Proof of MSC implementation of MSC4108 version 2025

This is an implementation of the latest version of MSC4108 (version 2025). The implementation is independent of the existing experimental MSC4108 support in Synapse which is for the 2024 version.

It is configured using:

```yaml
experimental_features:
  msc4108v2025_mode: <mode>
```

Where mode is one of:

- `off` (default): feature is disabled
- `authenticated`: enabled and requires an authenticated user to create a rendezvous
- `public`: enabled and

rust/src/lib.rs

@@ -11,6 +11,7 @@ pub mod http;
 pub mod http_client;
 pub mod identifier;
 pub mod matrix_const;
+pub mod msc4108v2025_rendezvous;
 pub mod push;
 pub mod rendezvous;
 pub mod segmenter;
@@ -54,6 +55,7 @@ fn synapse_rust(py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
     events::register_module(py, m)?;
     http_client::register_module(py, m)?;
     rendezvous::register_module(py, m)?;
+    msc4108v2025_rendezvous::register_module(py, m)?;
     segmenter::register_module(py, m)?;
 
     Ok(())

rust/src/msc4108v2025_rendezvous/mod.rs

@@ -0,0 +1,327 @@
+/*
+ * This file is licensed under the Affero General Public License (AGPL) version 3.
+ *
+ * Copyright (C) 2025 Element Creations, Ltd
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * See the GNU Affero General Public License for more details:
+ * <https://www.gnu.org/licenses/agpl-3.0.html>.
+ */
+
+use std::{
+    collections::BTreeMap,
+    time::{Duration, SystemTime},
+};
+
+use bytes::Bytes;
+use http::{Response, StatusCode};
+use pyo3::{
+    pyclass, pymethods,
+    types::{PyAnyMethods, PyModule, PyModuleMethods},
+    Bound, IntoPyObject, Py, PyAny, PyObject, PyResult, Python,
+};
+use ulid::Ulid;
+
+use self::session::Session;
+use crate::{
+    errors::{NotFoundError, SynapseError},
+    http::{http_request_from_twisted, http_response_to_twisted},
+    UnwrapInfallible,
+};
+
+mod session;
+
+#[pyclass]
+struct MSC4108v2025RendezvousHandler {
+    clock: PyObject,
+    sessions: BTreeMap<Ulid, Session>,
+    capacity: usize,
+    max_content_length: u64,
+    ttl: Duration,
+}
+
+impl MSC4108v2025RendezvousHandler {
+    /// Check the length of the data parameter and throw error if invalid.
+    fn check_data_length(&self, data: &str) -> PyResult<()> {
+        let data_length = data.len() as u64;
+        if data_length > self.max_content_length {
+            return Err(SynapseError::new(
+                StatusCode::PAYLOAD_TOO_LARGE,
+                "Payload too large".to_owned(),
+                "M_TOO_LARGE",
+                None,
+                None,
+            ));
+        }
+        Ok(())
+    }
+
+    /// Evict expired sessions and remove the oldest sessions until we're under the capacity.
+    fn evict(&mut self, now: SystemTime) {
+        // First remove all the entries which expired
+        self.sessions.retain(|_, session| !session.expired(now));
+
+        // Then we remove the oldest entries until we're under the limit
+        while self.sessions.len() > self.capacity {
+            self.sessions.pop_first();
+        }
+    }
+}
+
+#[pymethods]
+impl MSC4108v2025RendezvousHandler {
+    #[new]
+    #[pyo3(signature = (homeserver, /, capacity=100, max_content_length=4*1024, eviction_interval=60*1000, ttl=2*60*1000))]
+    fn new(
+        py: Python<'_>,
+        homeserver: &Bound<'_, PyAny>,
+        capacity: usize,
+        max_content_length: u64,
+        eviction_interval: u64,
+        ttl: u64,
+    ) -> PyResult<Py<Self>> {
+        let clock = homeserver
+            .call_method0("get_clock")?
+            .into_pyobject(py)
+            .unwrap_infallible()
+            .unbind();
+
+        // Construct a Python object so that we can get a reference to the
+        // evict method and schedule it to run.
+        let self_ = Py::new(
+            py,
+            Self {
+                clock,
+                sessions: BTreeMap::new(),
+                capacity,
+                max_content_length,
+                ttl: Duration::from_millis(ttl),
+            },
+        )?;
+
+        let evict = self_.getattr(py, "_evict")?;
+        homeserver.call_method0("get_clock")?.call_method(
+            "looping_call",
+            (evict, eviction_interval),
+            None,
+        )?;
+
+        Ok(self_)
+    }
+
+    fn _evict(&mut self, py: Python<'_>) -> PyResult<()> {
+        let clock = self.clock.bind(py);
+        let now: u64 = clock.call_method0("time_msec")?.extract()?;
+        let now = SystemTime::UNIX_EPOCH + Duration::from_millis(now);
+        self.evict(now);
+
+        Ok(())
+    }
+
+    fn handle_post(&mut self, py: Python<'_>, twisted_request: &Bound<'_, PyAny>) -> PyResult<()> {
+        let request = http_request_from_twisted(twisted_request)?;
+
+        let clock = self.clock.bind(py);
+        let now: u64 = clock.call_method0("time_msec")?.extract()?;
+        let now = SystemTime::UNIX_EPOCH + Duration::from_millis(now);
+
+        // We trigger an immediate eviction if we're at 2x the capacity
+        if self.sessions.len() >= self.capacity * 2 {
+            self.evict(now);
+        }
+
+        // Generate a new ULID for the session from the current time.
+        let id = Ulid::from_datetime(now);
+
+        // parse JSON body out of request
+        let json: serde_json::Value =
+            serde_json::from_slice(&request.into_body()).map_err(|_| {
+                SynapseError::new(
+                    StatusCode::BAD_REQUEST,
+                    "Invalid JSON in request body".to_owned(),
+                    "M_INVALID_PARAM",
+                    None,
+                    None,
+                )
+            })?;
+
+        let data: String = json["data"].as_str().map(|s| s.to_owned()).ok_or_else(|| {
+            SynapseError::new(
+                StatusCode::BAD_REQUEST,
+                "Missing 'data' field in JSON body".to_owned(),
+                "M_INVALID_PARAM",
+                None,
+                None,
+            )
+        })?;
+
+        self.check_data_length(&data)?;
+
+        let session = Session::new(id, data, now, self.ttl);
+
+        let response_body = serde_json::to_string(&session.post_response()).map_err(|_| {
+            SynapseError::new(
+                StatusCode::INTERNAL_SERVER_ERROR,
+                "Failed to serialize response".to_owned(),
+                "M_UNKNOWN",
+                None,
+                None,
+            )
+        })?;
+        let mut response = Response::new(response_body.as_bytes());
+        *response.status_mut() = StatusCode::OK;
+        http_response_to_twisted(twisted_request, response)?;
+
+        self.sessions.insert(id, session);
+
+        Ok(())
+    }
+
+    fn handle_get(
+        &mut self,
+        py: Python<'_>,
+        twisted_request: &Bound<'_, PyAny>,
+        id: &str,
+    ) -> PyResult<()> {
+        let now: u64 = self.clock.call_method0(py, "time_msec")?.extract(py)?;
+        let now = SystemTime::UNIX_EPOCH + Duration::from_millis(now);
+
+        let id: Ulid = id.parse().map_err(|_| NotFoundError::new())?;
+        let session = self
+            .sessions
+            .get(&id)
+            .filter(|s| !s.expired(now))
+            .ok_or_else(NotFoundError::new)?;
+
+        let response_body = serde_json::to_string(&session.get_response()).map_err(|_| {
+            SynapseError::new(
+                StatusCode::INTERNAL_SERVER_ERROR,
+                "Failed to serialize response".to_owned(),
+                "M_UNKNOWN",
+                None,
+                None,
+            )
+        })?;
+        let mut response = Response::new(response_body.as_bytes());
+        *response.status_mut() = StatusCode::OK;
+        http_response_to_twisted(twisted_request, response)?;
+
+        Ok(())
+    }
+
+    fn handle_put(
+        &mut self,
+        py: Python<'_>,
+        twisted_request: &Bound<'_, PyAny>,
+        id: &str,
+    ) -> PyResult<()> {
+        let request = http_request_from_twisted(twisted_request)?;
+
+        // parse JSON body out of request
+        let json: serde_json::Value =
+            serde_json::from_slice(&request.into_body()).map_err(|_| {
+                SynapseError::new(
+                    StatusCode::BAD_REQUEST,
+                    "Invalid JSON in request body".to_owned(),
+                    "M_INVALID_PARAM",
+                    None,
+                    None,
+                )
+            })?;
+
+        let sequence_token: String = json["sequence_token"]
+            .as_str()
+            .map(|s| s.to_owned())
+            .ok_or_else(|| {
+                SynapseError::new(
+                    StatusCode::BAD_REQUEST,
+                    "Missing 'sequence_token' field in JSON body".to_owned(),
+                    "M_INVALID_PARAM",
+                    None,
+                    None,
+                )
+            })?;
+
+        let data: String = json["data"].as_str().map(|s| s.to_owned()).ok_or_else(|| {
+            SynapseError::new(
+                StatusCode::BAD_REQUEST,
+                "Missing 'data' field in JSON body".to_owned(),
+                "M_INVALID_PARAM",
+                None,
+                None,
+            )
+        })?;
+
+        self.check_data_length(&data)?;
+
+        let now: u64 = self.clock.call_method0(py, "time_msec")?.extract(py)?;
+        let now = SystemTime::UNIX_EPOCH + Duration::from_millis(now);
+
+        let id: Ulid = id.parse().map_err(|_| NotFoundError::new())?;
+        let session = self
+            .sessions
+            .get_mut(&id)
+            .filter(|s| !s.expired(now))
+            .ok_or_else(NotFoundError::new)?;
+
+        if !session.sequence_token().eq(&sequence_token) {
+            return Err(SynapseError::new(
+                StatusCode::CONFLICT,
+                "sequence_token does not match".to_owned(),
+                "IO_ELEMENT_MSC4108_CONCURRENT_WRITE",
+                None,
+                None,
+            ));
+        }
+
+        session.update(data, now);
+
+        let response_body = serde_json::to_string(&session.put_response()).map_err(|_| {
+            SynapseError::new(
+                StatusCode::INTERNAL_SERVER_ERROR,
+                "Failed to serialize response".to_owned(),
+                "M_UNKNOWN",
+                None,
+                None,
+            )
+        })?;
+        let mut response = Response::new(response_body.as_bytes());
+        *response.status_mut() = StatusCode::OK;
+        http_response_to_twisted(twisted_request, response)?;
+
+        Ok(())
+    }
+
+    fn handle_delete(&mut self, twisted_request: &Bound<'_, PyAny>, id: &str) -> PyResult<()> {
+        let _request = http_request_from_twisted(twisted_request)?;
+
+        let id: Ulid = id.parse().map_err(|_| NotFoundError::new())?;
+        let _session = self.sessions.remove(&id).ok_or_else(NotFoundError::new)?;
+
+        let mut response = Response::new(Bytes::new());
+        *response.status_mut() = StatusCode::OK;
+        http_response_to_twisted(twisted_request, response)?;
+
+        Ok(())
+    }
+}
+
+pub fn register_module(py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()> {
+    let child_module = PyModule::new(py, "msc4108v2025_rendezvous")?;
+
+    child_module.add_class::<MSC4108v2025RendezvousHandler>()?;
+
+    m.add_submodule(&child_module)?;
+
+    // We need to manually add the module to sys.modules to make `from
+    // synapse.synapse_rust import rendezvous` work.
+    py.import("sys")?
+        .getattr("modules")?
+        .set_item("synapse.synapse_rust.msc4108v2025_rendezvous", child_module)?;
+
+    Ok(())
+}

rust/src/msc4108v2025_rendezvous/session.rs

@@ -0,0 +1,112 @@
+/*
+ * This file is licensed under the Affero General Public License (AGPL) version 3.
+ *
+ * Copyright (C) 2025 Element Creations, Ltd
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * See the GNU Affero General Public License for more details:
+ * <https://www.gnu.org/licenses/agpl-3.0.html>.
+ */
+
+use std::time::{Duration, SystemTime, UNIX_EPOCH};
+
+use base64::{engine::general_purpose::URL_SAFE_NO_PAD, Engine as _};
+use serde::Serialize;
+use sha2::{Digest, Sha256};
+use ulid::Ulid;
+
+/// A single session, containing data, metadata, and expiry information.
+pub struct Session {
+    id: Ulid,
+    hash: [u8; 32],
+    data: String,
+    last_modified: SystemTime,
+    expires: SystemTime,
+}
+
+#[derive(Serialize)]
+pub struct PostResponse {
+    id: String,
+    sequence_token: String,
+    expires_ts: u64,
+}
+
+#[derive(Serialize)]
+pub struct GetResponse {
+    data: String,
+    sequence_token: String,
+    expires_ts: u64,
+}
+
+#[derive(Serialize)]
+pub struct PutResponse {
+    sequence_token: String,
+}
+
+impl Session {
+    /// Create a new session with the given data and time-to-live.
+    pub fn new(id: Ulid, data: String, now: SystemTime, ttl: Duration) -> Self {
+        let hash = Sha256::digest(&data).into();
+        Self {
+            id,
+            hash,
+            data,
+            expires: now + ttl,
+            last_modified: now,
+        }
+    }
+
+    /// Returns true if the session has expired at the given time.
+    pub fn expired(&self, now: SystemTime) -> bool {
+        self.expires <= now
+    }
+
+    /// Update the session with new data and last modified time.
+    pub fn update(&mut self, data: String, now: SystemTime) {
+        self.hash = Sha256::digest(&data).into();
+        self.data = data;
+        self.last_modified = now;
+    }
+
+    /// The sequence token for the session.
+    pub fn sequence_token(&self) -> String {
+        let encoded = URL_SAFE_NO_PAD.encode(self.hash);
+        format!("\"{encoded}\"")
+            .parse()
+            .expect("base64-encoded hash as sequence token should be valid")
+    }
+
+    pub fn get_response(&self) -> GetResponse {
+        GetResponse {
+            data: self.data.clone(),
+            sequence_token: self.sequence_token(),
+            expires_ts: self
+                .expires
+                .duration_since(UNIX_EPOCH)
+                .unwrap_or_default()
+                .as_millis() as u64,
+        }
+    }
+
+    pub fn post_response(&self) -> PostResponse {
+        PostResponse {
+            id: self.id.to_string(),
+            sequence_token: self.sequence_token(),
+            expires_ts: self
+                .expires
+                .duration_since(UNIX_EPOCH)
+                .unwrap_or_default()
+                .as_millis() as u64,
+        }
+    }
+
+    pub fn put_response(&self) -> PutResponse {
+        PutResponse {
+            sequence_token: self.sequence_token(),
+        }
+    }
+}

synapse/config/experimental.py

@@ -523,7 +523,7 @@ class ExperimentalConfig(Config):
             "msc4069_profile_inhibit_propagation", False
         )
 
-        # MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code
+        # MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code - 2024 version:
         self.msc4108_enabled = experimental.get("msc4108_enabled", False)
 
         self.msc4108_delegation_endpoint: Optional[str] = experimental.get(
@@ -548,6 +548,27 @@ class ExperimentalConfig(Config):
                 ("experimental", "msc4108_delegation_endpoint"),
             )
 
+        # MSC4108: Mechanism to allow OAuth 2.0 API sign in and E2EE set up via QR code - 2025 version:
+        msc4108v2025_mode = experimental.get("msc4108v2025_mode", "off")
+
+        if ["off", "public", "authenticated"].count(msc4108v2025_mode) != 1:
+            raise ConfigError(
+                "msc4108v2025_mode must be one of 'off', 'public' or 'authenticated'",
+                ("experimental", "msc4108v2025_mode"),
+            )
+        self.msc4108v2025_enabled: bool = msc4108v2025_mode != "off"
+        self.msc4108v2025_requires_authentication: bool = (
+            msc4108v2025_mode == "authenticated"
+        )
+
+        if self.msc4108v2025_enabled and not (
+            config.get("matrix_authentication_service") or {}
+        ).get("enabled", False):
+            raise ConfigError(
+                "MSC4108 2025 version requires matrix_authentication_service to be enabled",
+                ("experimental", "msc4108v2025_enabled"),
+            )
+
         # MSC4133: Custom profile fields
         self.msc4133_enabled: bool = experimental.get("msc4133_enabled", False)
 

synapse/rest/client/rendezvous.py

@@ -68,9 +68,55 @@ class MSC4108RendezvousServlet(RestServlet):
         self._handler.handle_post(request)
 
 
+class MSC4108v2025CreateRendezvousServlet(RestServlet):
+    PATTERNS = client_patterns(
+        "/io.element.msc4108/rendezvous$", releases=[], v1=False, unstable=True
+    )
+
+    def __init__(self, hs: "HomeServer") -> None:
+        super().__init__()
+        self._handler = hs.get_msc4108v2025_rendezvous_handler()
+        self.auth = hs.get_auth()
+        self.require_authentication = (
+            hs.config.experimental.msc4108v2025_requires_authentication
+        )
+
+    async def on_POST(self, request: SynapseRequest) -> None:
+        if self.require_authentication:
+            # This will raise if the user is not authenticated
+            await self.auth.get_user_by_req(request)
+        self._handler.handle_post(request)
+
+
+class MSC4108v2025UpdateRendezvousServlet(RestServlet):
+    PATTERNS = client_patterns(
+        "/io.element.msc4108/rendezvous/(?P<rendezvous_id>[^/]+)$",
+        releases=[],
+        v1=False,
+        unstable=True,
+    )
+
+    def __init__(self, hs: "HomeServer") -> None:
+        super().__init__()
+        self._handler = hs.get_msc4108v2025_rendezvous_handler()
+
+    def on_GET(self, request: SynapseRequest, rendezvous_id: str) -> None:
+        self._handler.handle_get(request, rendezvous_id)
+
+    def on_PUT(self, request: SynapseRequest, rendezvous_id: str) -> None:
+        self._handler.handle_put(request, rendezvous_id)
+
+    def on_DELETE(self, request: SynapseRequest, rendezvous_id: str) -> None:
+        self._handler.handle_delete(request, rendezvous_id)
+
+
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     if hs.config.experimental.msc4108_enabled:
         MSC4108RendezvousServlet(hs).register(http_server)
 
     if hs.config.experimental.msc4108_delegation_endpoint is not None:
         MSC4108DelegationRendezvousServlet(hs).register(http_server)
+
+    if hs.config.experimental.msc4108v2025_enabled:
+        MSC4108v2025CreateRendezvousServlet(hs).register(http_server)
+        MSC4108v2025UpdateRendezvousServlet(hs).register(http_server)

synapse/rest/client/versions.py

@@ -161,7 +161,7 @@ class VersionsRestServlet(RestServlet):
                     "org.matrix.msc4069": self.config.experimental.msc4069_profile_inhibit_propagation,
                     # Allows clients to handle push for encrypted events.
                     "org.matrix.msc4028": self.config.experimental.msc4028_push_encrypted_events,
-                    # MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code
+                    # MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code - 2024 version
                     "org.matrix.msc4108": (
                         self.config.experimental.msc4108_enabled
                         or (
@@ -169,6 +169,10 @@ class VersionsRestServlet(RestServlet):
                             is not None
                         )
                     ),
+                    # MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code - 2025 version
+                    "io.element.msc4108": (
+                        self.config.experimental.msc4108v2025_enabled
+                    ),
                     # MSC4140: Delayed events
                     "org.matrix.msc4140": bool(self.config.server.max_event_delay_ms),
                     # Simplified sliding sync

synapse/server.py

@@ -170,6 +170,7 @@ from synapse.state import StateHandler, StateResolutionHandler
 from synapse.storage import Databases
 from synapse.storage.controllers import StorageControllers
 from synapse.streams.events import EventSources
+from synapse.synapse_rust.msc4108v2025_rendezvous import MSC4108v2025RendezvousHandler
 from synapse.synapse_rust.rendezvous import RendezvousHandler
 from synapse.types import DomainSpecificString, ISynapseReactor
 from synapse.util import SYNAPSE_VERSION
@@ -1156,6 +1157,10 @@ class HomeServer(metaclass=abc.ABCMeta):
     def get_rendezvous_handler(self) -> RendezvousHandler:
         return RendezvousHandler(self)
 
+    @cache_in_self
+    def get_msc4108v2025_rendezvous_handler(self) -> MSC4108v2025RendezvousHandler:
+        return MSC4108v2025RendezvousHandler(self)
+
     @cache_in_self
     def get_outbound_redis_connection(self) -> "ConnectionHandler":
         """

synapse/synapse_rust/msc4108v2025_rendezvous.pyi

@@ -0,0 +1,30 @@
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 Element Creations, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+from twisted.web.iweb import IRequest
+
+from synapse.server import HomeServer
+
+class MSC4108v2025RendezvousHandler:
+    def __init__(
+        self,
+        homeserver: HomeServer,
+        /,
+        capacity: int = 100,  # This should be configurable
+        max_content_length: int = 4 * 1024,  # MSC4108 specifies maximum of 4KB
+        eviction_interval: int = 60 * 1000,
+        ttl: int = 2 * 60 * 1000,  # MSC4108 specifies minimum of 120 seconds
+    ) -> None: ...
+    def handle_post(self, request: IRequest) -> None: ...
+    def handle_get(self, request: IRequest, session_id: str) -> None: ...
+    def handle_put(self, request: IRequest, session_id: str) -> None: ...
+    def handle_delete(self, request: IRequest, session_id: str) -> None: ...

tests/rest/client/test_msc4108v2025_rendezvous.py

@@ -0,0 +1,632 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 Element Creations, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+
+import json
+import urllib.parse
+from typing import Any, Mapping
+from unittest.mock import Mock
+
+from twisted.internet.testing import MemoryReactor
+
+from synapse.rest import admin
+from synapse.rest.client import login, rendezvous
+from synapse.server import HomeServer
+from synapse.types import UserID
+from synapse.util.clock import Clock
+
+from tests import unittest
+from tests.unittest import checked_cast, override_config
+from tests.utils import HAS_AUTHLIB
+
+msc4108_endpoint = "/_matrix/client/unstable/io.element.msc4108/rendezvous"
+
+
+class RendezvousServletTestCase(unittest.HomeserverTestCase):
+    """
+    Test the experimental MSC4108 rendezvous endpoint with the latest behaviour.
+    """
+
+    servlets = [
+        admin.register_servlets,
+        login.register_servlets,
+        rendezvous.register_servlets,
+    ]
+
+    def make_homeserver(self, reactor: MemoryReactor, clock: Clock) -> HomeServer:
+        self.hs = self.setup_test_homeserver()
+        return self.hs
+
+    def setup_mock_oauth(self) -> None:
+        """
+        This isn't a very elegant way to mock the OAuth API, but it works for our purposes.
+        """
+
+        # Import this here so that we've checked that authlib is available.
+        from synapse.api.auth.mas import MasDelegatedAuth
+
+        self.auth = checked_cast(MasDelegatedAuth, self.hs.get_auth())
+
+        self._rust_client = Mock(spec=["post"])
+        self._rust_client.post = self._mock_oauth_response
+        self.auth._rust_http_client = self._rust_client
+
+    async def _mock_oauth_response(
+        self,
+        url: str,
+        response_limit: int,
+        headers: Mapping[str, str],
+        request_body: str,
+    ) -> bytes:
+        # get the token from the request body which is form encoded
+        parsed_body = urllib.parse.parse_qs(request_body)
+        token = parsed_body.get("token", [""])[0]
+
+        if not token.startswith("mock_token_"):
+            return bytes(json.dumps({"active": False}).encode("utf-8"))
+        token = token.replace("mock_token_", "")
+
+        username, device_id = token.split("_", 1)
+        user_id = UserID(username, self.hs.hostname)
+        store = self.hs.get_datastores().main
+
+        # Check th user exists in the store
+        user_info = await store.get_user_by_id(user_id=user_id.to_string())
+        if user_info is None:
+            return bytes(json.dumps({"active": False}).encode("utf-8"))
+
+        # Check the device exists in the store
+        device = await store.get_device(
+            user_id=user_id.to_string(), device_id=device_id
+        )
+        if device is None:
+            return bytes(json.dumps({"active": False}).encode("utf-8"))
+
+        return bytes(
+            json.dumps(
+                {
+                    "active": True,
+                    "scope": "urn:matrix:client:device:"
+                    + device_id
+                    + " urn:matrix:client:api:*",
+                    "username": username,
+                }
+            ).encode("utf-8")
+        )
+
+    def register_oauth_user(self, username: str, device_id: str) -> str:
+        # Provision the user and the device
+        store = self.hs.get_datastores().main
+        user_id = UserID(username, self.hs.hostname)
+
+        self.get_success(store.register_user(user_id=user_id.to_string()))
+        self.get_success(
+            store.store_device(
+                user_id=user_id.to_string(),
+                device_id=device_id,
+                initial_device_display_name=None,
+            )
+        )
+        # Generate an access token for the device
+        return "mock_token_" + username + "_" + device_id
+
+    def test_disabled(self) -> None:
+        channel = self.make_request("POST", msc4108_endpoint, {}, access_token=None)
+        self.assertEqual(channel.code, 404)
+
+    @override_config(
+        {
+            "experimental_features": {
+                "msc4108v2025_mode": "off",
+            },
+        }
+    )
+    def test_off(self) -> None:
+        channel = self.make_request("POST", msc4108_endpoint, {}, access_token=None)
+        self.assertEqual(channel.code, 404)
+
+    @unittest.skip_unless(HAS_AUTHLIB, "requires authlib")
+    @override_config(
+        {
+            "disable_registration": True,
+            "matrix_authentication_service": {
+                "enabled": True,
+                "secret": "secret_value",
+                "endpoint": "https://issuer",
+            },
+            "experimental_features": {
+                "msc4108v2025_mode": "public",
+            },
+        }
+    )
+    def test_rendezvous_public(self) -> None:
+        """
+        Test the MSC4108 rendezvous endpoint, including:
+            - Creating a session
+            - Getting the data back
+            - Updating the data
+            - Deleting the data
+            - Sequence token handling
+        """
+        # We can post arbitrary data to the endpoint
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "foo=bar"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        rendezvous_id = channel.json_body["id"]
+        sequence_token = channel.json_body["sequence_token"]
+        expires_ts = channel.json_body["expires_ts"]
+        self.assertGreater(expires_ts, self.hs.get_clock().time_msec())
+
+        session_endpoint = msc4108_endpoint + f"/{rendezvous_id}"
+
+        # We can get the data back
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["data"], "foo=bar")
+        self.assertEqual(channel.json_body["sequence_token"], sequence_token)
+        self.assertEqual(channel.json_body["expires_ts"], expires_ts)
+
+        # We can update the data
+        channel = self.make_request(
+            "PUT",
+            session_endpoint,
+            {"sequence_token": sequence_token, "data": "foo=baz"},
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+        old_sequence_token = sequence_token
+        new_sequence_token = channel.json_body["sequence_token"]
+
+        # If we try to update it again with the old etag, it should fail
+        channel = self.make_request(
+            "PUT",
+            session_endpoint,
+            {"sequence_token": old_sequence_token, "data": "bar=baz"},
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 409)
+        self.assertEqual(
+            channel.json_body["errcode"], "IO_ELEMENT_MSC4108_CONCURRENT_WRITE"
+        )
+
+        # We should get the updated data
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["data"], "foo=baz")
+        self.assertEqual(channel.json_body["sequence_token"], new_sequence_token)
+        self.assertEqual(channel.json_body["expires_ts"], expires_ts)
+
+        # We can delete the data
+        channel = self.make_request(
+            "DELETE",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+
+        # If we try to get the data again, it should fail
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 404)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    @unittest.skip_unless(HAS_AUTHLIB, "requires authlib")
+    @override_config(
+        {
+            "disable_registration": True,
+            "matrix_authentication_service": {
+                "enabled": True,
+                "secret": "secret_value",
+                "endpoint": "https://issuer",
+            },
+            "experimental_features": {
+                "msc4108v2025_mode": "authenticated",
+            },
+        }
+    )
+    def test_rendezvous_requires_authentication(self) -> None:
+        """
+        Test the MSC4108 rendezvous endpoint when configured with the mode authenticated, including:
+            - Creating a session
+            - Getting the data back
+            - Updating the data
+            - Deleting the data
+            - Sequence token handling
+        """
+        self.setup_mock_oauth()
+        alice_token = self.register_oauth_user("alice", "device1")
+
+        # This should fail without authentication:
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "foo=bar"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 401)
+
+        # This should work as we are now authenticated
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "foo=bar"},
+            access_token=alice_token,
+        )
+        self.assertEqual(channel.code, 200)
+        rendezvous_id = channel.json_body["id"]
+        sequence_token = channel.json_body["sequence_token"]
+        expires_ts = channel.json_body["expires_ts"]
+        self.assertGreater(expires_ts, self.hs.get_clock().time_msec())
+
+        session_endpoint = msc4108_endpoint + f"/{rendezvous_id}"
+
+        # We can get the data back without authentication
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["data"], "foo=bar")
+        self.assertEqual(channel.json_body["sequence_token"], sequence_token)
+        self.assertEqual(channel.json_body["expires_ts"], expires_ts)
+
+        # We can update the data without authentication
+        channel = self.make_request(
+            "PUT",
+            session_endpoint,
+            {"sequence_token": sequence_token, "data": "foo=baz"},
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+        new_sequence_token = channel.json_body["sequence_token"]
+
+        # We should get the updated data without authentication
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["data"], "foo=baz")
+        self.assertEqual(channel.json_body["sequence_token"], new_sequence_token)
+        self.assertEqual(channel.json_body["expires_ts"], expires_ts)
+
+        # We can delete the data without authentication
+        channel = self.make_request(
+            "DELETE",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+
+        # If we try to get the data again, it should fail
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 404)
+        self.assertEqual(channel.json_body["errcode"], "M_NOT_FOUND")
+
+    @unittest.skip_unless(HAS_AUTHLIB, "requires authlib")
+    @override_config(
+        {
+            "disable_registration": True,
+            "matrix_authentication_service": {
+                "enabled": True,
+                "secret": "secret_value",
+                "endpoint": "https://issuer",
+            },
+            "experimental_features": {
+                "msc4108v2025_mode": "public",
+            },
+        }
+    )
+    def test_expiration(self) -> None:
+        """
+        Test that entries are evicted after a TTL.
+        """
+        # Start a new session
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "foo=bar"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        session_endpoint = msc4108_endpoint + "/" + channel.json_body["id"]
+
+        # Sanity check that we can get the data back
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["data"], "foo=bar")
+
+        # Advance the clock, TTL of entries is 2 minutes
+        self.reactor.advance(120)
+
+        # Get the data back, it should be gone
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 404)
+
+    @unittest.skip_unless(HAS_AUTHLIB, "requires authlib")
+    @override_config(
+        {
+            "disable_registration": True,
+            "matrix_authentication_service": {
+                "enabled": True,
+                "secret": "secret_value",
+                "endpoint": "https://issuer",
+            },
+            "experimental_features": {
+                "msc4108v2025_mode": "public",
+            },
+        }
+    )
+    def test_capacity(self) -> None:
+        """
+        Test that a capacity limit is enforced on the rendezvous sessions, as old
+        entries are evicted at an interval when the limit is reached.
+        """
+        # Start a new session
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "foo=bar"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        session_endpoint = msc4108_endpoint + "/" + channel.json_body["id"]
+
+        # Sanity check that we can get the data back
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["data"], "foo=bar")
+
+        # We advance the clock to make sure that this entry is the "lowest" in the session list
+        self.reactor.advance(1)
+
+        # Start a lot of new sessions
+        for _ in range(100):
+            channel = self.make_request(
+                "POST",
+                msc4108_endpoint,
+                {"data": "foo=bar"},
+                access_token=None,
+            )
+            self.assertEqual(channel.code, 200)
+
+        # Get the data back, it should still be there, as the eviction hasn't run yet
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 200)
+
+        # Advance the clock, as it will trigger the eviction
+        self.reactor.advance(59)
+
+        # Get the data back, it should be gone
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 404)
+
+    @unittest.skip_unless(HAS_AUTHLIB, "requires authlib")
+    @override_config(
+        {
+            "disable_registration": True,
+            "matrix_authentication_service": {
+                "enabled": True,
+                "secret": "secret_value",
+                "endpoint": "https://issuer",
+            },
+            "experimental_features": {
+                "msc4108v2025_mode": "public",
+            },
+        }
+    )
+    def test_hard_capacity(self) -> None:
+        """
+        Test that a hard capacity limit is enforced on the rendezvous sessions, as old
+        entries are evicted immediately when the limit is reached.
+        """
+        # Start a new session
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "foo=bar"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        session_endpoint = msc4108_endpoint + "/" + channel.json_body["id"]
+        # We advance the clock to make sure that this entry is the "lowest" in the session list
+        self.reactor.advance(1)
+
+        # Sanity check that we can get the data back
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["data"], "foo=bar")
+
+        # Start a lot of new sessions
+        for _ in range(200):
+            channel = self.make_request(
+                "POST",
+                msc4108_endpoint,
+                {"data": "foo=bar"},
+                access_token=None,
+            )
+            self.assertEqual(channel.code, 200)
+
+        # Get the data back, it should already be gone as we hit the hard limit
+        channel = self.make_request(
+            "GET",
+            session_endpoint,
+            access_token=None,
+        )
+
+        self.assertEqual(channel.code, 404)
+
+    @unittest.skip_unless(HAS_AUTHLIB, "requires authlib")
+    @override_config(
+        {
+            "disable_registration": True,
+            "matrix_authentication_service": {
+                "enabled": True,
+                "secret": "secret_value",
+                "endpoint": "https://issuer",
+            },
+            "experimental_features": {
+                "msc4108v2025_mode": "public",
+            },
+        }
+    )
+    def test_data_type(self) -> None:
+        """
+        Test that the data field is restricted to string.
+        """
+        invalid_datas: list[Any] = [123214, ["asd"], {"asd": "asdsad"}, None]
+
+        # We cannot post invalid non-string data field values to the endpoint
+        for invalid_data in invalid_datas:
+            channel = self.make_request(
+                "POST",
+                msc4108_endpoint,
+                {"data": invalid_data},
+                access_token=None,
+            )
+            self.assertEqual(channel.code, 400)
+            self.assertEqual(channel.json_body["errcode"], "M_INVALID_PARAM")
+
+        # Make a valid request
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "test"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        rendezvous_id = channel.json_body["id"]
+        sequence_token = channel.json_body["sequence_token"]
+
+        session_endpoint = msc4108_endpoint + f"/{rendezvous_id}"
+
+        # We can't update the data with invalid data
+        for invalid_data in invalid_datas:
+            channel = self.make_request(
+                "PUT",
+                session_endpoint,
+                {"sequence_token": sequence_token, "data": invalid_data},
+                access_token=None,
+            )
+            self.assertEqual(channel.code, 400)
+            self.assertEqual(channel.json_body["errcode"], "M_INVALID_PARAM")
+
+    @unittest.skip_unless(HAS_AUTHLIB, "requires authlib")
+    @override_config(
+        {
+            "disable_registration": True,
+            "matrix_authentication_service": {
+                "enabled": True,
+                "secret": "secret_value",
+                "endpoint": "https://issuer",
+            },
+            "experimental_features": {
+                "msc4108v2025_mode": "public",
+            },
+        }
+    )
+    def test_max_length(self) -> None:
+        """
+        Test that the data max length is restricted.
+        """
+        too_long_data = "a" * 5000  # MSC4108 specifies 4KB max length
+
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": too_long_data},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 413)
+        self.assertEqual(channel.json_body["errcode"], "M_TOO_LARGE")
+
+        # Make a valid request
+        channel = self.make_request(
+            "POST",
+            msc4108_endpoint,
+            {"data": "test"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        rendezvous_id = channel.json_body["id"]
+        sequence_token = channel.json_body["sequence_token"]
+
+        session_endpoint = msc4108_endpoint + f"/{rendezvous_id}"
+
+        # We can't update the data with invalid data
+        channel = self.make_request(
+            "PUT",
+            session_endpoint,
+            {"sequence_token": sequence_token, "data": too_long_data},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 413)
+        self.assertEqual(channel.json_body["errcode"], "M_TOO_LARGE")