# Private and Secure Distribution of Targeted Advertisements to Mobile Phones
## 6 Evaluation
### User Privacy against Other Users
#### 6.2.4. Agent Delivers Ads  
Upon meeting the Agent, the Requester first receives his/her ADMs. Then, he/she asks for the Bundles where his/her ads are stored based on the information he/she obtains from the ADMs. Finally, he/she recovers his/her ads, which can either be ads that are meant just for him/her or shared ads, i.e., ads that are addressed to multiple users. Recall that shared ads are placed within Bundles in such a way that they are mixed with other ads which are addressed to the respective users individually. Ads that are addressed to only one user, either the Agent of the Requester, remain completely private from the other user who cannot decrypt them without having the appropriate decryption key \(DK_{i}^{AD}\), which can only be obtained from an ADM. However, the shared ads can be read by both users,and therefore, if one of the users is able to identify which of the ads within a Bundle are shared, he/she will immediately learn the interests of the other user. Upon receiving a Bundle, the Requester has no means of identifying the shared ads, as they are mixed with the other ads, and therefore, the privacy of the Agent and the privacy of other Requesters who use the same Agent are preserved. From the Agent's perspective, when the Requester asks for a specific Bundle, then the Agent can assume there is a possibility that some of the ads within the Bundle may be shared. Despite this, the Agent has no way of being absolutely certain of that possibility, as the Requester may be asking for the Bundle just because he/she wants to obtain his/her own ads, which are individually addressed ads that are not visible to the Agent. If the demanded Bundle were to include no individually-addressed ads for the Requester, the entire content of the Bundle would be visible to the Agent. Then, the Agent would have been able to infer that at least one of the contained ads is shared and that would put the Requester's privacy at risk. For this reason, the Ad-Dealer composes Bundles is such a way that this is never the case. Even in the extreme scenario where the exact same ARMs were sent by both users and, therefore, all of the ads can be shared, the Ad-Dealer prevents this from happening by supplementing additional ads. One last possibility we need to consider is that a user acts completely maliciously by fabricating his/her UIP, i.e., he/she carefully composes his/her UIP in such a way that only specific interests are marked. A malicious Requester would pose no threat for an Agent as the former does not obtain any information that would enable him/her to identify the shared adverts within a Bundle even if the Requester's UIP is fabricated. A malicious Agent, however, may threaten the privacy of a Requester by employing a fabricated UIP. Consider a scenario where a malicious Agent engages in a profile comparison with a potential Requester by listing the groups of his/her UIP in order L = [A,B,C,D,E], and the Requester sends him/her two ARMs. At this point, the Agent may assume that it is highly likely that the ARMs are referring to interests that can be found in one of the high-ranked groups, such as A or B. The Agent then composes \(ARM_{(x)}\) for a specific interest \(x\), which can be found in one of his/her highly-ranked groups, such as Group A. Afterwards, he/she creates multiple additional ARMs for more interests that belong to low-ranking groups (e.g., Group E or D) and then sends all of them to the Ad-Dealer. If \(ARM_{(x)}\) is the same as one of the two ARMs that were sent by the Requester, the Ad-Dealer will retrieve a single shared advert and address it to both users. The Ad-Dealer will also fetch additional adverts for the Requester's second ARM, as well as for the remaining ARMs of the Agent. Since both the Requester and the Agent have at least one individually-addressed advert, the Ad-Dealer can compose all of the ads into a single Bundle and send it back to the Agent.