tests: Add root key rotation tests

Add one test with 1 subtests for various root key rotation situations.

The test data definition format is a bit tricky but I tried to document
that in the test function docstring.

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

tests/test_updater_key_rotations.py

@@ -0,0 +1,211 @@
+#!/usr/bin/env python
+
+# Copyright 2021, New York University and the TUF contributors
+# SPDX-License-Identifier: MIT OR Apache-2.0
+
+"""Test ngclient Updater key rotation handling"""
+
+from dataclasses import dataclass
+from typing import List, Optional, Type
+import os
+import sys
+import tempfile
+import unittest
+
+from securesystemslib.signer import SSlibSigner
+
+from tuf.api.metadata import Key
+from tuf.exceptions import UnsignedMetadataError
+from tuf.ngclient import Updater
+
+from tests import utils
+from tests.repository_simulator import RepositorySimulator
+from tests.utils import run_sub_tests_with_dataset
+
+
+@dataclass
+class RootVersion:
+    keys: List[int]
+    threshold: int
+    signatures: List[int]
+    result: Optional[Type[Exception]] = None
+
+
+class TestUpdaterKeyRotations(unittest.TestCase):
+    """Test ngclient root rotation handling"""
+    # set dump_dir to trigger repository state dumps
+    dump_dir: Optional[str] = None
+
+    def setUp(self) -> None:
+        self.sim = None
+        self.metadata_dir = None
+        self.subtest_count = 0
+        # pylint: disable-next=consider-using-with
+        self.temp_dir = tempfile.TemporaryDirectory()
+
+        # Pre-create a bunch of keys and signers
+        self.keys: List[Key] = []
+        self.signers: List[SSlibSigner] = []
+        for _ in range(10):
+            key, signer = RepositorySimulator.create_key()
+            self.keys.append(key)
+            self.signers.append(signer)
+
+    def tearDown(self) -> None:
+        self.temp_dir.cleanup()
+
+    def setup_subtest(self) -> None:
+        self.subtest_count += 1
+
+        # Setup repository for subtest: make sure no roots have been published
+        self.sim = RepositorySimulator()
+        self.sim.signed_roots.clear()
+        self.sim.root.version = 0
+
+        if self.dump_dir is not None:
+            # create subtest dumpdir
+            name = f"{self.id().split('.')[-1]}-{self.subtest_count}"
+            self.sim.dump_dir = os.path.join(self.dump_dir, name)
+            os.mkdir(self.sim.dump_dir)
+
+    def _run_refresh(self) -> None:
+        """Create new updater, run refresh"""
+        if self.sim.dump_dir is not None:
+            self.sim.write()
+
+        # bootstrap with initial root
+        self.metadata_dir = tempfile.mkdtemp(dir=self.temp_dir.name)
+        with open(os.path.join(self.metadata_dir, "root.json"), "bw") as f:
+            f.write(self.sim.signed_roots[0])
+
+        updater = Updater(
+            self.metadata_dir,
+            "https://example.com/metadata/",
+            fetcher=self.sim,
+        )
+        updater.refresh()
+
+    root_rotation_cases = {
+        "1-of-1 key rotation": [
+            RootVersion([1], 1, [1]),
+            RootVersion([2], 1, [2, 1]),
+            RootVersion([2], 1, [2]),
+        ],
+        "1-of-1 key rotation, unused signatures": [
+            RootVersion([1], 1, [3, 1, 4]),
+            RootVersion([2], 1, [3, 2, 1, 4]),
+            RootVersion([2], 1, [3, 2, 4]),
+        ],
+        "1-of-1 key rotation fail: not signed with old key": [
+            RootVersion([1], 1, [1]),
+            RootVersion([2], 1, [2, 3, 4], UnsignedMetadataError),
+        ],
+        "1-of-1 key rotation fail: not signed with new key": [
+            RootVersion([1], 1, [1]),
+            RootVersion([2], 1, [1, 3, 4], UnsignedMetadataError),
+        ],
+        "3-of-5, sign with different keycombos": [
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 2, 4]),
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 4, 1]),
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 1, 3]),
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 1, 3]),
+        ],
+        "3-of-5, one key rotated": [
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 2, 4]),
+            RootVersion([0, 1, 3, 4, 5], 3, [0, 4, 1]),
+        ],
+        "3-of-5, one key rotate fails: not signed with 3 new keys": [
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 2, 4]),
+            RootVersion([0, 1, 3, 4, 5], 3, [0, 2, 4], UnsignedMetadataError),
+        ],
+        "3-of-5, one key rotate fails: not signed with 3 old keys": [
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 2, 4]),
+            RootVersion([0, 1, 3, 4, 5], 3, [0, 4, 5], UnsignedMetadataError),
+        ],
+        "3-of-5, one key rotated, with intermediate step": [
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 2, 4]),
+            RootVersion([0, 1, 3, 4, 5], 3, [0, 2, 4, 5]),
+            RootVersion([0, 1, 3, 4, 5], 3, [0, 4, 5]),
+        ],
+        "3-of-5, all keys rotated, with intermediate step": [
+            RootVersion([0, 1, 2, 3, 4], 3, [0, 2, 4]),
+            RootVersion([5, 6, 7, 8, 9], 3, [0, 2, 4, 5, 6, 7]),
+            RootVersion([5, 6, 7, 8, 9], 3, [5, 6, 7]),
+        ],
+        "1-of-3 threshold increase to 2-of-3": [
+            RootVersion([1, 2, 3], 1, [1]),
+            RootVersion([1, 2, 3], 2, [1, 2]),
+        ],
+        "1-of-3 threshold bump to 2-of-3 fails: new threshold not reached": [
+            RootVersion([1, 2, 3], 1, [1]),
+            RootVersion([1, 2, 3], 2, [2], UnsignedMetadataError),
+        ],
+        "2-of-3 threshold decrease to 1-of-3": [
+            RootVersion([1, 2, 3], 2, [1, 2]),
+            RootVersion([1, 2, 3], 1, [1, 2]),
+            RootVersion([1, 2, 3], 1, [1]),
+        ],
+        "2-of-3 threshold decr. to 1-of-3 fails: old threshold not reached": [
+            RootVersion([1, 2, 3], 2, [1, 2]),
+            RootVersion([1, 2, 3], 1, [1], UnsignedMetadataError),
+        ],
+        "1-of-2 threshold increase to 2-of-2": [
+            RootVersion([1], 1, [1]),
+            RootVersion([1, 2], 2, [1, 2]),
+        ],
+    }
+
+    @run_sub_tests_with_dataset(root_rotation_cases)
+    def test_root_rotation(self, root_versions: List[RootVersion]) -> None:
+        """Test Updater.refresh() with various sequences of root updates
+
+        Each RootVersion in the list describes root keys and signatures of a
+        remote root metadata version. As an example:
+            RootVersion([1,2,3], 2, [1,2])
+        defines a root that contains keys 1, 2 and 3 with threshold 2. The
+        metadata is signed with keys 1 and 2.
+
+        Assert that refresh() result is expected and that local root on disk is
+        the expected one after all roots have been loaded from remote using the
+        standard client update workflow.
+        """
+        self.setup_subtest()
+
+        # Publish all remote root versions defined in root_versions
+        for rootver in root_versions:
+            # clear root keys, signers
+            self.sim.root.roles["root"].keyids.clear()
+            self.sim.signers["root"].clear()
+
+            self.sim.root.roles["root"].threshold = rootver.threshold
+            for i in rootver.keys:
+                self.sim.root.add_key("root", self.keys[i])
+            for i in rootver.signatures:
+                self.sim.add_signer("root", self.signers[i])
+            self.sim.root.version += 1
+            self.sim.publish_root()
+
+        # run client workflow, assert success/failure
+        expected_result = root_versions[-1].result
+        if expected_result is None:
+            self._run_refresh()
+            expected_local_root = self.sim.signed_roots[-1]
+        else:
+            # failure expected: local root should be the root before last
+            with self.assertRaises(expected_result):
+                self._run_refresh()
+            expected_local_root = self.sim.signed_roots[-2]
+
+        # assert local root on disk is expected
+        with open(os.path.join(self.metadata_dir, "root.json"), "rb") as f:
+            self.assertEqual(f.read(), expected_local_root)
+
+
+if __name__ == "__main__":
+    if "--dump" in sys.argv:
+        TestUpdaterKeyRotations.dump_dir = tempfile.mkdtemp()
+        print(f"Repository dumps in {TestUpdaterKeyRotations.dump_dir}")
+        sys.argv.remove("--dump")
+
+    utils.configure_test_logging(sys.argv)
+    unittest.main()