updater: verify newly downloaded root metadata with its signatures

Per the detailed client workflow in the specification step 1.2

"Version N+1 of the root metadata file MUST have been signed by:
(1) a threshold of keys specified in the trusted root metadata file
(version N), and
(2) a threshold of keys specified in the new root metadata file being
validated (version N+1)."

Number 2 is implemented here as this step was not being performed by the
Updater. Unfortunately we can't use existing signature verification
methods in tuf.sig, because tuf.sig.signature_status() does not verify
signatures for keys which are not listed in keydb (and tuf.sig.verify
uses tuf.sig.signature_status)

Therefore this patch introduces a method for verifying signatures with
root keys listed in the signable being verified.

Signed-off-by: Joshua Lock <[email protected]>

tuf/client/updater.py

@@ -1370,6 +1370,45 @@ def verify_target_file(target_file_object):
 
 
 
+  def _verify_root_self_signed(self, signable):
+    """
+    Verify the root metadata in signable is signed by a threshold of keys,
+    where the threshold and valid keys are defined by itself
+    """
+    threshold = signable['signed']['roles']['root']['threshold']
+    keyids = signable['signed']['roles']['root']['keyids']
+    keys = signable['signed']['keys']
+    signatures = signable['signatures']
+    signed = securesystemslib.formats.encode_canonical(
+        signable['signed']).encode('utf-8')
+    validated = 0
+
+    for signature in signatures:
+      keyid = signature['keyid']
+
+      # At this point we are verifying that the root metadata is signed by a
+      # threshold of keys listed in the current root role, therefore skip
+      # keys with a keyid that is not listed in the current root role.
+      if keyid not in keyids:
+        continue
+
+      key = keys[keyid]
+      # The ANYKEY_SCHEMA check in verify_signature expects the keydict to
+      # include a keyid
+      key['keyid'] = keyid
+      valid_sig = securesystemslib.keys.verify_signature(key, signature, signed)
+
+      if valid_sig:
+        validated = validated + 1
+
+    if validated >= threshold:
+      return True
+    return False
+
+
+
+
+
   def _verify_metadata_file(self, metadata_file_object,
       metadata_role):
     """
@@ -1439,6 +1478,20 @@ def _verify_metadata_file(self, metadata_file_object,
     if not valid:
       raise securesystemslib.exceptions.BadSignatureError(metadata_role)
 
+    # For root metadata, verify the downloaded root metadata object with the
+    # new threshold of new signatures contained within the downloaded root
+    # metadata object
+    # NOTE: we perform the checks on root metadata here because this enables
+    # us to perform the check before the tempfile is persisted. Furthermore,
+    # by checking here we can easily perform the check for each download
+    # mirror. Whereas if we check after _verify_metadata_file we may be
+    # persisting invalid files and we cannot try copies of the file from other
+    # mirrors.
+    if valid and metadata_role == 'root':
+      valid = self._verify_root_self_signed(metadata_signable)
+      if not valid:
+        raise securesystemslib.exceptions.BadSignatureError(metadata_role)
+