\begin{figure}[th]
    \begin{minipage}[b]{0.45\linewidth}
      \begin{center}
	\begin{tikzpicture}[scale = 0.4,
            transform shape,
            msglabel/.style    = { text = Black, yshift = .3cm,
                                   sloped, midway },
            okmsg/.style       = { ->, color = MidnightBlue, thick,
                                   >=stealth },
            rstmsg/.style      = { ->, color = BrickRed, thick,
                                   >=stealth }
	  ]
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h1) at (-4, 0) {};
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h2) at (4, 0) {};
	  \node[above = 0cm of h1] {Wallet};
	  \node[above = 0cm of h2] {Exchange};

	  \path[->, color = MidnightBlue, very thick, >=stealth]
	    (-5, 4.5) edge
	    node[rotate=90, text = Black, yshift = .3cm] {Time}
	    (-5, -4.5);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h1.east)+(0,3)$) edge
	    node[text = Black, yshift = .3cm, sloped] {GET {\tt /keys}}
	    ($(h2.west)+(0,2)$);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h2.west)+(0,0.5)$) edge
	    node[text = Black, yshift = .3cm, sloped] {200 OK: $S_T(DK,A_{DK},M), S_M(SK)$}
	    ($(h1.east)+(0,-0.5)$);
	  \node at (5.3, 0) {};
	\end{tikzpicture}
      \end{center}
    \end{minipage}
    \hspace{0.4cm}
    \begin{minipage}[b]{0.45\linewidth}
      \tiny
      \begin{description}
      \item[$T$] financial regulator key Necessarily pinned
      \item[$DK$] RSA public key (``denomination key'')
      \item[$A_{DK}$] Value of coins signed by $DK$
      \item[$M$] Offline master key of exchange
      \item[$SK$] Online signing key of exchange
      \end{description}
    \end{minipage}
  \end{figure}