# Private and Secure Distribution of Targeted Advertisements to Mobile Phones
## 3 Related Work
### Literature Review  
The P2 PMM-system [6] relies on a trusted proxy that is referred to as the Intermediary Services Provider (ISP). The ISP is entrusted to store the sensitive information of the user and directly answer requests with adverts that it has obtained from merchants. Although the ISP has no immediate interest to expose any information to the merchants, this method assumes that the ISP can be fully trusted. Obliviad [7] accomplishes a similar goal by replacing the proxy with a hardware device that is placed on the Ad-Network side. The device receives the requests from the client and then sends a number of matching ads that are obtained from the Ad-Network's database. The system accounts for click reports and maintains privacy by deploying a Private Information Retrieval (PIR) mechanism, which allows the client to access the Ad-Network's database, while preventing the Ad-Network from learning about the query and the resulting answer [8]. Aside from requiring additional computational power, this architecture does not guarantee that the operator of the Ad-Network will not bypass security by physically tampering with the device. Adnostic [9] composes a local interest profile that does not get disclosed to other parties. When the user visits a website, he/she is sent a number of ads that are relevant to the contextual theme of that particular website. For example, if the user visits a travel website, the ads can be for holidays to various destinations. The ad that is the most relevant according to the user's interest profile is then selected and displayed, while the rest are discarded. Although this method may be considered secure, it produces unnecessary overhead. It can also be argued that targeting is not very effective, since the ads that are sent to the user are based on a very general interest assumption. [PERSON] et al. [10] follow a similar approach as [9]. They propose a role reversal scheme where the ad providers send to the user a series of interest profiles along with a set of matching adverts. This approach may potentially be more effective than Adnostic [9], since the users have a wider variety of ads to choose from. However, the generated overhead due to unused ads still degrades efficiency. Privad [11] is based on the selection from an advert pool, but also incorporates a proxy. A trusted third party, the Dealer, operates between the user and the Ad-Network. The user selects a general interest category and sends it to the Ad-Network through the Dealer (proxy). Upon receiving the message, the Ad-Network uses the same path to respond with a wide variety of relevant ads. The user's device sorts through these ads and selects the most relevant to be displayed. Although this method is simple to incorporate into the existing model, it assumes the existence of a fully-trusted third party. The Dealer is also a single point of failure, and if compromised, the security of the entire system can be bypassed. PervAd [12; 13] provides personalized ads through broadcasting. Users who maintain a local interest profile can collect relevant ads as they move into proximity of customized WiFi access points. The system minimizes overhead by first sharing some contextual information about the available ads, thus allowing users to selectively download only specific content. The interest profile is specified by the user himself/herself, and the downloading process is performed anonymously. Even though this method achieves a substantial level of privacy, it is highly impractical as users need to physically travel to specific locations. The schemes in [14; 15] also use broadcasting, but at the same time make use of opportunistic networking. In both schemes, the users download adverts directly from local businesses. As users come into proximity, their devices connect and exchange ads based on contextual information. A mechanism for keeping track and rewarding points for interactions is also established.