# Behavioural Verification: Preventing Report Fraud in Decentralized Advert Distribution Systems
## 1 Introduction  
Digital advertising is a form of marketing which capitalizes on the popularity of digital media Publishers such as websites and software applications in order to present consumers with promotional material from retailers who are referred as Advertisers. Ad-Networks such as Google AdSense and Yahoo! AdNet, operate as a middleman between Advertisers and Publishers. Their purpose is to generate revenue by presenting users who browse a Publisher's platform with relevant adverts of the Advertiser's products and services. In more detail, Ad-Networks are granted full control of certain areas within the Publisher's interface for the purpose of featuring adverts. When a user accesses a Publisher's digital domain (whether website or software application), a request is sent to the Ad-Network who selects the most prominent advert from one of the Advertisers and displays it along side the Publisher's content where it may be viewed by the user. The Ad-Network then claims a monetary reward by filing a report to the corresponding Advertiser and afterwards awards a commission to the Publisher for providing his platform. The ever increasing expansion of the marketing industry makes digital advertising highly profitable for the Ad-Networks and it provides for a tenable source of profit for Publishers that is also beneficial for users who can get services at a lower price. It should therefore come as no surprise that according to eMarketer, the total spending on digital advertising in the US is estimated to exceedS77 billion for the year 2017 with most of that money being spent specifically on adverts that target mobile users [1]. Despite being highly beneficial for all stakeholders, the Ad-Network advertising model is far from ideal as it is susceptible to fraud. According to a report released by the Investigative Advertising Bureau (IAB) in December 2015, the cost of digital advertising fraud is estimated to be over $8.2 billion with 56% of that (up to $4.6 billion) being attributed to non-human traffic [2]. Ad-Networks attempt to limit the problem through the enforcement of policy-based filtering mechanisms (which detect suspicious traffic), but this approach has not been entirely effective [3]. The Advertisers are the victims of the fraud and although they still benefit from digital ads, they are also forced to tolerate the contemporary state of affairs as they have no other choice but to trust the Ad-Networks. This research introduces an advert reporting system which utilizes opportunistic networking, blockchain-inspired architecture and client-side processing to entirely substitute Ad-Networks with a community of independent advert-distributors called **Ad-Dealers**. The Ad-Dealers have a physical presence in publicly accessible areas and serve as anonymous communication gateways to Advertisers. Users who appear within proximity of an Ad-Dealer are able to establish a secure connection with the Advertisers in order to transfer information. Opportunistic networking is exploited to further extend the reach of the Ad-Dealers over a social network of mobile users who voluntarily ferry data for each other, while still maintaining privacy through the use of encryption. This enables the system to propagate adverts from the Ad-Dealers to targeted users but also **allows the users to anonymously submit Ad-Reports which they compose themselves**. Although the majority of users have no immediate benefit from submitting fraudulent Ad-Reports, this does not ensure that a filled report corresponds to real user activity. As users need to remain anonymous, identifying dishonest (malicious) users through traditional methods such as digital signatures is problematic. To address this limitation, our contribution is a mechanism which enables the verification of reports that were submitted by honest users without compromising their identity. What constitutes a user's honesty for our system is the manner upon which they access adverts on their mobile devices.