diff --git a/packages-exp/auth-exp/src/core/util/event_id.test.ts b/packages-exp/auth-exp/src/core/util/event_id.test.ts
new file mode 100644
index 0000000000..510b66b5c2
--- /dev/null
+++ b/packages-exp/auth-exp/src/core/util/event_id.test.ts
@@ -0,0 +1,33 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { expect } from 'chai';
+import { _generateEventId } from './event_id';
+
+describe('core/util/event_id', () => {
+  it('sub-15 digit id', () => {
+    expect(_generateEventId('', 10)).to.have.length(10);
+  });
+
+  it('15 digit id', () => {
+    expect(_generateEventId('', 15)).to.have.length(15);
+  });
+
+  it('above-15 digit id', () => {
+    expect(_generateEventId('', 20)).to.have.length(20);
+  });
+});
diff --git a/packages-exp/auth-exp/src/core/util/event_id.ts b/packages-exp/auth-exp/src/core/util/event_id.ts
index ef5abe1839..6e2e02fca6 100644
--- a/packages-exp/auth-exp/src/core/util/event_id.ts
+++ b/packages-exp/auth-exp/src/core/util/event_id.ts
@@ -15,6 +15,10 @@
  * limitations under the License.
  */
 
-export function _generateEventId(prefix?: string): string {
-  return `${prefix ? prefix : ''}${Math.floor(Math.random() * 1000000000)}`;
+export function _generateEventId(prefix = '', digits = 10): string {
+  let random = '';
+  for (let i = 0; i < digits; i++) {
+    random += Math.floor(Math.random() * 10);
+  }
+  return prefix + random;
 }
diff --git a/packages-exp/auth-exp/src/platform_browser/messagechannel/sender.ts b/packages-exp/auth-exp/src/platform_browser/messagechannel/sender.ts
index f8f4147c52..1452c9857f 100644
--- a/packages-exp/auth-exp/src/platform_browser/messagechannel/sender.ts
+++ b/packages-exp/auth-exp/src/platform_browser/messagechannel/sender.ts
@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+import { _generateEventId } from '../../core/util/event_id';
 import {
   _SenderRequest,
   _EventType,
@@ -32,10 +33,6 @@ interface MessageHandler {
   onMessage: EventListenerOrEventListenerObject;
 }
 
-function generateEventId(prefix = '', digits = 20): string {
-  return `${prefix}${Math.floor(Math.random() * Math.pow(10, digits))}`;
-}
-
 /**
  * Interface for sending messages and waiting for a completion response.
  *
@@ -91,7 +88,7 @@ export class Sender {
     let completionTimer: any;
     let handler: MessageHandler;
     return new Promise<_ReceiverMessageResponse<T>>((resolve, reject) => {
-      const eventId = generateEventId();
+      const eventId = _generateEventId('', 20);
       messageChannel.port1.start();
       const ackTimer = setTimeout(() => {
         reject(new Error(_MessageError.UNSUPPORTED_EVENT));