Set margin with enumitem

Use leftmargin=*

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\documentclass{article}
\usepackage{enumitem}
\usepackage{showframe}
\begin{document}
\begin{enumerate}[label=\bfseries Step \arabic*:,leftmargin=*]
\item Elliptic Key Creation :
\item Exchange of Public Elliptic keys
\item MasterSecret Computation
\end{enumerate}
\end{document}

Or set a proper itemindent if you want to control the indent.

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\documentclass{article}
\usepackage{enumitem}
\usepackage{showframe}
\begin{document}
\begin{enumerate}[label=\bfseries Step~\arabic*:,itemindent=3em]
\item Elliptic Key Creation :
\item Exchange of Public Elliptic keys
\item MasterSecret Computation
\end{enumerate}
\end{document}