diff --git a/keyserver/addons/rust-node-addon/index.js b/keyserver/addons/rust-node-addon/index.js
--- a/keyserver/addons/rust-node-addon/index.js
+++ b/keyserver/addons/rust-node-addon/index.js
@@ -1,18 +1,10 @@
 // @flow
 
-const { platform, arch } = process;
+import type { RustNativeBindingAPI } from '../../src/lib/types/rust-binding-types.js';
 
-type RustAPI = {
-  +registerUser: (
-    userId: string,
-    deviceId: string,
-    username: string,
-    password: string,
-    userPublicKey: string,
-  ) => Promise<string>,
-};
+const { platform, arch } = process;
 
-async function getRustAPI(): Promise<RustAPI> {
+async function getRustAPI(): Promise<RustNativeBindingAPI> {
   let nativeBinding = null;
   if (platform === 'darwin' && arch === 'x64') {
     // $FlowFixMe
@@ -31,11 +23,10 @@
   }
 
   if (!nativeBinding) {
-    throw new Error('Failed to load native binding');
+    throw new Error('Failed to load Rust native binding');
   }
 
-  const { registerUser } = nativeBinding.default;
-  return { registerUser };
+  return nativeBinding.default;
 }
 
 export { getRustAPI };
diff --git a/lib/types/rust-binding-types.js b/lib/types/rust-binding-types.js
new file mode 100644
--- /dev/null
+++ b/lib/types/rust-binding-types.js
@@ -0,0 +1,30 @@
+// @flow
+
+type tunnelBrokerOnReceiveCallback = (
+  err: Error | null,
+  payload: string,
+) => any;
+
+interface TunnelbrokerClientClass {
+  new(
+    deviceId: string,
+    onReceiveCallback: tunnelBrokerOnReceiveCallback,
+  ): TunnelbrokerClientClass;
+  publish(toDeviceId: string, payload: string): Promise<void>;
+}
+
+type RustNativeBindingAPI = {
+  +registerUser: (
+    userId: string,
+    deviceId: string,
+    username: string,
+    password: string,
+    userPublicKey: string,
+  ) => Promise<string>,
+  +TunnelbrokerClient: (
+    deviceId: string,
+    onReceiveCallback: tunnelBrokerOnReceiveCallback,
+  ) => TunnelbrokerClientClass,
+};
+
+export type { RustNativeBindingAPI };