What to upload
The PDF is the proof.
Best
A sealed PDF
The Google Doc as it looked at seal, or the Authmarks sealed copy, downloaded from the record. Either one is checked byte for byte against what Authmarks signed.
Also works
A detached .c2pa file
The signature saved on its own. It confirms the seal exists, but it can't vouch for a separate copy of the document.
Won't work
A plain export
Downloading the Google Doc again, or printing it, makes a new unsealed file. Ask the author for the sealed PDF instead.
How the check works
Tamper-evident, not locked.
A seal has to be checkable by anyone without seeing the document. So Authmarks signs a C2PA Content Credential (the open standard backed by Adobe, Microsoft and the BBC) into each sealed PDF, and keeps the fingerprint of every file it signed.
- At seal time, Authmarks exports the Google Doc, signs it with the Authmarks seal key and stores the fingerprint of every signed file.
- When you check a file, your browser computes its SHA-256 fingerprint locally. Only that fingerprint is sent. Edit one character, re-save or re-export, and the fingerprint no longer matches.
- A copy that names a real seal but isn’t one of its signed files is reported as changed, so a look-alike can’t borrow a real seal.
- Nothing private is shown. Anyone can confirm a file is sealed and when. The writing itself, the replay and the score stay with the writer and the people they share them with.
- Offline, without us: the free open-source
c2patoolreads the signature in the PDF. Check it chains to the Authmarks root certificate.