Digital Signature. document changed, invalid signature.

Forum|Forum|4 years ago
October 22, 2021
1 reply
19821 views

Hi, I have a problem when I digitally sign a document in Reader.

Reader states that "the document has changed after signing" but no changes have been made.

I use Reader DC 2021 007 20091 and Windows 10 20H2.

Why does Reader say so and state that the signature is invalid?

Security digital signatures and esignatures

Amal.

Legend

Hi there

Hope you are doing well and sorry for the trouble. As described, when I digitally sign a document in Reader. Reader states that "the document has changed after signing.

Is this a behavior with a particular PDF file or with all the PDFs that you digitally sign? Please try with a different PDF file and check. If the file is stored on a shared network drive, please download it to your compouter first and try signing it and check.

You may also try to sign the PDF file online via Document Cloud https://documentcloud.adobe.com/link/home/ and see if that helps.

Regards

Amal

S

SeamountainAuthor

Participating Frequently

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Hi Amal,

It does not matter if the document is stored locally on my computer or on a network device, I get the same problem with Adobe saying that the document has been modified, with the result that the signature is invalid.

I did not test to sign online because it is not an alternative for me.

I have seen some posts from 2017 about SHA1 and SHA256 and if I change the windows registry so that Adobe will use SHA1, I will not get the error that the document has been changed.

So what is it that makes Adobe think the document has changed if I use SHA256?

MikelKlink

Participating Frequently

There are some theoretical exploits, generally involving invalid objects and misuse of white space, which I have never seen in real life. Rewriting all the unsigned incremental sections (including the linearization section) prevents these exploits (unless you have an evil signing program).

Well, exploits usually don't need incremental updates, at least not before signing. But rewriting the file indeed gets rid of the dangers by many possible exploits.

Nonetheless, rewriting is not required by the specifications in question, so validators must be good enough to detect all the exploits. Which they aren't yet. But they've gotten much better since https://www.pdf-insecurity.org/ started.

And YES to

unless you have an evil signing program.

The biggest danger most likely is in trusting unknown programs, in particular unknown remote services, to sign documents using your local smartcard or other sscd.

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