Implementation of MSC3967: Don't require UIA for initial upload of cross signing keys (#15077)

5 files changed, 182 insertions, 9 deletions

diff --git a/changelog.d/15077.feature b/changelog.d/15077.feature
new file mode 100644
index 0000000000..384e751056
--- /dev/null
+++ b/changelog.d/15077.feature
@@ -0,0 +1 @@
+Experimental support for MSC3967 to not require UIA for setting up cross-signing on first use.
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index bc38fae0b6..7c81f055b6 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -194,3 +194,6 @@ class ExperimentalConfig(Config):
         self.msc3966_exact_event_property_contains = experimental.get(
             "msc3966_exact_event_property_contains", False
         )
+
+        # MSC3967: Do not require UIA when first uploading cross signing keys
+        self.msc3967_enabled = experimental.get("msc3967_enabled", False)
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 43cbece21b..4e9c8d8db0 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -1301,6 +1301,20 @@ class E2eKeysHandler:
 
         return desired_key_data
 
+    async def is_cross_signing_set_up_for_user(self, user_id: str) -> bool:
+        """Checks if the user has cross-signing set up
+
+        Args:
+            user_id: The user to check
+
+        Returns:
+            True if the user has cross-signing set up, False otherwise
+        """
+        existing_master_key = await self.store.get_e2e_cross_signing_key(
+            user_id, "master"
+        )
+        return existing_master_key is not None
+
 
 def _check_cross_signing_key(
     key: JsonDict, user_id: str, key_type: str, signing_key: Optional[VerifyKey] = None
diff --git a/synapse/rest/client/keys.py b/synapse/rest/client/keys.py
index 7873b363c0..32bb8b9a91 100644
--- a/synapse/rest/client/keys.py
+++ b/synapse/rest/client/keys.py
@@ -312,15 +312,29 @@ class SigningKeyUploadServlet(RestServlet):
         user_id = requester.user.to_string()
         body = parse_json_object_from_request(request)
 
-        await self.auth_handler.validate_user_via_ui_auth(
-            requester,
-            request,
-            body,
-            "add a device signing key to your account",
-            # Allow skipping of UI auth since this is frequently called directly
-            # after login and it is silly to ask users to re-auth immediately.
-            can_skip_ui_auth=True,
-        )
+        if self.hs.config.experimental.msc3967_enabled:
+            if await self.e2e_keys_handler.is_cross_signing_set_up_for_user(user_id):
+                # If we already have a master key then cross signing is set up and we require UIA to reset
+                await self.auth_handler.validate_user_via_ui_auth(
+                    requester,
+                    request,
+                    body,
+                    "reset the device signing key on your account",
+                    # Do not allow skipping of UIA auth.
+                    can_skip_ui_auth=False,
+                )
+            # Otherwise we don't require UIA since we are setting up cross signing for first time
+        else:
+            # Previous behaviour is to always require UIA but allow it to be skipped
+            await self.auth_handler.validate_user_via_ui_auth(
+                requester,
+                request,
+                body,
+                "add a device signing key to your account",
+                # Allow skipping of UI auth since this is frequently called directly
+                # after login and it is silly to ask users to re-auth immediately.
+                can_skip_ui_auth=True,
+            )
 
         result = await self.e2e_keys_handler.upload_signing_keys_for_user(user_id, body)
         return 200, result
diff --git a/tests/rest/client/test_keys.py b/tests/rest/client/test_keys.py
index 741fecea77..8ee5489057 100644
--- a/tests/rest/client/test_keys.py
+++ b/tests/rest/client/test_keys.py
@@ -14,12 +14,21 @@
 
 from http import HTTPStatus
 
+from signedjson.key import (
+    encode_verify_key_base64,
+    generate_signing_key,
+    get_verify_key,
+)
+from signedjson.sign import sign_json
+
 from synapse.api.errors import Codes
 from synapse.rest import admin
 from synapse.rest.client import keys, login
+from synapse.types import JsonDict
 
 from tests import unittest
 from tests.http.server._base import make_request_with_cancellation_test
+from tests.unittest import override_config
 
 
 class KeyQueryTestCase(unittest.HomeserverTestCase):
@@ -118,3 +127,135 @@ class KeyQueryTestCase(unittest.HomeserverTestCase):
 
         self.assertEqual(200, channel.code, msg=channel.result["body"])
         self.assertIn(bob, channel.json_body["device_keys"])
+
+    def make_device_keys(self, user_id: str, device_id: str) -> JsonDict:
+        # We only generate a master key to simplify the test.
+        master_signing_key = generate_signing_key(device_id)
+        master_verify_key = encode_verify_key_base64(get_verify_key(master_signing_key))
+
+        return {
+            "master_key": sign_json(
+                {
+                    "user_id": user_id,
+                    "usage": ["master"],
+                    "keys": {"ed25519:" + master_verify_key: master_verify_key},
+                },
+                user_id,
+                master_signing_key,
+            ),
+        }
+
+    def test_device_signing_with_uia(self) -> None:
+        """Device signing key upload requires UIA."""
+        password = "wonderland"
+        device_id = "ABCDEFGHI"
+        alice_id = self.register_user("alice", password)
+        alice_token = self.login("alice", password, device_id=device_id)
+
+        content = self.make_device_keys(alice_id, device_id)
+
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/device_signing/upload",
+            content,
+            alice_token,
+        )
+
+        self.assertEqual(channel.code, HTTPStatus.UNAUTHORIZED, channel.result)
+        # Grab the session
+        session = channel.json_body["session"]
+        # Ensure that flows are what is expected.
+        self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
+
+        # add UI auth
+        content["auth"] = {
+            "type": "m.login.password",
+            "identifier": {"type": "m.id.user", "user": alice_id},
+            "password": password,
+            "session": session,
+        }
+
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/device_signing/upload",
+            content,
+            alice_token,
+        )
+
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)
+
+    @override_config({"ui_auth": {"session_timeout": "15m"}})
+    def test_device_signing_with_uia_session_timeout(self) -> None:
+        """Device signing key upload requires UIA buy passes with grace period."""
+        password = "wonderland"
+        device_id = "ABCDEFGHI"
+        alice_id = self.register_user("alice", password)
+        alice_token = self.login("alice", password, device_id=device_id)
+
+        content = self.make_device_keys(alice_id, device_id)
+
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/device_signing/upload",
+            content,
+            alice_token,
+        )
+
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)
+
+    @override_config(
+        {
+            "experimental_features": {"msc3967_enabled": True},
+            "ui_auth": {"session_timeout": "15s"},
+        }
+    )
+    def test_device_signing_with_msc3967(self) -> None:
+        """Device signing key follows MSC3967 behaviour when enabled."""
+        password = "wonderland"
+        device_id = "ABCDEFGHI"
+        alice_id = self.register_user("alice", password)
+        alice_token = self.login("alice", password, device_id=device_id)
+
+        keys1 = self.make_device_keys(alice_id, device_id)
+
+        # Initial request should succeed as no existing keys are present.
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/device_signing/upload",
+            keys1,
+            alice_token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)
+
+        keys2 = self.make_device_keys(alice_id, device_id)
+
+        # Subsequent request should require UIA as keys already exist even though session_timeout is set.
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/device_signing/upload",
+            keys2,
+            alice_token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.UNAUTHORIZED, channel.result)
+
+        # Grab the session
+        session = channel.json_body["session"]
+        # Ensure that flows are what is expected.
+        self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
+
+        # add UI auth
+        keys2["auth"] = {
+            "type": "m.login.password",
+            "identifier": {"type": "m.id.user", "user": alice_id},
+            "password": password,
+            "session": session,
+        }
+
+        # Request should complete
+        channel = self.make_request(
+            "POST",
+            "/_matrix/client/v3/keys/device_signing/upload",
+            keys2,
+            alice_token,
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)