pgf-umlsd rectangles and text

I have several issues with this package to draw my sequence diagram:
How can it is possible position correctly the rectangle according to the position of the other elements? For example I defined the Client A and I would to put the rectangle under it. Further, between the message exchange, I have other rectangles to depict (A side and B side) with a lot of text to put inside. I tried to use the newline but it doesn’t work.

documentclass[a4paper,12pt]{article}
usepackage{pgf-umlsd}
begin{document}
begin{figure}
    centering
    begin{sequencediagram}
    tikzstyle{inststyle}+=[bottom color=white, top color=white, rounded corners=3mm]
    newinst[0]{a}{Client A}
    newinst[5]{b}{Server B}
    node [draw] (s) at (a) {$k_i, k_r, k_1, k_2 = textsf{kdf}(g^{x,y},n_i|n_r)$};
    begin{call}{a}{$[k_i, k_r, k_1, k_2k_i, k_r, k_1, k_2,k_i, k_r, k_1, k_2],aa$}{b}{$[k_i, k_r, k_1, k_2k_i, k_r, k_1, k_2k_i,  k_r, k_1, k_2]$}
    end{call}
    end{sequencediagram}
    caption{Caption}
    label{fig:my_label}
end{figure}
end{document}

What I would to have: