diff --git a/keyserver/src/socket/tunnelbroker.js b/keyserver/src/socket/tunnelbroker.js
--- a/keyserver/src/socket/tunnelbroker.js
+++ b/keyserver/src/socket/tunnelbroker.js
@@ -2,12 +2,14 @@
 
 import WebSocket from 'ws';
 
+import { type SessionRequestMessage } from 'lib/types/tunnelbroker-messages.js';
+
 function createTunnelbrokerWebsocket() {
   try {
     const tunnelbrokerSocket = new WebSocket('ws://localhost:51001');
     tunnelbrokerSocket.on('open', () => {
       // TODO: Replace keyserver details with actual details
-      const message = {
+      const message: SessionRequestMessage = {
         type: 'sessionRequest',
         accessToken: 'foobar',
         deviceId: 'foo',
diff --git a/lib/types/tunnelbroker-messages.js b/lib/types/tunnelbroker-messages.js
new file mode 100644
--- /dev/null
+++ b/lib/types/tunnelbroker-messages.js
@@ -0,0 +1,9 @@
+// @flow
+
+export type SessionRequestMessage = {
+  +type: 'sessionRequest',
+  +deviceID: string,
+  +accessToken: string,
+  +notifyToken?: string,
+  +deviceType: 'mobile' | 'web' | 'keyserver',
+};