How can I insert a framed box inside a table?

1. The optional argument of \tabular or array isn't a placement option. The optional argument set the vertical alignment of tabular. Working parameters are b, t and c.

2. fbox works like a hbox and you can use vertical space inside a single hbox. You can combine two rules for specifying the width and height.

   \fbox{\rule{2cm}{0pt}\rule{0pt}{5cm}}
   

   You can also set a single minipage inside fbox with a width and length.

Here is one possibility:

\documentclass{scrartcl}
\usepackage{array}
\begin{document}
\begin{tabular}{cm{5cm}m{5cm}c}
  fingerprint &  &  & fingerprint \\
  \smash{\fbox{\rule{2cm}{0pt}\rule[-3cm]{0pt}{3cm}}} & Signature 1 & \rule{5cm}{.5pt} &  \smash{\fbox{\rule{2cm}{0pt}\rule[-3cm]{0pt}{3cm}}}  \\[1cm]
  & Signature2 & \rule{5cm}{.5pt} &   \\
\end{tabular}
\end{document}

Or use TikZ directly

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node[minimum height=2.9cm,draw,minimum width=2.1cm,label={[inner sep=3mm]90:\textsf{fingerprint 1}}] {};
\node[minimum height=2.9cm,draw,minimum width=2.1cm,label={[inner sep=3mm]90:\textsf{fingerprint 2}}] at (10cm,0) {};
\node (sig1) at (3,0.8) {\textsf{signature 1}};\draw (sig1.east) -- ++(3.7cm,0);
\node (sig2) at (3,-.5) {\textsf{signature 2}};\draw (sig2.east) -- ++(3.7cm,0);
\end{tikzpicture}
\end{document}

You can use \framebox with its optional width argument and then inside of it a \rule{0pt}{<height>} as follows:

\framebox[<width>]{\rule{0pt}{<height>}}

Note that the amount of \fboxsep is added to all sides.