try.tex

\documentclass[tikz,border=3mm]{standalone}
\usepackage{pgf-umlsd}
\usetikzlibrary{shadows,positioning}
\tikzset{every shadow/.style={fill=none,shadow xshift=0pt,shadow yshift=0pt}}
\begin{document}
	\sffamily
    \small
    \begin{sequencediagram}
	\newinst{A}{Alice}
	\newinst[6]{B}{Bob}
	\mess[1]{A}{$g$, $p$, $g^a$ mod $p$}{B}
	\mess[1]{B}{$g^b$ mod $p$}{A}
    \end{sequencediagram}
\end{document}