Since no one has replied, maybe someone can tell me if the things I'm asking are simply not possible.
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The answer to all the three questions that you have is NO
sorry but when you insert a digital signature it locks the document , and again it is not possible to remove the digital signature field.
Please open the signed pdf file and click on file and go to properties and check the security tab...
--- Tausif Ahmad
In the simplest sense a digital signature is a special checksum of all the bytes in the bytes in the PDF combined with all the bytes in the digital signature. If the document is changed in any way the checksum changes, so the signature becomes invalid. A valid signature ensures that the document hasn't changed since the signature was applied. That stuff that's shown on the signature field is irrelevant. It's just a pretty picture. The real stuff is going on inside the PDF where you can't see it.
Obviously it gets a lot more complext then this. The signature can be selectively applied so that the certain types of changes are allowed. A signature does not add real protection to a PDF, but Acrobat plays along with this scheme by trying to not allow changes that would invalidate the signature, and logging all modifications to the PDF.
1) There are ways to maintain the visual appearance of the signature on the page. If the permissions have been set to allow it the PDF can be flattened, which also partially removes much of the stuff that's going on internally. But this isn't always possible. I had to solve this problem once for a client, so I wrote a plug-in that strips the signature permissions from the PDF (I'll post this to www.pdfscripting.com sometime soon), after which the the PDF can be flattened, keeping the appearance. But of course, after this point the document has to be considered invalid. For example, if the document was a contract, the flattened version would be good for use in an analysis of say all contracts for a particular year, or for copying legal language to another contract, but it couldn't be used as the basis of a legal dispute.
2) I don't understand this one. Do you mean you're deleting the signature field and some of the internal digital signature stuff is still hanging around causing problems? If you own the signature (or the permissions allow), then a standard form reset will "Un-Sign" the PDF. Inserting PDF pages into another PDF will strip out all this info as well.
3) The stamp appearance is set at the time the stamp is applied, it cannot be changed after this point. It's a lot like a PDF stamp.
The source for PDF Scripting Info
Thom and Tausif,
Thank you for your replies, and to Thom especially for your detailed explanation.
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With regard to the very first question you asked "Is it possible to maintain digital signatures when inserting signed PDFs into another PDF?" the answer is yes, if you are adding the file in as a file attachment, but no if you are trying to add the pages into another document. That is, a file attachment maintains it original integrity not unlike attaching a file to an e-mail. But, as mentioned above, if you are trying to interleaf pages either from an unsigned document into a signed document that would break the signature. Conversely, if you are trying to interleaf pages from a signed document into and unsigned document it will work, but the signature won't migrate into the destination doc.
Regarding question three, people in the US think it's a European time format, and Europeans think Adobe is being US-centric. It's actually neither of those, but rather the universal "generalized time format" as defined in the ASN.1 X.208 specification. Everything in the PKI universe is managed through specifications and the time format is no different. So the short answer is no, it's not changeable. Acrobat has to be able to process the time in order to do path validation (which is more complicate than imaginable). Using generalized time is the only way to make that work on all platforms, in all languages, and in all time zones.
Thanks to you and Thom I now have a much better idea about how digital signatures work. I have the need to add a cover page to a signed document, but looks like that is not possible. I appreciate your time.
I have the same need, and I know some people have worked around this. I don't need an encrypted digital signature that checksums the bytes. I need the components of it stamped onto each page of a multiple page document. How can I accomplish this short of printing them all out with signature, scanning then having the scanner combine them into one document?
How can I add a file as a file attachment to an electronically signed document? After the document is signed it seems to not allow me. Can you please explain how this would be done?
It can't be done. A digital signature locks the file. What you can do is attach both the signed file and the attachment to another PDF, either as attachments or as a part of a Portfolio.