Given a pdf that is to be made available to the general public who will download them, fill them in and upload them to our server. How can one be sure that this pdf when returned was created by the original publisher and remains unchanged apart from the fact that it's fields have been filled in?
Can certificates (fill & sign) be used to achieve this? Access / Edit control is not required, anyone should be able to fill them in, but trustworthyness that the rest of the file remains unmodified is required.
You're talking about a certifying signature, which is part of the Security tools in Acrobat, not part of the fill & sign system. It protects the document, allowing only specific types of change - but to do it you need a digital ID, and if it's a document going out to the public then that ID should be recognized by Adobe's Trust List - which means buying a hardware-backed certificate from one of the official suppliers. It's not cheap.
Signing can be used to verify that a file was not modified, and check against a certificate to see who signed it; that's what it is for.
Some kinds of signing allow form filling, I believe (but haven't tried, sorry).
Your processes need to check, or your staff need to be trained to check the ACTUAL signature - not just to look at the page. Many people believe the signature is a mark on a page and leave themselves open to fraud. Receipt of a file looking like the original and claiming to be signed means nothing - I'm sure this is your concern too but training is needed to make others share the concern.
Verifying a signed pdf is not an issue, that's all good. It's a matter of determining whether there's a way (as you say) to sign but allow form filling. Signing is required in order to verify that nothing else but the form data has changed.
You're talking about a certifying signature, which is part of the Security tools in Acrobat, not part of the fill & sign system. It protects the document, allowing only specific types of change - but to do it you need a digital ID, and if it's a document going out to the public then that ID should be recognized by Adobe's Trust List - which means buying a hardware-backed certificate from one of the official suppliers. It's not cheap.
Can't you can create your own digital ID for free though?
Yes, but Acrobat/Reader will show a big error banner for anyone who hasn't manually installed that self-signed certificate, and you're talking about a form that is "made available to the general public" so the last thing you want is a warning message saying the file "has problems". Self-signed = scammer.
That is why the vendors on the Adobe Approved Trust List (AATL) can charge a lot for the certificate. It's legal proof of the signer's identity that can't be faked.
"I wonder how can you say "Receipt of a file looking like the original and claiming to be signed means nothing""
The essence of my point is that digital signing puts a mark on a page in the PDF. Time and again we hear from people obsessed with these marks, and clearly using them to verify signatures. Sometimes the people implementing the signatures don't understand the crucial difference, sometimes they understand clearly but the end users look at the page anyway. It requires training and consistent procedures to get people to actually check certificates using the Signature view.
I think it was a terrible mistake to have digital signatures associated with a mark on the page.
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