diff --git a/packages/firestore/src/platform/platform.ts b/packages/firestore/src/platform/platform.ts
index f206ab1a77e..1ab582ea05f 100644
--- a/packages/firestore/src/platform/platform.ts
+++ b/packages/firestore/src/platform/platform.ts
@@ -43,6 +43,13 @@ export interface Platform {
   /** Converts a binary string to a Base64 encoded string. */
   btoa(raw: string): string;
 
+  /**
+   * Generates `nBytes` of random bytes.
+   *
+   * If `nBytes < 0` , an error will be thrown.
+   */
+  randomBytes(nBytes: number): Uint8Array;
+
   /** The Platform's 'window' implementation or null if not available. */
   readonly window: Window | null;
 
diff --git a/packages/firestore/src/platform_browser/browser_platform.ts b/packages/firestore/src/platform_browser/browser_platform.ts
index 48b19339e29..38133db4df5 100644
--- a/packages/firestore/src/platform_browser/browser_platform.ts
+++ b/packages/firestore/src/platform_browser/browser_platform.ts
@@ -20,9 +20,11 @@ import { Platform } from '../platform/platform';
 import { Connection } from '../remote/connection';
 import { JsonProtoSerializer } from '../remote/serializer';
 import { ConnectivityMonitor } from './../remote/connectivity_monitor';
+
 import { NoopConnectivityMonitor } from '../remote/connectivity_monitor_noop';
 import { BrowserConnectivityMonitor } from './browser_connectivity_monitor';
 import { WebChannelConnection } from './webchannel_connection';
+import { debugAssert } from '../util/assert';
 
 // Implements the Platform API for browsers and some browser-like environments
 // (including ReactNative).
@@ -72,4 +74,23 @@ export class BrowserPlatform implements Platform {
   btoa(raw: string): string {
     return btoa(raw);
   }
+
+  randomBytes(nBytes: number): Uint8Array {
+    debugAssert(nBytes >= 0, `Expecting non-negative nBytes, got: ${nBytes}`);
+
+    // Polyfills for IE and WebWorker by using `self` and `msCrypto` when `crypto` is not available.
+    const crypto =
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      typeof self !== 'undefined' && (self.crypto || (self as any)['msCrypto']);
+    const bytes = new Uint8Array(nBytes);
+    if (crypto) {
+      crypto.getRandomValues(bytes);
+    } else {
+      // Falls back to Math.random
+      for (let i = 0; i < nBytes; i++) {
+        bytes[i] = Math.floor(Math.random() * 256);
+      }
+    }
+    return bytes;
+  }
 }
diff --git a/packages/firestore/src/platform_node/node_platform.ts b/packages/firestore/src/platform_node/node_platform.ts
index 927209acd5a..2fe74f7d68b 100644
--- a/packages/firestore/src/platform_node/node_platform.ts
+++ b/packages/firestore/src/platform_node/node_platform.ts
@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+import { randomBytes } from 'crypto';
 import { inspect } from 'util';
 
 import { DatabaseId, DatabaseInfo } from '../core/database_info';
@@ -27,6 +28,7 @@ import { NoopConnectivityMonitor } from './../remote/connectivity_monitor_noop';
 
 import { GrpcConnection } from './grpc_connection';
 import { loadProtos } from './load_protos';
+import { debugAssert } from '../util/assert';
 
 export class NodePlatform implements Platform {
   readonly base64Available = true;
@@ -75,4 +77,10 @@ export class NodePlatform implements Platform {
   btoa(raw: string): string {
     return new Buffer(raw, 'binary').toString('base64');
   }
+
+  randomBytes(nBytes: number): Uint8Array {
+    debugAssert(nBytes >= 0, `Expecting non-negative nBytes, got: ${nBytes}`);
+
+    return randomBytes(nBytes);
+  }
 }
diff --git a/packages/firestore/src/util/misc.ts b/packages/firestore/src/util/misc.ts
index 8a46a11292c..35a7d40baf4 100644
--- a/packages/firestore/src/util/misc.ts
+++ b/packages/firestore/src/util/misc.ts
@@ -16,6 +16,7 @@
  */
 
 import { debugAssert } from './assert';
+import { PlatformSupport } from '../platform/platform';
 
 export type EventHandler<E> = (value: E) => void;
 export interface Indexable {
@@ -27,11 +28,27 @@ export class AutoId {
     // Alphanumeric characters
     const chars =
       'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
+    // The largest byte value that is a multiple of `char.length`.
+    const maxMultiple = Math.floor(256 / chars.length) * chars.length;
+    debugAssert(
+      0 < maxMultiple && maxMultiple < 256,
+      `Expect maxMultiple to be (0, 256), but got ${maxMultiple}`
+    );
+
     let autoId = '';
-    for (let i = 0; i < 20; i++) {
-      autoId += chars.charAt(Math.floor(Math.random() * chars.length));
+    const targetLength = 20;
+    while (autoId.length < targetLength) {
+      const bytes = PlatformSupport.getPlatform().randomBytes(40);
+      for (let i = 0; i < bytes.length; ++i) {
+        // Only accept values that are [0, maxMultiple), this ensures they can
+        // be evenly mapped to indices of `chars` via a modulo operation.
+        if (autoId.length < targetLength && bytes[i] < maxMultiple) {
+          autoId += chars.charAt(bytes[i] % chars.length);
+        }
+      }
     }
-    debugAssert(autoId.length === 20, 'Invalid auto ID: ' + autoId);
+    debugAssert(autoId.length === targetLength, 'Invalid auto ID: ' + autoId);
+
     return autoId;
   }
 }
diff --git a/packages/firestore/test/util/test_platform.ts b/packages/firestore/test/util/test_platform.ts
index 1d1f8346d77..cd9d458f63d 100644
--- a/packages/firestore/test/util/test_platform.ts
+++ b/packages/firestore/test/util/test_platform.ts
@@ -266,6 +266,10 @@ export class TestPlatform implements Platform {
   btoa(raw: string): string {
     return this.basePlatform.btoa(raw);
   }
+
+  randomBytes(nBytes: number): Uint8Array {
+    return this.basePlatform.randomBytes(nBytes);
+  }
 }
 
 /** Returns true if we are running under Node. */