(backend) wip introduce a token exchange endpoint

src/backend/core/api/feature_flag.py

@@ -18,6 +18,7 @@ class FeatureFlag:
         "application": "APPLICATION_ENABLED",
         "roomkit": "ROOMKIT_ENABLED",
         "connection_test": "CONNECTION_TEST_ENABLED",
+        "user_access_token": "USER_ACCESS_TOKEN_ENABLED",
     }
 
     @classmethod

src/backend/core/api/serializers.py

@@ -599,3 +599,25 @@ class ExternalProcessEventSerializer(BaseValidationOnlySerializer):
     # useless bad requests
     type = serializers.CharField(required=False, allow_null=True, allow_blank=True)
     status = serializers.CharField(required=False, allow_null=True, allow_blank=True)
+
+
+class TransitCodeSerializer(BaseValidationOnlySerializer):
+    """Validate the single-use transit code sent to the exchange endpoint."""
+
+    # todo if I can pass the max length directly to the char field
+    code = serializers.CharField(max_length=255, trim_whitespace=True)
+
+    def validate_code(self, value):
+        """Reject codes whose length cannot match a generated one.
+
+        `secrets.token_urlsafe(nbytes)` produces (4 * nbytes + 2) // 3
+        url-safe characters. Checking the length against the configured
+        TRANSIT_CODE_NBYTES makes malformed codes fail fast with a 400,
+        before any cache lookup.
+        """
+        expected_length = (4 * settings.TRANSIT_CODE_NBYTES + 2) // 3
+
+        if len(value) != expected_length:
+            raise serializers.ValidationError("Invalid transit code format.")
+
+        return value

src/backend/core/api/throttling.py

@@ -97,3 +97,14 @@ class ConnectionTestAnonRateThrottle(MonitoredAnonRateThrottle):
     """Throttle anonymous users requesting connection test tokens."""
 
     scope = "connection_test"
+
+
+class ExchangeAccessTokenAnonRateThrottle(MonitoredAnonRateThrottle):
+    """Throttle anonymous transit code exchange attempts.
+
+    Abuse mitigation only, not a security boundary: DRF throttling is
+    best-effort. The security of the exchange rests on the codes'
+    entropy and single use.
+    """
+
+    scope = "exchange_access_token"

src/backend/core/api/viewsets.py

@@ -75,6 +75,7 @@ from core.recording.worker.mediator import (
     WorkerServiceMediator,
 )
 from core.services.invitation import InvitationService
+from core.services.jwt_token import JwtTokenService
 from core.services.livekit_events import (
     LiveKitEventsService,
     LiveKitWebhookError,
@@ -100,6 +101,7 @@ from core.services.room_roles import (
 )
 from core.services.subtitle import SubtitleException, SubtitleService
 from core.tasks.connection_test import delete_connection_test_room
+from core.services.transit_code import TransitCodeService
 from core.tasks.file import process_file_deletion
 from core.utils import generate_token
 
@@ -237,6 +239,76 @@ class UserViewSet(
             self.serializer_class(request.user, context=context).data
         )
 
+    @decorators.action(
+        detail=False,
+        methods=["post"],
+        url_path="exchange-access-token",
+        permission_classes=[],
+        throttle_classes=[throttling.ExchangeAccessTokenAnonRateThrottle],
+    )
+    @FeatureFlag.require("user_access_token")
+    def exchange_access_token(self, request):
+        """Exchange a single-use transit code for a user access token.
+
+        The endpoint is unauthenticated: the transit code itself, an opaque
+        random string obtained through the external API and delivered to
+        the embedded frontend via a URL fragment, is the credential. Each
+        code can be exchanged exactly once (consuming it deletes it from
+        the cache); replaying a consumed code is denied and logged.
+
+        The issued JWT authenticates the user the code was minted for on
+        the whole core API, exactly like a session cookie would (similar
+        to lib-jitsi-meet's token authentication), and never appears in
+        any URL. Role-based permissions apply unchanged.
+        """
+        serializer = serializers.TransitCodeSerializer(data=request.data)
+        serializer.is_valid(raise_exception=True)
+
+        code_data = TransitCodeService().consume_code(serializer.validated_data["code"])
+
+        if code_data is None:
+            logger.warning("Invalid, expired or already used transit code")
+            raise drf_exceptions.PermissionDenied(
+                "Invalid, expired or already used transit code."
+            )
+
+        # Re-check the user at exchange time so that a deactivation after
+        # the transit code was minted is taken into account.
+        try:
+            user = models.User.objects.get(id=code_data["user_id"], is_active=True)
+        except models.User.DoesNotExist as excpt:
+            raise drf_exceptions.PermissionDenied(
+                "This account can no longer access the application."
+            ) from excpt
+
+        token_service = JwtTokenService(
+            secret_key=settings.USER_ACCESS_TOKEN_SECRET_KEY,
+            algorithm=settings.USER_ACCESS_TOKEN_ALG,
+            issuer=settings.USER_ACCESS_TOKEN_ISSUER,
+            audience=settings.USER_ACCESS_TOKEN_AUDIENCE,
+            expiration_seconds=settings.USER_ACCESS_TOKEN_TTL,
+            token_type=settings.USER_ACCESS_TOKEN_TYPE,
+        )
+
+        # todo - discuss wether it's the relevant scope
+        data = token_service.generate_jwt(
+            user,
+            "user:access",
+            {
+                "token_type": "user_access",
+                "client_id": code_data.get("client_id", "unknown"),
+            },
+        )
+
+        # Log for auditing
+        logger.info(
+            "User access token issued from transit code: user_id=%s, client_id=%s",
+            user.id,
+            code_data.get("client_id", "unknown"),
+        )
+
+        return drf_response.Response(data)
+
 
 class RoomViewSet(
     mixins.CreateModelMixin,