Watermarked — Gmail attachment watermarking by mleroy

Watermarked — watermarking for Gmail attachments

Watermarked automatically adds a watermark to images and PDF files that you attach to a Gmail message, right before sending. The watermark identifies the recipient of the message.

The goal is simple: if a confidential document leaks, the watermark makes it possible to know which recipient this particular copy had been sent to.

How it works

Open a compose window in Gmail and enter at least one recipient.
Attach an image (PNG, JPEG…) or a PDF, by drag-and-drop or via the paperclip icon.
A small red WM button appears on the attachment. Click on it.
The attachment is replaced by its watermarked version, renamed <name>-wm.<extension>.
Send your message normally.
Important: the attachment must be added after the extension is installed/enabled. The file's bytes are captured at the moment it is dropped into the compose window.

What the watermark does

On images: the recipient's text is drawn diagonally, semi-transparent, across the entire surface of the image. The metadata (recipient, send date, send time) is rewritten into the file — comment segment for JPEG, tEXt text chunks for PNG.
On PDFs: each page receives the same semi-transparent diagonal watermark. The document's metadata fields (subject, keywords, modification date) are filled in with the recipient and the timestamp. The author field is explicitly cleared.

Privacy

No data is collected, stored on a server, or transmitted to a third party.
No network request is made. All processing (watermark rendering, metadata rewriting) happens locally in your browser.
The extension only runs on mail.google.com.
Only the activeTab permission is requested. No access to history, other tabs, or cookies.
No information about the sender (address, name, signature) is added to the files.
Code is available here : https://github.com/LeroyM084/watermarked