\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}

\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
\begin{preview}
\tikzstyle{vertex}=[draw,fill=black!15,circle,minimum size=20pt,inner sep=0pt]
\tikzstyle{encrypt}=[draw,fill=black!15,rectangle,minimum size=20pt,inner sep=0pt]
\begin{tikzpicture}
    \newcommand{\n}{3}
    \foreach \nr in {1, ..., \n}{
        \node (M\nr) at (0,{(\n-\nr)*2}) {$C_\nr$};
        \node (C\nr) at (4,{(\n-\nr)*2}) {$M_\nr$};
        \node (E\nr)[encrypt] at (2,{(\n-\nr)*2}) {$D$};
        \node (K\nr) at (2,{(\n-\nr)*2+1}) {$K$};

        \draw[->,very thick] (M\nr) -- (E\nr);
        \draw[->,very thick] (K\nr) -- (E\nr);
        \draw[->,very thick] (E\nr) -- (C\nr);
    }

   \node (Mx) at (0,-1) {$\vdots$};
   \node (Ex) at (2,-1) {$\vdots$};
   \node (Cx) at (4,-1) {$\vdots$};
\end{tikzpicture}
\end{preview}
\end{document}
