# Behavioural Verification: Preventing Report Fraud in Decentralized Advert Distribution Systems
## 3 Preliminaries  
The **Advert Distribution System (ADS)**[17] that we offered in our previous work, combines anonymous download technologies and opportunistic networking in order to deliver adverts to users without exposing their consumer interests. The system that we present in this paper expands the functionality of ADS by enabling users to also submit reports about the adverts that they have viewed.  
As both systems share much of the same architectural elements and stakeholders, in this section we provide a fundamental overview of ADS which will be required in order to better comprehend the extended advert reporting system that will be described later on. As depicted in Figure 1, ADS establishes a communication link between **Advertisers**, who are retailers that wish to promote their products and **Users** who represent consumers. The **Ad-Dealers** are local broadcasting stations who serve as communication gateways between users and Advertisers in order to distribute adverts upon request. The role of the Ad-Dealer may be cast to regional entities such as shopping malls, WiFi hotspots and local businesses.  
Users run a software client on their smartphone devices that allows them to connect anonymously to Ad-Dealers within their proximity through the use of specialized networking equipment. This allows the mobile clients to freely request and consequently download adverts which are then displayed by **Publishers** who are digital advertising platforms such as websites and mobile apps.  
Furthermore, users can also communicate with the Ad-Dealers indirectly through opportunistic networking by forwarding Ad-Requests to an Agent over Bluetooth of WiFi Direct. As shown in the diagram, the Agent physically ferries the Ad-Requests to an Ad-Dealer, collects the requested adverts and conveys them back to the user. In practice, **Agents** are also mobile users who have the additional role of propagating Ad-Requests and adverts on behalf of other users within their social community. This setup benefits members of the community who do not visit Ad-Dealers on a regular basis but can obtain their adverts through other users (Agents) within their social circles.  
User privacy against the Ad-Dealers is ensured by the use of anonymous connections (no use of IP address or MAC) and the fact that Agents operate as a form of partially trusted proxy between the users and Ad-Dealers. Ad-Requests are encrypted with a public key which is incorporated into the users' client software and can only be deciphered by the Ad-Dealers who share knowledge of a secret decryption key. The adverts are also encrypted by the Ad-Dealers with a key which is provided to them by the users themselves within the equivalent Ad-Requests. This preserves the privacy of users against Agents who have no access to the content of Ad-Requests nor the adverts and also guarantees security against a rogue Agent who might inject malicious adverts into the network. Security throughout the entire system is also enforced by strong authentication mechanisms as to prevent malicious users from impersonating Ad-Dealers, Agents and other users (In our previous work,we provide a detailed description of how the opportunistic network is established and the mechanisms which are employed to ensure security and anonymity. For this reason, this paper assumes existence of a secure opportunistic link and focuses exclusively on the submission of Ad-Reports).