How do I create a PDF file that can be digitally signed?

Question

We are using Acrobat digital signatures in my organization.

With Acrobat Professional you can put in a signature block that allows signing with a client-side digital signature. This is something that can be then done by someone using Acrobat Reader. This is not the same as adding a digital signature to the document—this is adding a space where a signature can be added by someone else to indicate acceptance of the produced PDF.

How do I create a PDF file with LaTeX that can be digitally signed?

Answer 1

In order to be able to sign a document using Adobe Reader or Adobe Acrobat, you need a so-called signature field in your PDF, which is a special kind of form field. In order to add it to a file you have the choice between two different LaTeX packages:

The digsig package

This is a small package that enhances the form capabilities of the hyperref package to support signature fields. It is not available on CTAN, but you can find it on the author's website. To use it, simply add \usepackage{digsig} to the preamble of your document (if you're already using hyperref, load it after \usepackage{hyperref}). Now you can add signature fields to your document using the \digsigfield macro, which expects the width and height of the form field and a freely chosen name. Note that you have to enclose the command in a Form environment, like this:

\documentclass{article}
\usepackage{digsig}
\begin{document}
\begin{Form}
  \digsigfield{5cm}{3cm}{My signature}
\end{Form}
\end{document}

The eforms package

eforms is another package to create PDF forms, with support for signature fields. You can find it on CTAN, but it is not included in TeX Live, so you may have to install it manually. It is very similar in use as the digsig package. However, there's a bug in the eforms driver file for pdfTeX (it includes code which is only valid when you use latex->dvips->ps2pdf, causing errors when used with pdflatex). The following code already includes a fix for this:

\documentclass{article}
\usepackage[pdftex]{eforms}
\makeatletter
\def\eq@SigField{\centerWidget\eq@rectH
    \if\autoCenter n\eqcenterWidget=0pt\fi
    \leavevmode\hbox{\pdfstartlink user{\common@SigFieldCode}%
    \lower\eqcenterWidget\Bbox{\eq@rectW}{\eq@rectH}\pdfendlink}%
    \endgroup
}
\makeatother
\begin{document}
\sigField{My signature}{5cm}{3cm}
\end{document}

In both cases, the output is a rectangular field you can fill with a digital signature using e.g. Adobe Reader^* or Adobe Acrobat:

Note that many other PDF viewers don't support these fields and won't display nothing at all. A "live" example of a signature field can be found in the eforms manual on page 15.

^*In order to digitally sign a PDF document containing a signature field using Adobe Reader, this feature has to be enabled first with the commercial product Adobe Acrobat Pro.