How do I sign a PDF in Okular? I tried searching for about 20 mins, but I can't find any answers...

I have a .png of my signature, and I basically want to insert it into the document on the dotted line. An electronic timestamp etc would also be nice, but is not required.

I found this article, but it uses GIMP instead of Okular. Also, it doesn't appear to timestamp the document.

The article is a bit old (2010), so I wonder if Okular has a PDF signer yet? This post was also helpful, but does not have the turn-key solution that I am looking for.

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    This is an old question that got no response. It got bumped by the tag edit. I assume it's been overtaken by events, but Okular is just a viewer. AFAIK, it can't edit/modify a document (other than possibly save it in a different format). – fixer1234 Oct 27 '15 at 20:24
  • Take a look at How do I digitally sign a PDF? – dma_k Sep 21 '17 at 9:49

How do I sign a PDF in Okular?

To my knowledge, currently none of Okular's backends support electronic signatures, although that feature has been requested a number of times.

As an alternative, a PDF (or any other file) may be signed using a detached signature with GPG or any of its numerous frontends (such as Kleopatra or Kgpg in KDE).

I have a .png of my signature, and I basically want to insert it into the document on the dotted line

Do not do this.

Anyone having access to your PNG or any document where it is used, such as the PDF you are intending to embed this on, will have a perfect, infinitely reproducible copy of your autograph signature. It offers no security at all and in fact is detrimental to it: upon seeing your scribble on a document, recipients may be tempted to assume that it is authentic and not bother to check for an actual (and legally valid) electronic signature. Acrobat is, or at least used to be, a major offender in this regard—I have seen documents trivially "forged" because of this ill-conceived feature.

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  • Poppler has recently gained some support for verifying signatures, but not yet for creating them. As for a PNG of the scribble, AFAIK in some countries even a scan of a paper signed document officially counts as a valid form of electronic signature (though lesser than a cryptographic one)... – user1686 May 23 '16 at 11:53
  • Is it even possible to sign and verify a PDF document using Free Software? Let me know if this sounds off, but I would rather insert a PNG of my signature than use a non-FOSS program to sign/verify my signature because a proprietary signature system may not be compatible on all platforms. I'd be very interested to hear more about the progress of signature signing/verification in Poppler, as well as the many FOSS programs that use Poppler. – modulitos May 28 '16 at 5:44
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    How is sending a pdf, digitally signed with a .png which I scanned myself, worse w/r/t security than sending a signed hardcopy that can be scanned by anyone? – Matthias Kuhn Jun 10 '16 at 12:08
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    The question does not ask for your personal opinions about security. – vaer-k May 16 '18 at 19:23

Okular is limited by it's backend poppler. Within the last few months poppler has slowly been adding support for the nss backend. https://bugs.freedesktop.org/show_bug.cgi?id=16770

However, until this functionality is exposed in it's API (also being worked on) and until that API is utilized in Okular, it won't be able to sign/verify signed PDFs.

The cli tool pdfisg that comes with recent version of poppler can read signatures and determine if the signature is valid and if the cert issuer is trusted.

For signing, there is a FOSS java app called PortableSigner that can sign PDF documents. http://portablesigner.sourceforge.net/

Recent versions of LibreOffice also feature document signing.

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  • You are right, it works with LibreOffice Draw. – jadelord Mar 31 '17 at 17:59

So, it's been about 1.5 years since I asked this question, and I still haven't found an optimal solution for this issue. The main shortcoming is finding a FOSS program to cryptographically sign and verify a PDF document.

I think the accepted answer details the situation best, as well as why Okular falls short.

So, I just wanted to share my current solution, which inserts a png of my image using GIMP. Basically, I am following the steps outlined in the article from my question. But I also use a GIMP plug-in to handle multi-page PDF, outline here: http://www.zyxware.com/articles/4438/how-to-export-multiple-layers-from-gimp-into-a-pdf-file And here is the plug-in: https://www.dropbox.com/s/jzdkgv2f0jrbw6i/export-layers-to-pdf.py?dl=0

I know that solution is not cryptographically correct, but it works for me! But if someone can outline a FOSS solution that can cryptographically sign a PDF document, I will gladly accept that answer!

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  • Try xournal; it handles the scenario of embedding a PNG/SVG signature in a more straightforward way than GIMP in my experience. – user30747 Dec 15 '18 at 17:37

There are no FOSS PDF readers that can sign an Adobe PDF. The reason for this is that signable PDFs created with Adobe tools use Adobe proprietary extensions of the PDF format. PDF is an open format but in this case Adobe have created their own extensions which are owned, patented and licensed by them in order to prevent others from being able to do this.

The only way that a PDF can be signed using FOSS software is to wrap the PDF inside of an open digital signature file format.

You could also investigate remote signing services, where your PDF will be presented to you as HTML which you can then sign. You can sign weakly with an adopted picture of something that resembles your hand written signature - which is a very bad idea as noted by other answers and comments. You can sign more strongly with a cryptographic key - usually with the proviso that the remote signing service hold your key. I do not know if any remote signing services allow you to hold the key, for example, on a 2-factor token device or encrypted on your own hard disk.

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LibreOffice developer blog post about signing existing PDF files in LO 5.3 or newer: http://vmiklos.hu/blog/pdf-sign.html

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    Welcome to Super User. External links can break or be unavailable, in which case your answer would not be useful. Please include the essential information within your answer and use the link for attribution and further reading. Thanks. – fixer1234 Jan 15 '17 at 8:57

IMHO pdfsig is a good choice to check signatures of PDF. If you are willing to use ruby ​​look at the origami gem, here you will find an example to embed a signature in a PDF with your self-signed certificate.

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