# Private and Secure Distribution of Targeted Advertisements to Mobile Phones
## 3 Related Work
### Classification Assessment  
Our assessment of the related work is shown in Table 1. The previously-proposed systems still have certain shortcomings, which we will now analyse in more detail.  
Systems that make use of a proxy can be easily incorporated into the current architecture and achieve an adequate level of privacy, without seriously reducing the effectiveness and the efficiency of the system. Regardless, such models assume the existence of a trusted or partially trusted third party that can act as the proxy. This is not entirely realistic and also creates a single point of failure that threatens the integrity of the system if compromised.  
Systems that are based on the selection from a pool of adverts, maintain privacy by taking advantage of the computational capability of mobile devices. These systems do not require any trust between the participants, and depending on the method that is used to populate the pool, they can achieve a satisfactory level of privacy. Although these systems are not too difficult to introduce, they suffer greatly in terms of targeting effectiveness and resource efficiency. The information that is shared with the Ad-Network may be too generic to effectively retrieve adverts that perfectly correspond to the user's interests. A significant amount of overhead is also generated. This is due to the fact that the advert selection needs to be performed locally and also additional ads need to be downloaded and stored only to be discarded afterwards.  
Models that allow users to directly download content from broadcasting stations offer the highest level of security so far. In some of these systems, the user does not even need to share any information, and therefore, privacy is only dependent on the capability of mobile devices to connect to stations anonymously. These systems require the use of specialized equipment that may be costly or impractical to incorporate. Their effectiveness to deliver accurate content depends on the amount of information that the user shares with the station, the availability of relevant adverts and the user's ability to physically travel to the specified locations. Resource efficiency is also limited, as these models can be very demanding in terms of storage, processing power and battery life. When enhanced with opportunistic networking, the effectiveness of the system may increase, but this will also raise trust issues among the users. Users need strong incentives to actively participate in such systems and efficiency will be reduced even further as additional strain will be put on the mobile devices.  
\begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Trusted Proxy** & **Pool of Ads** & **Direct Download** \\ \hline Privacy vs. Ad-Network & \(\times\) & \(\times\) & \(+\) \\ Privacy vs. other users & & & \(-\) \\ Security vs. attacker & \(+\) & \(+\) & \(-\) \\ Targeting effectiveness & \(+\) & \(-\) & \(+\) \\ Practicality/usability & \(+\) & \(-\) & \(-\) \\ Resource conservation & \(+\) & \(-\) & \(\times\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Assessment of categories of ad-delivery systems (good: “\(+\)”; adequate: “\(\times\)”; poor: “\(-\)”).