How to convert signed documents to PDF/A?

I often get the question whether it is possible to convert digitally signed documents to PDF/A. Because there's no short answer to this I thought it would be helpful to explore the topic a bit into more detail. What technical means does PDF offer to perform such a conversion?

Generally spoken, the conversion of a PDF document to a PDF/A document is done by deleting and updating existing objects and adding new objects. Theoretically, this could be performed by using the incremental update mechanism of PDF which would not break the existing digital signature. Practically, however, the most important viewer, Acrobat Reader, only accepts very limited updates, such as adding comments, once a document is digitally signed. Thus, the incremental update feature cannot be used for this purpose and, unfortunately, there is no other suitable mechanism.

But how can the problem be solved then?

There are several possibilities. A simple solution would be to convert the document to a PDF/A-3 file and attach the original file to it. However, in some archiving environments, PDF/A-3 is not permitted. In these cases the conversion tool can create a page containing the verification protocol of the digital signature, convert the original file without the signature to a PDF/A file, merge both files and resign the result.

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