I am trying to draw a flowchart with tikz. Below is my try. I would like the vertical arrow with superscript "a" to be connected with the horizontal arrow with superscript "n" and be more centered in the middle of the node "Step 3". Could you please tell me how I can do that?
\documentclass[a4paper, 11pt]{article}
\usepackage[a4paper,left=3cm,right=3cm,top=3cm,bottom=3cm]{geometry}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{clrscode}
\usepackage{enumitem}
\usepackage{fancyhdr}
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage[utf8]{inputenc}
\usepackage{latexsym}
\usepackage{lmodern}
\usepackage{mathtools}
\usepackage{mdframed}
\usepackage{pgf}
\usepackage{tcolorbox}
\usepackage{tikz}
\usepackage{tikzpeople}
\usepackage[absolute,overlay]{textpos}
\usepackage{xcolor}
\usetikzlibrary{arrows}
\usetikzlibrary{positioning}
\usetikzlibrary{shapes.multipart}
\usetikzlibrary{shapes.geometric}
\tikzstyle{process} = [rectangle, minimum width=5cm, text width=5cm, minimum height=1cm, draw=black]
\tikzstyle{arrow} = [thick,->,>=stealth]
\tikzstyle{name} = [rectangle, minimum width=5cm, text width=5cm, minimum height=1cm, draw=black]
\DeclareGraphicsExtensions{.pdf,.png,.jpg, .tif}
\pagestyle{fancy}
\fancyhf{}
\renewcommand{\headrulewidth}{0pt}
\fancyfoot[LE,RO]{\thepage}
\renewcommand{\familydefault}{\sfdefault}
\newcommand{\legendre}[2]{\genfrac{(}{)}{}{}{#1}{#2}}
\newcommand{\jacobi}[2]{\genfrac{(}{)}{}{}{#1}{#2}}
\newcommand{\congruent}[3]{$#1 \equiv #2 \text{ } \left( \text{mod } #3 \right)$}
\newcommand{\ZNZ}[1][n]{\mathbb{Z}/{#1}\mathbb{Z}}
\newcommand{\ZNZX}[1][n]{(\mathbb{Z}/{#1}\mathbb{Z})^\times}
\newcommand{\QR}[1][n]{\mathcal{QR}_{#1}}
\newcommand{\QNR}[1][n]{\mathcal{QNR}_{#1}}
\newcommand{\QNRP}{\mathcal{QNR}_n^{+1}}
\newcommand{\JNP}{\mathcal{J}_n^{+1}}
\newcommand{\JNM}{\mathcal{J}_n^{-1}}
\begin{document}
\begin{tikzpicture}[node distance = 1cm]
\node[name](alice){\textbf{Alice}};
\node[process, below = of alice](step1){\textbf{Step 1:} Pick two distinct large Blum primes $p$ and $q$ and set $n \coloneqq pq$.};
\end{tikzpicture}
\end{document}

