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

\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
\begin{preview}
% thanks to http://tex.stackexchange.com/a/75838/5645
\tikzset{XOR/.style={fill=black!15,draw,minimum size=13pt,circle,append after command={
        [shorten >=\pgflinewidth, shorten <=\pgflinewidth,]
        (\tikzlastnode.north) edge (\tikzlastnode.south)
        (\tikzlastnode.east) edge (\tikzlastnode.west)
        }
    }
}

\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}) {$M_\nr$};
        \node (x\nr)[XOR]       at (2,{(\n-\nr)*2}) {};
        \node (E\nr)[encrypt]   at (4,{(\n-\nr)*2}) {$E$};
        \node (C\nr)            at (6,{(\n-\nr)*2}) {$C_\nr$};

        \node (K\nr)            at (4,{(\n-\nr)*2+1}) {$K$};

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

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

    \foreach \nr in {2, ..., \n}{
        \pgfmathtruncatemacro{\tmp}{\nr-1} 
        \draw[->,very thick] (E\tmp) -- (4, {(\n-\tmp)*2-0.5}) -- (2, {(\n-\nr)*2+1.5}) -- (x\nr);
    }

   \node (Mx) at (0,-1) {$\vdots$};
   \node (xx) at (2,-1) {$\vdots$};
   \node (Ex) at (4,-1) {$\vdots$};
   \node (Cx) at (6,-1) {$\vdots$};
\end{tikzpicture}
\end{preview}
\end{document}
