\begin{figure}[h!]
  \centering
  %\footnotesize % Apply footnotesize to all text
  \setmsckeyword{}
  \drawframe{no}    % uncomment to not draw a frame
  \begin{msc}[
    /msc/title top distance=0cm,
    /msc/first level height=.1cm,
    /msc/last level height=0.7cm,  % Slightly reduced
    /msc/head height=0cm,
    /msc/instance width=0cm,
    /msc/head top distance=0.5cm,
    /msc/foot distance=-0.0cm,
    /msc/instance width=0cm,
    /msc/condition height=0.1cm  % Reduced condition height
    ]{}
    %%%%%%%%%%%%%%%%%% CONFIG %%%%%%%%%%%%%%%%%%%%%%%%%
    \setlength{\instwidth}{0\mscunit} % to remove default box below agents 
    \setlength{\instdist}{4.5cm}  % default value between agents

    %%%%%%%%%%%%%%%%%% AGENTS %%%%%%%%%%%%%%%%%%%%%%%%%
    \declinst{A}{              % Alice
      \begin{tabular}[c]{c}
        Alice \\
      \end{tabular}
    }{}
    \declinst{B}{              % Bob
      \begin{tabular}[c]{c}
        Bob  
      \end{tabular}
    }{}

    \nextlevel[0.25]


\action*{
  \footnotesize
  \begin{tabular}{@{}l@{}}
    \textsf{(pk\textsubscript{A}, sk\textsubscript{A}) = DH\_Gen()} \\
    \textsf{(spk\textsubscript{A}, ssk\textsubscript{A}) = SignGen()} \\
    \textsf{sig\_pk\textsubscript{A} = Sign(spk\textsubscript{A}, pk\textsubscript{A})}
  \end{tabular}
}{A}

\action*{
  \footnotesize
  \begin{tabular}{@{}l@{}}
    \textsf{(pk\textsubscript{B}, sk\textsubscript{B}) = DH\_Gen()} \\
    \textsf{(spk\textsubscript{B}, ssk\textsubscript{B}) = SignGen()} \\
    \textsf{sig\_pk\textsubscript{B} = Sign(spk\textsubscript{B}, pk\textsubscript{B})}
  \end{tabular}
}{B}

    \nextlevel[3]

  \condition{{{\footnotesize out-of-band verification of \textsf{spk\textsubscript{A}, spk\textsubscript{B}}}}}{B,A}

    \nextlevel[2.5]
    \mess{\footnotesize\textsf{spk\textsubscript{A}, sig\_pk\textsubscript{A}, pk\textsubscript{A}}}{A}{B}
    \nextlevel[1.25]
    \mess{\footnotesize\textsf{spk\textsubscript{B}, sig\_pk\textsubscript{B}, pk\textsubscript{B}}}{B}{A}

  \nextlevel[0.75]

\action*{
  \footnotesize
  \begin{tabular}{@{}l@{}}
    \textsf{if CheckSign(spk\textsubscript{B}, pk\textsubscript{B}, sig\_pk\textsubscript{B})} \\
    \textsf{key = DH(sk\textsubscript{A}, pk\textsubscript{B})}
  \end{tabular}
}{A}

\action*{
  \footnotesize
  \begin{tabular}{@{}l@{}}
    \textsf{if CheckSign(spk\textsubscript{A}, pk\textsubscript{A}, sig\_pk\textsubscript{A})} \\
    \textsf{key = DH(sk\textsubscript{B}, pk\textsubscript{A})}
  \end{tabular}
}{B}

  \nextlevel[1]

   \end{msc}
\end{figure}