THEMIS: Decentralized and Trustless Ad Platform with Reporting Integrity

Online advertising fuels the (seemingly) free internet. However, although users can access a website free of charge, there is a heavy cost on their privacy due to the deployed user tracking. To make matters worse, in the current ad ecosystem there is no transparency: apart from users, publishers and advertisers, there are numerous dubious middlemen exploiting the ad revenues and the users' data beyond anyone's control. In this paper, we propose THEMIS: a novel privacy-by-design ad platform that is decentralized and requires zero trust from both users and advertisers. THEMIS (i) provides auditability to all participants, (ii) rewards users for interacting with ads, and (iii) allows advertisers to verify the performance and billing reports of their ad campaigns. To assess the feasibility and practicability of our approach, we implemented a prototype using smart contracts and zero-knowledge schemes. Preliminary results of our evaluation show that during ad reward payouts, THEMIS can support more than 7M concurrent users on a single side-chain, a number that grows linearly by running multiple instances of THEMIS in parallel.