\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
\foreach \x in {0, 1, 2} {
% Draw the PRF instances.
\node (f\x) at ($\x*(3.0cm,0)$) [minimum size=1.25cm, rounded corners=1ex, draw] {\Large $F_k$};

% Draw nodes for ciphertext, XOR (circle only), and plaintext, respectively.
\node (c\x) [below of=f\x, node distance=2.5cm] {$c_\x$};
\node (p\x) [below of=f\x, node distance=1.5cm, circle, draw] {};
\node (m\x) [left of=p\x, node distance=1.0cm] {$m_\x$};

% Draw the cross'' of the XORs.
\draw[-] (p\x.north) -- (p\x.south);
\draw[-] (p\x.east) -- (p\x.west);

% Arrow: Plaintext input'd to XOR.
\draw[-latex] (m\x) -- (p\x);
% Arrow: PRF output to XOR.
\draw[-latex] (f\x) -- (p\x);
% Arrow: XOR output is ciphertext.
\draw[-latex] (p\x) -- (c\x);
}

% Inputs to the PRFs...
\node (n0) [above of=f0, node distance=1.5cm] {$nonce\|ctr$};
\node (n1) [above of=f1, node distance=1.5cm] {$nonce\|(ctr+1)$};
\node (n2) [above of=f2, node distance=1.5cm] {$nonce\|(ctr+2)$};

% ... and respective arrows.
\draw[-latex] (n0) -- (f0);
\draw[-latex] (n1) -- (f1);
\draw[-latex] (n2) -- (f2);
\end{tikzpicture}
\end{document}