Various models can be used to implement a secure data exchange between the users of an \ac{OSN} via additional network. The solution strategies shown below differ primarily in the question of where data is stored and how it can be found.

\begin{figure}[h!]
	\centering
	\includegraphics[width=1.0\textwidth]{solution-strategy-architecture}
	\caption{Architectures for secure data exchange among users: (a) by the use of an additional server, (b) via a P2P network connecting all users or (c) via a hybrid P2P network with servers acting as super-peers}
	\label{fig:solution-strategy-architecture}
\end{figure}

One possibility is to use an extra infrastructure to store the data, as shown in Figure \ref{fig:solution-strategy-architecture}.a. An additional server stores and distributes the private data to be protected. Using a server has the advantage that the data are always available and there are no dependencies to other hybrid \ac{OSN} users. Furthermore, resources only have to be available centrally and not locally on the user's device. At the central location, the data can be indexed and explicitly queried. However, the operation and maintenance of one or more servers are problematic. In principle, the question for the service provider has to be clarified, because the reliability of the infrastructure is essential. FaceCloak (see Chapter \ref{sec:facecloak}) used an architecture based on this structure.

Instead of operating a separate, additional server, it would also be possible to use a third-party, existing infrastructure. These include, for example, blockchains or \ac{P2P} file-sharing networks that could be used for data exchange. Since no influence can be exerted on existing infrastructure, its use entails further restrictions and potential risks.

A decentralized solution strategy would create a network among users of the hybrid application (see Figure \ref{fig:solution-strategy-architecture}.b). No extra infrastructure would have to be operated. The users would then have a typical peer role. By using this model, it is difficult to keep data available and accessible even if the user is permanently or temporarily offline. The problem needs to be solved.
Furthermore, the resources on the devices are limited, so that effective and economical solutions are needed. Another challenge is the addressing of peers. Since they typically do not have a static \ac{IP} address, solutions have to be found for accessibility. Since there is no central, global index, finding data is even more difficult.

Adding servers to the \ac{P2P} network would create a hybrid solution (see Figure X). In this model, the servers would take on the role of a super peer, permanently reachable at a fixed address, thus stabilizing the \ac{P2P} network. The problem of data availability could be limited by storing much of the data at super peers. The problem of addressing would also be solved by establishing connections to other peers via the known super peers. However, the problem would remain with the cost and maintenance of the servers.

Table \ref{tab:solution-strategy-architecture-comparison} lists the advantages and disadvantages of the different strategies for the hybrid \ac{OSN} architecture.

% Own infrastructure
\newcommand{\advantageoi}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item Availability of data
			\item Finding the data
			\item Resources only have to be available centrally
			\item No dependencies among hybrid \ac{OSN} users
		\end{itemize}
		\hspace{1mm}
\end{minipage}}

\newcommand{\disadvantageoi}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item Expenses
			\item Who operates the infrastructure?
			\item Compliance with legal requirements
		\end{itemize}
\end{minipage}}

% Own network
\newcommand{\advantageon}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item Resources scale with increasing number of users
		\end{itemize}
\end{minipage}}

\newcommand{\disadvantageon}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item Availability of data
			\item Finding the data
			\item Addressing the peers
			\item Local resources limited
		\end{itemize}
		\hspace{1mm}
\end{minipage}}

% Hybrid solution
\newcommand{\advantagehn}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item Availability of data
			\item Peer discovery
		\end{itemize}
\end{minipage}}

\newcommand{\disadvantagehn}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item Expenses
			\item Who operates the infrastructure?
			\item Finding the data
		\end{itemize}
		\hspace{1mm}
\end{minipage}}

% External infrastructure
\newcommand{\advantageei}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item Ideally no costs
			\item Resources are provided by the external infrastructure
		\end{itemize} 
\end{minipage}}

\newcommand{\disadvantageei}{\begin{minipage} [t] {0.3\textwidth} 
		\begin{itemize}
			\item No influence on future development
			\item Dependence on infrastructure entails risks
		\end{itemize}
		\hspace{1mm}
\end{minipage}}


\begin{table}[h!]
	\centering
	\begin{tabularx}{\textwidth}{X|l|l|}
		\cline{2-3}
		& \textbf{Advantages} & \textbf{Disadvantages} \\ \hline
		\multicolumn{1}{|l|}{\textbf{\begin{tabular}[c]{@{}l@{}}Own infrastructure\\ (centralized)\end{tabular}}}        & \advantageoi                    & \disadvantageoi                       \\ \hline
		\multicolumn{1}{|l|}{\textbf{\begin{tabular}[c]{@{}l@{}}P2P network\\ (decentralized)\end{tabular}}}             & \advantageon                    & \disadvantageon                       \\ \hline
		\multicolumn{1}{|l|}{\textbf{\begin{tabular}[c]{@{}l@{}}Hybrid P2P network\\ (decentralized)\end{tabular}}}      & \advantagehn                    & \disadvantagehn                       \\ \hline
		\multicolumn{1}{|l|}{\textbf{External infrastructure}}                                                           & \advantageei                    & \disadvantageei                       \\ \hline
	\end{tabularx}
	\caption{Advantages and disadvantages of the different solution strategies for the hybrid \ac{OSN} architecture.}
	\label{tab:solution-strategy-architecture-comparison}
\end{table}