Unable to apply a Document time stamp to a Certified PDF

[Horvath Wolf]

Applying a Document time stamp works on digitally signed documents, even if the document has been locked after applying a digital signature. This is expected behavior in order to comply with the requirements of PAdES-B-LTA.

However, if a document was first certified with a digital signature, I won't be able to apply a Document time stamp to it afterwards. Why is that? Am I missing something?

Applying a Document time stamp on such documents while retaining compliance, in my case with eIDAS, is still achievable with other tools based on the DSS framework, however not through Adobe Reader/Acrobat.

Is there a way to achieve this functionality solely by using Adobe PDF solutions?

[Amal.]

Hi Ivan,

Hope you are doing well and sorry to hear that.

It is the designed behavior of Adobe Acrobat/Reader DC application. Once the PDF file is signed and the signature process is completed it gets locked for further editing. For more information about editing a signed PDF please check  the help page https://helpx.adobe.com/acrobat/kb/edit-signed-PDF.html

Regards

Amal

[Horvath Wolf]

Hi Amal, and thank you for answering my question. I understand now that it is not possible to apply a document time stamp to a certified/signature-locked PDF. That is unfortunate, though, because while I understand that locking the document is meant to disable/discourage any changes being made to it, I don't really see how applying a document time stamp would constitute a change to the document. In other words, I don't think that applying a document time stamp would result in invalidating the applied signature(s), but I may be wrong there...

[md555]

Actually this is definitely a serious bug.

Digitally signed documents become invalid after the signer's certificate expires. To extend document's validity, it's necessary to add verification info and fresh document timestamp.

If Acrobat disallows this, all documents signed in Acrobat become invalid after certificate expiration, without any possibility to extend their validity.

[Horvath Wolf]

I agree this is a serious issue for locked documents.

I just tested this again on Acrobat Pro DC version 2022.001.20085 on a blank document which I digitally signed and locked in the process of signing it. After applying the signature, there is no right-click option to "Add Signature Information", and the options to sign/timestamp/certify are all greyed-out and unavailable.

I hope this "designed behaviour" either gets corrected in the future updates, or at least Adobe gives us an explanation of why is it designed like this.

Screenshots:

[md555]

We advise our users to never use "Lock document after signing".

[Amal.]

Hi there

Occasionally, you or the signer may choose to lock the document after signing. In this case, the document becomes read-only and no further changes are allowed; even a signature can’t be added in the document. The Lock document after signing option is useful if you’re the last signer or the only one signer.  as described in the help page shared earlier.

Regards

Amal