I would like to draw something similar to the following image using tikzpicture. I succeed in drawing the arrows but I could not put numbers at the beginning of them.
I used the idea in Drawing network protocols to draw the arrows.
Can you help me about it? Thanks!
Below you can see what I have done.

\begin{tikzpicture}
\tikzstyle{every node}=[font=\tiny]
\matrix (m)[matrix of nodes, column sep=2cm,row sep=8mm, nodes={draw=none, anchor=center,text depth=0pt} ]{
{\normalsize \textbf{$ P_i $}} & $ (pk,v,g, \Z_p, t_1, t_2, id) $ and names of parties are public & {\normalsize \textbf{$ P_j $}}\\[-4mm]
$ x_i\in_r \Z_p $ & & $ x_j\in_r \Z_p$ \\[-7mm]
& $h_i=g^{x_i}$, $ PK log(g,h)\{\xi: h=g^{\xi}\} $ & \\
$ h = \sum g^{x_i} = g^x $ & $h_j=g^{x_j}$, $ PK log(g,h)\{\xi: h=g^{\xi}\} $ & $ h = \sum g^{x_i} = g^x $ \\
& \textbf{. . . . . . . . . . . . . . . . . . . . . . . . . . . . .} & \\
$ r_i \in \Z_p $ & $ V^1_i = VE((g^{r_i},s_ih^{r_i}), h;\emptyset)\{(v_i,s_i)\in R\} $ &
\\ };
\draw[shorten <=-1cm,shorten >=-1cm] (m-1-1.south east)--(m-1-1.south west){};
\draw[shorten <=-1cm,shorten >=-1cm] (m-1-3.south east)--(m-1-3.south west);
\draw[shorten <=-1cm,shorten >=-1cm,-latex] (m-3-2.south west)--(m-3-2.south east);
\draw[shorten <=-1cm,shorten >=-1cm,-latex](m-4-2.south east)--(m-4-2.south west);
\draw[shorten <=-1cm,shorten >=-1cm,-latex] (m-6-2.south west)--(m-6-2.south east);
\end{tikzpicture}

|[circle,inner sep=1pt]|1). BTW: welcome to TeX.SX! – Claudio Fiandrino May 13 '14 at 13:23