified: Metadata
-) -> bool:
-    """Verify 'unverified' with keys and threshold defined in delegator"""
-    role = None
-    keys = {}
-    if isinstance(delegator.signed, Root):
-        keys = delegator.signed.keys
-        role = delegator.signed.roles.get(role_name)
-    elif isinstance(delegator.signed, Targets):
-        if delegator.signed.delegations:
-            keys = delegator.signed.delegations.keys
-            # role names are unique: first match is enough
-            roles = delegator.signed.delegations.roles
-            role = next((r for r in roles if r.name == role_name), None)
-    else:
-        raise ValueError("Call is valid only on delegator metadata")
-
-    if role is None:
-        raise ValueError(f"Delegated role {role_name} not found")
-
-    # verify that delegate is signed by correct threshold of unique keys
-    unique_keys = set()
-    for keyid in role.keyids:
-        key = keys[keyid]
-        try:
-            key.verify_signature(unverified)
-            unique_keys.add(key.keyval["public"])
-        except Exception as e:  # pylint: disable=broad-except
-            # TODO specify the Exceptions (see issue #1351)
-            logger.info("verify failed: %s", e)
-
-    return len(unique_keys) >= role.threshold
-
 
 class TrustedMetadataSet(abc.Mapping):
     """Internal class to keep track of trusted metadata in Updater
@@ -207,20 +171,14 @@ def update_root(self, data: bytes):
 
         if self.root is not None:
             # We are not loading initial trusted root: verify the new one
-            if not verify_with_threshold(self.root, "root", new_root):
-                raise exceptions.UnsignedMetadataError(
-                    "New root is not signed by root", new_root.signed
-                )
+            self.root.verify_delegate("root", new_root)
 
             if new_root.signed.version != self.root.signed.version + 1:
                 raise exceptions.ReplayedMetadataError(
                     "root", new_root.signed.version, self.root.signed.version
                 )
 
-        if not verify_with_threshold(new_root, "root", new_root):
-            raise exceptions.UnsignedMetadataError(
-                "New root is not signed by itself", new_root.signed
-            )
+        new_root.verify_delegate("root", new_root)
 
         self._trusted_set["root"] = new_root
         logger.debug("Updated root")
@@ -270,10 +228,7 @@ def update_timestamp(self, data: bytes):
                 f"Expected 'timestamp', got '{new_timestamp.signed.type}'"
             )
 
-        if not verify_with_threshold(self.root, "timestamp", new_timestamp):
-            raise exceptions.UnsignedMetadataError(
-                "New timestamp is not signed by root", new_timestamp.signed
-            )
+        self.root.verify_delegate("timestamp", new_timestamp)
 
         # If an existing trusted timestamp is updated,
         # check for a rollback attack
@@ -339,10 +294,7 @@ def update_snapshot(self, data: bytes):
                 f"Expected 'snapshot', got '{new_snapshot.signed.type}'"
             )
 
-        if not verify_with_threshold(self.root, "snapshot", new_snapshot):
-            raise exceptions.UnsignedMetadataError(
-                "New snapshot is not signed by root", new_snapshot.signed
-            )
+        self.root.verify_delegate("snapshot", new_snapshot)
 
         if (
             new_snapshot.signed.version
@@ -408,7 +360,7 @@ def update_delegated_targets(
         if self.snapshot is None:
             raise RuntimeError("Cannot load targets before snapshot")
 
-        delegator = self.get(delegator_name)
+        delegator: Optional[Metadata] = self.get(delegator_name)
         if delegator is None:
             raise RuntimeError("Cannot load targets before delegator")
 
@@ -438,11 +390,7 @@ def update_delegated_targets(
                 f"Expected 'targets', got '{new_delegate.signed.type}'"
             )
 
-        if not verify_with_threshold(delegator, role_name, new_delegate):
-            raise exceptions.UnsignedMetadataError(
-                f"New {role_name} is not signed by {delegator_name}",
-                new_delegate,
-            )
+        delegator.verify_delegate(role_name, new_delegate)
 
         if new_delegate.signed.version != meta.version:
             raise exceptions.BadVersionNumberError(