How it works and its limits

Sidik Dokumen hides a unique code inside the file. That code is linked to a recipient record on our server. Here is what it survives and what it doesn't.

What it survives

  • The file is forwarded as is by email, Google Drive or a USB stick.
  • The file is renamed.
  • Metadata is removed with common tools. PDF, Word, Excel, PowerPoint and PNG files still carry other marks inside.
  • The file is opened and saved again. Usually some marks remain, depending on the app.
  • A PNG image is cropped, as long as the crop isn't too small.

What removes the mark

  • The content is retyped into a new document.
  • The document is printed, then scanned or photocopied.
  • The screen is photographed or screenshotted.
  • The content is copied and pasted into a new file.
  • A JPEG image is re-compressed, for example when sent through a chat app. JPEG is only marked in metadata, so its mark is weak.
  • The file is converted to another format, for example Word to PDF in another app.

Formats and durability

  • PDFStrong
  • DOCXStrong
  • XLSXStrong
  • PPTXStrong
  • PNGStrong
  • JPEGWeak

JPEG is only marked in its metadata. The mark is lost when the image is re-compressed, sent through a chat app, or screenshotted. For images that matter, use PNG.

The data we keep

The recipient names and notes you write are stored in our database, because that is what a check with the owner key shows.

From the files, we keep only the original filename, size, type and digital fingerprint (SHA-256). File contents are not stored in the database.

Originals are deleted as soon as the copies are made. Marked copies are deleted when their download window ends (24 hours free, 7 days Lite, 30 days Pro). Files you check are deleted as soon as the check finishes.

From the owner key, we keep only its fingerprint. Without the key, nobody can reveal recipient names through the Check page, including us.

If you buy a plan, we keep your email, a fingerprint of your password (not the password), your order history and which documents were marked through your account. Owner keys are still never stored.

IP addresses are only recorded in scrambled (hashed) form, to limit requests per hour.

If our server key changes

Every mark is signed with a secret server key. If that key is changed, old copies can no longer be recognised. That's why it is never changed without a strong reason.

Watermark a document