Digital signatures

Our administration now requests all documents to be digitally signed, with a certificate that each employee receives from the DFN. Windows and Mac users employ Acrobat Reader for this purpose, but what software can be used under Linux?

My first choice was LibreOffice, which has offered this functionality already for several years. Signing a pristine pdf works well indeed, apart from the fact that LibeOffice does not create a placeholder for the signature. However, signing a document that has already been signed by Window users turned out to be simply not possible (here's the 12 years old bug report – and here's the inverse one that is just as old).

The next candidate was Okular, the PDF viewer of KDE, which has been sponsored by the University of Dresden to implement this functionality. But only half-way, it seems to me. I could sign most (but not all) documents, but I couldn't configure the placeholder at all. As the font size in the signature box does not scale with the size of the box, the name of the person signing is often cut off. How difficult can it be to implement such a very basic and obvious requirement?

Finally, I turned to Master PDF Editor, which I've occasionally used in the past for annotations when Evince did not yet offer this possibility. I was actually not surprised that this feature-rich PDF editor also offers digital signatures, but I was pleased that the software comes with its own certificate storage and that the signature placeholder is highly configurable. For example, one can configure the placeholder to include one's own analogue signature as a background.

Alas, using the Master PDF editor without any restrictions requires purchasing a license. The free version is unlimited, but leaves a watermark in documents that have been digitally signed or otherwise altered with the software. The licence is very fairly priced, but as the software is developed in Russia, even asking for one is frowned upon and politically inopportune. Fortunately, nobody has yet complained about the watermark in documents I have digitally signed. 😈