diff --git a/.eslintrc.json b/.eslintrc.json
index 3ad61648..15bb96ea 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -193,6 +193,7 @@
                 "ext/js/data/json-schema.js",
                 "ext/js/data/options-util.js",
                 "ext/js/data/permissions-util.js",
+                "ext/js/data/sandbox/string-util.js",
                 "ext/js/dom/simple-dom-parser.js",
                 "ext/js/general/cache-map.js",
                 "ext/js/general/object-property-accessor.js",
diff --git a/ext/background.html b/ext/background.html
index 57caa671..6259e519 100644
--- a/ext/background.html
+++ b/ext/background.html
@@ -36,6 +36,7 @@
 <script src="/js/data/json-schema.js"></script>
 <script src="/js/data/options-util.js"></script>
 <script src="/js/data/permissions-util.js"></script>
+<script src="/js/data/sandbox/string-util.js"></script>
 <script src="/js/dom/native-simple-dom-parser.js"></script>
 <script src="/js/general/cache-map.js"></script>
 <script src="/js/general/object-property-accessor.js"></script>
diff --git a/ext/info.html b/ext/info.html
index 5872dda4..74d669c4 100644
--- a/ext/info.html
+++ b/ext/info.html
@@ -68,6 +68,7 @@
 <script src="/js/comm/cross-frame-api.js"></script>
 <script src="/js/data/anki-util.js"></script>
 <script src="/js/data/permissions-util.js"></script>
+<script src="/js/data/sandbox/string-util.js"></script>
 <script src="/js/dom/document-focus-controller.js"></script>
 <script src="/js/dom/html-template-collection.js"></script>
 <script src="/js/pages/settings/backup-controller.js"></script>
diff --git a/ext/js/data/sandbox/string-util.js b/ext/js/data/sandbox/string-util.js
new file mode 100644
index 00000000..37e021c9
--- /dev/null
+++ b/ext/js/data/sandbox/string-util.js
@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Class containing generic string utility functions.
+ */
+class StringUtil {
+    /**
+     * Decodes the contents of an ArrayBuffer using UTF8.
+     * @param arrayBuffer The input ArrayBuffer.
+     * @returns A UTF8-decoded string.
+     */
+    static arrayBufferUtf8Decode(arrayBuffer) {
+        try {
+            return new TextDecoder('utf-8').decode(arrayBuffer);
+        } catch (e) {
+            return decodeURIComponent(escape(this.arrayBufferToBinaryString(arrayBuffer)));
+        }
+    }
+
+    /**
+     * Converts the contents of an ArrayBuffer to a base64 string.
+     * @param arrayBuffer The input ArrayBuffer.
+     * @returns A base64 string representing the binary content.
+     */
+    static arrayBufferToBase64(arrayBuffer) {
+        return btoa(this.arrayBufferToBinaryString(arrayBuffer));
+    }
+
+    /**
+     * Converts the contents of an ArrayBuffer to a binary string.
+     * @param arrayBuffer The input ArrayBuffer.
+     * @returns A string representing the binary content.
+     */
+    static arrayBufferToBinaryString(arrayBuffer) {
+        const bytes = new Uint8Array(arrayBuffer);
+        try {
+            return String.fromCharCode(...bytes);
+        } catch (e) {
+            let binary = '';
+            for (let i = 0, ii = bytes.byteLength; i < ii; ++i) {
+                binary += String.fromCharCode(bytes[i]);
+            }
+            return binary;
+        }
+    }
+}
diff --git a/ext/js/media/audio-downloader.js b/ext/js/media/audio-downloader.js
index e3b507b4..4bc4cf1b 100644
--- a/ext/js/media/audio-downloader.js
+++ b/ext/js/media/audio-downloader.js
@@ -19,6 +19,7 @@
  * JsonSchema
  * NativeSimpleDOMParser
  * SimpleDOMParser
+ * StringUtil
  */
 
 class AudioDownloader {
@@ -257,7 +258,7 @@ class AudioDownloader {
             throw new Error('Could not retrieve audio');
         }
 
-        const data = this._arrayBufferToBase64(arrayBuffer);
+        const data = StringUtil.arrayBufferToBase64(arrayBuffer);
         const contentType = response.headers.get('Content-Type');
         return {data, contentType};
     }
@@ -288,10 +289,6 @@ class AudioDownloader {
         return digest;
     }
 
-    _arrayBufferToBase64(arrayBuffer) {
-        return btoa(String.fromCharCode(...new Uint8Array(arrayBuffer)));
-    }
-
     _createSimpleDOMParser(content) {
         if (typeof NativeSimpleDOMParser !== 'undefined' && NativeSimpleDOMParser.isSupported()) {
             return new NativeSimpleDOMParser(content);
diff --git a/ext/js/pages/settings/backup-controller.js b/ext/js/pages/settings/backup-controller.js
index 02ad368c..cf1a6b0c 100644
--- a/ext/js/pages/settings/backup-controller.js
+++ b/ext/js/pages/settings/backup-controller.js
@@ -18,6 +18,7 @@
 /* global
  * DictionaryController
  * OptionsUtil
+ * StringUtil
  */
 
 class BackupController {
@@ -317,17 +318,8 @@ class BackupController {
         return warnings;
     }
 
-    _utf8Decode(arrayBuffer) {
-        try {
-            return new TextDecoder('utf-8').decode(arrayBuffer);
-        } catch (e) {
-            const binaryString = String.fromCharCode.apply(null, new Uint8Array(arrayBuffer));
-            return decodeURIComponent(escape(binaryString));
-        }
-    }
-
     async _importSettingsFile(file) {
-        const dataString = this._utf8Decode(await this._readFileArrayBuffer(file));
+        const dataString = StringUtil.arrayBufferUtf8Decode(await this._readFileArrayBuffer(file));
         const data = JSON.parse(dataString);
 
         // Type check
diff --git a/ext/settings.html b/ext/settings.html
index cd2376d6..3260810c 100644
--- a/ext/settings.html
+++ b/ext/settings.html
@@ -3438,6 +3438,7 @@
 <script src="/js/data/json-schema.js"></script>
 <script src="/js/data/options-util.js"></script>
 <script src="/js/data/permissions-util.js"></script>
+<script src="/js/data/sandbox/string-util.js"></script>
 <script src="/js/dom/document-focus-controller.js"></script>
 <script src="/js/dom/document-util.js"></script>
 <script src="/js/dom/dom-data-binder.js"></script>
diff --git a/ext/sw.js b/ext/sw.js
index 6394260e..6443c57f 100644
--- a/ext/sw.js
+++ b/ext/sw.js
@@ -33,6 +33,7 @@ self.importScripts(
     '/js/data/json-schema.js',
     '/js/data/options-util.js',
     '/js/data/permissions-util.js',
+    '/js/data/sandbox/string-util.js',
     '/js/dom/simple-dom-parser.js',
     '/js/general/cache-map.js',
     '/js/general/object-property-accessor.js',