Make the signature disappear after the modification of a certified document

Question

Hallo,
    I would like to certify a document to make the people who receive it aware that it is a unique and original document and to discourage people in modifing it.
I've tried using certificate function and, when I activate the certification, the document became protected from editing and my digital signature appears on it.
That's ok but, with one of the many programs used to unlock the protected .pdf documents, I was able to edit the document, my digital signature didn't disappear and the document seem to be original apart from the signature functions which aren't enabled anymore.
I would like a strong visual change on the document, like the disappearing of the signature or the appearing of a mark, in order to tell people:"If you see/don't see this mark the document could have been modified and you don't have to rely on it", is it possible?
I'm using Adobe Acrobat 8 Pro.
Thank you,
Alberto Bellini

Answer 1

Hi Chris,
Just to be clear, I'm only talking about what Acrobat does. To begin with, Acrobat will respect (enforce) the permissions that correspond to a Digital Signature (be it a Certifying or Regular signature). If somehow you do something to the file that is not allowed by the permission set, then the signsature will be rendered invalid.
The question I have is what application is making the change. When it comes to third party software (i.e. not made by Adobe) as long as it can open the file the user can do whatever they want. You can always open a PDF in a text editor and make a change. Whether or not the third party software processes the signature or obeys the permissions is a separate question.
Finally, back to your question. If you do make a change using third party software that is outside the scope of the signature, when you reopen the file in Acrobat (and when I say Acrobat I really mean both Acrobat and Reader, but I'm too lazy to type both) it will show up as invalid. If you made changes that are within the scope of the signature, then the signature would be valid with changes. It would be just as if you made an allowable change using Acrobat.
For example, if adding form data was an allowable change for a Certifying signature, and your ability to write in PDF is strong enough to add the data to the correct object using the third party software, and update the XREF table while keeping the signature byte range intact, when opening the file in Acrobat you would get a valid signature.
Steve