Base64 Decoder — Chrome Extension

Quickly decode Base64-encoded text from webpages or via the popup UI.

What this repository contains

A small Chrome extension that provides three main ways to decode Base64:

• Auto-decode: a content script (content.js) scans page text nodes and replaces Base64-looking tokens with decoded text (or links if the decoded text is a URL).
• Context menu: a right-click "Decode Base64" item (registered by background.js) decodes the selected text and opens a new tab with the decoded result and copy/download buttons.
• Popup UI: popup.html + popup.js expose a small UI to paste Base64, decode it to UTF-8 or binary hex, copy to clipboard, or download.

Files

• manifest.json — Chrome Extension manifest (Manifest V3). Declares permissions, content scripts, icons, and the service worker.
• content.js — Content script that walks DOM text nodes, heuristically finds Base64-like tokens, decodes them (if valid), and replaces them inline. Converts decoded URLs to clickable links.
• background.js — Service worker that creates the context menu and handles decoding of selected text, opening a new tab with results.
• popup.html — Popup UI markup for manual decode operations.
• popup.js — Logic for the popup: decode button, paste/copy/download/clear actions, and handling messages (for selections forwarded to popup).
• styles.css — Styling for the popup UI.
• icons/ — Icons used by the extension.

Key features

• Automatically decodes Base64-like tokens you encounter on pages and replaces them inline.
• Right-click a page selection and choose "Decode Base64" to quickly see (and copy/download) decoded content in a new tab.
• Popup provides manual decoding with convenient copy/download options and the ability to export binary as a file.
• If decoded bytes are valid UTF-8, the extension shows text; otherwise it shows hex bytes.

How it works (technical summary)

• Detection: content.js uses a heuristic isProbablyBase64() which checks for a Base64 character set and that the token length is divisible by 4. The script also ignores short tokens (the UI requires tokens > 12 characters for auto-replaces).
• Decoding: The code uses atob() to convert from Base64 to a binary string, turns that into a Uint8Array, then tries TextDecoder('utf-8') to detect UTF-8 text. If decoding to UTF-8 fails, bytes are shown as hex.
• URLs: After decoding, if the resulting text looks like a URL, content.js creates an <a> element so the link is clickable.
• Context menu: background.js registers a context menu item and decodes the selection when clicked, opening a data:text/html tab showing decoded text or hex, plus copy/download buttons.

Usage

1. Load the extension (developer mode):

   • Open chrome://extensions
   • Enable "Developer mode"
   • Click "Load unpacked" and select this repository's folder

2. Popup UI:

   • Click the extension action (toolbar icon) to open the popup.
   • Paste or type Base64 into the input box.
   • Click "Decode". If decoded bytes form UTF-8 text, that text is shown. Otherwise the hex bytes are shown.
   • Use Copy or Download to export results. Use the file type select to choose .txt, .json, or .bin.

3. Context menu:

   • Select Base64 text on any webpage, right-click, choose "Decode Base64".
   • A new tab opens with the decoded result and buttons to copy or download.

4. Auto-decode on pages:

   • When a page loads, content.js scans visible text nodes. Long Base64-like tokens are replaced inline with decoded text. Decoded URLs become clickable links.

Examples

• Input (Base64): SGVsbG8sIFdvcmxkIQ==

• Decoded: Hello, World!

• Binary (non-UTF8) will be shown as hex bytes like: ff d8 ff e0 00 10 4a 46 ... (and can be downloaded as .bin).

Permissions & privacy

• Permissions in manifest.json: contextMenus, clipboardWrite, scripting, activeTab.
• Everything runs locally in the browser. No remote servers are contacted by the extension. Opening a decoded result uses a data: URL in a new tab.
• Be cautious when decoding untrusted Base64 — decoded content may contain URLs or text you didn't expect. The extension does not execute decoded content; it only displays it. The popup/download functions create blobs or write to the clipboard.

Limitations & edge cases

• Heuristic detection may false-positive on non-Base64 tokens that match the character set and length constraints.
• Very large Base64 blobs may impact page performance if auto-decoded. The content script attempts to minimize impact by only decoding reasonably long tokens.
• If TextDecoder('utf-8') fails, the extension shows hex bytes — this is intentional to surface non-text binary content.
• The content script avoids modifying SCRIPT and STYLE nodes but may modify other places where replacing text could affect layout. Use with caution on complex pages.