examples: Use verification results in repo example

This is an example of using the verification resutls in a repository.

The only remaining tricky part is in _get_verification_result():
* has to figure out the delegating metadata (something we currently
  cannot provide in repository.Repository for the general case)
* Needs a special case for first root

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

examples/repository/_simplerepo.py

@@ -8,7 +8,7 @@
 import logging
 from collections import defaultdict
 from datetime import datetime, timedelta
-from typing import Dict, List
+from typing import Dict, List, Union
 
 from securesystemslib import keys
 from securesystemslib.signer import Key, Signer, SSlibKey, SSlibSigner
@@ -20,10 +20,13 @@
     Metadata,
     MetaFile,
     Root,
+    RootVerificationResult,
+    Signed,
     Snapshot,
     TargetFile,
     Targets,
     Timestamp,
+    VerificationResult,
 )
 from tuf.repository import Repository
 
@@ -89,6 +92,27 @@ def targets_infos(self) -> Dict[str, MetaFile]:
     def snapshot_info(self) -> MetaFile:
         return self._snapshot_info
 
+    def _get_verification_result(
+        self, role: str, md: Metadata
+    ) -> Union[VerificationResult, RootVerificationResult]:
+        """Verify roles metadata using the existing repository metadata"""
+        if role == Root.type:
+            assert isinstance(md.signed, Root)
+            root = self.root()
+            if root.version == 0:
+                # special case first root
+                root = md.signed
+            return md.signed.get_root_verification_result(
+                root, md.signed_bytes, md.signatures
+            )
+        if role in [Timestamp.type, Snapshot.type, Targets.type]:
+            delegator: Signed = self.root()
+        else:
+            delegator = self.targets()
+        return delegator.get_verification_result(
+            role, md.signed_bytes, md.signatures
+        )
+
     def open(self, role: str) -> Metadata:
         """Return current Metadata for role from 'storage' (or create a new one)"""
 
@@ -112,6 +136,14 @@ def close(self, role: str, md: Metadata) -> None:
         for signer in self.signer_cache[role]:
             md.sign(signer, append=True)
 
+        # Double check that we only write verified metadata
+        vr = self._get_verification_result(role, md)
+        if not vr:
+            raise ValueError(f"Role {role} failed to verify")
+        keyids = [keyid[:7] for keyid in vr.signed]
+        verify_str = f"verified with keys [{', '.join(keyids)}]"
+        logger.debug("Role %s v%d: %s", role, md.signed.version, verify_str)
+
         # store new metadata version, update version caches
         self.role_cache[role].append(md)
         if role == "snapshot":
@@ -130,8 +162,6 @@ def add_target(self, path: str, content: str) -> None:
         with self.edit_targets() as targets:
             targets.targets[path] = TargetFile.from_data(path, data)
 
-        logger.debug("Targets v%d", targets.version)
-
         # update snapshot, timestamp
         self.do_snapshot()
         self.do_timestamp()
@@ -157,8 +187,6 @@ def submit_delegation(self, rolename: str, data: bytes) -> bool:
             logger.info("Failed to add delegation for %s: %s", rolename, e)
             return False
 
-        logger.debug("Targets v%d", targets.version)
-
         # update snapshot, timestamp
         self.do_snapshot()
         self.do_timestamp()
@@ -177,8 +205,6 @@ def submit_role(self, role: str, data: bytes) -> bool:
                 if not targetpath.startswith(f"{role}/"):
                     raise ValueError(f"targets allowed under {role}/ only")
 
-            self.targets().verify_delegate(role, md.signed_bytes, md.signatures)
-
             if md.signed.version != self.targets(role).version + 1:
                 raise ValueError("Invalid version {md.signed.version}")
 
@@ -186,10 +212,19 @@ def submit_role(self, role: str, data: bytes) -> bool:
             logger.info("Failed to add new version for %s: %s", role, e)
             return False
 
+        # Check that we only write verified metadata
+        vr = self._get_verification_result(role, md)
+        if not vr:
+            logger.info("Role %s failed to verify", role)
+            return False
+
+        keyids = [keyid[:7] for keyid in vr.signed]
+        verify_str = f"verified with keys [{', '.join(keyids)}]"
+        logger.debug("Role %s v%d: %s", role, md.signed.version, verify_str)
+
         # Checks passed: Add new delegated role version
         self.role_cache[role].append(md)
         self._targets_infos[f"{role}.json"].version = md.signed.version
-        logger.debug("%s v%d", role, md.signed.version)
 
         # To keep it simple, target content is generated from targetpath
         for targetpath in md.signed.targets: