Merge pull request 'Fix R4: checkpoint binds private key to enrolled fingerprint' (#8) from leora/fix-r4-checkpoint-key-binding into sigil-initial

checkpoint() only checked that the signer name was enrolled, so a stranger
key could write a polluting checkpoint row. Refuse unless the private key's
fingerprint equals the enrolled fingerprint for that signer.

sigil/store.py

@@ -675,8 +675,15 @@ class Store:
 
         public = _keys.public_from_private(private)
         fingerprint = _keys.fingerprint(public)
-        if self.get_agent(signer) is None:
+        agent_row = self.get_agent(signer)
+        if agent_row is None:
             raise ValueError(f"{signer} is not enrolled, so cannot sign a checkpoint")
+        enrolled_fp = agent_row["fingerprint"] or ""
+        if not enrolled_fp or fingerprint != enrolled_fp:
+            raise ValueError(
+                f"private key fingerprint does not match enrolled key for {signer}; "
+                "refusing to sign a checkpoint with a stranger key"
+            )
         with self._write_lock:
             last = self.conn.execute(
                 "SELECT id, hash FROM events ORDER BY id DESC LIMIT 1"

tests/test_production.py

@@ -133,6 +133,15 @@ class TestSignedCheckpoints(unittest.TestCase):
         with self.assertRaises(ValueError):
             stranger.checkpoint(self.private, "not-enrolled")
 
+    def test_checkpoint_refuses_enrolled_name_with_wrong_key(self):
+        # R4: signer name is enrolled, but private key is not the enrolled one.
+        before = len(self.store.checkpoints())
+        other_private, _ = keys.generate()
+        with self.assertRaises(ValueError) as ctx:
+            self.store.checkpoint(other_private, "bigpickle")
+        self.assertIn("does not match enrolled key", str(ctx.exception))
+        self.assertEqual(len(self.store.checkpoints()), before)
+
     def test_checkpoint_after_rekey_is_flagged(self):
         secret = identity.seed_bootstrap(self.store, "bigpickle")
         identity.enrol_rekey(