\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}
    \draw[fill=black!15] (-0.4,-1) -- (1,-1) -- (1,1) -- (-0.4,1) --cycle;
    \draw[fill=black!35] (-0.35,0) -- (0.95,-0) -- (0.95,0.9) -- (-0.35,0.9) --cycle;
    \draw[fill=black!15] (3.5,-1) -- (5.5,-1) -- (5.5,1) -- (3.5,1) --cycle;
    \node (A) at (0.25,-0.5) {$\mathcal{A}$};
    \node[text width=2cm] (E) at (4.7,0) {Challenger\\$Enc(k, \cdot)$};
    \node[text width=1.5cm] (O) at (0.5,0.5) {\scriptsize{Orakel\\$Enc(k, \cdot)$}};


    \draw[->,thick] (1,0.7)  -- node[sloped, above=-0.1cm] {$\scriptstyle M^{(0)}$} (3.5,0.7);
    \draw[->,thick] (1,0.4)  -- node[sloped, above=-0.1cm] {$\scriptstyle M^{(1)}$} (3.5,0.4);
    \draw[<-,thick] (1,0.0)  -- node[sloped, above=-0.1cm] {$\scriptstyle C^* = Enc(k, M^{(b)})$} (3.5,0.0);

    \draw[->,thick] (0,-1) |- node[sloped, above=0.19cm,right=0.3cm] {rate $b$} (2,-2);
    \draw[<-,very thick] (-0.5,-2) -- node[sloped, above=-0.05cm] {Zeit} (-0.5,1.5);
\end{tikzpicture}
\end{preview}
\end{document}