  Online advertising fuels the (seemingly) free internet. However, although
users can access most websites free of charge, they need to pay a heavy cost on
their privacy and blindly trust third parties and intermediaries that absorb
great amounts of adrevenues and user data. This is one of the reasons users opt
out from advertising by resorting ad blockers thatin turn cost publishers
millions of dollars in lost adrevenues. Existing privacy-preserving advertising
approaches(e.g., Adnostic, Privad, Brave Ads) from both industry and academia
cannot guarantee the integrity of the performance analytics they provide to
advertisers, while they also rely on centralized management that users have to
trust without being able to audit. In this paper, we propose THEMIS, a novel
privacy-by-design ad platform that is decentralized and requires zero trust
from users. THEMIS (i) provides auditability to all participants, (ii) rewards
users for viewing ads, and (iii) allows advertisers to verify the performance
and billing reports of their ad campaigns. To demonstrate the feasibility and
practicability of our approach, we implemented a prototype of THEMIS using a
combination of smart contracts and zero-knowledge schemes. Performance
evaluation results show that during adreward payouts, THEMIS can support more
than 51M users on a single-sidechain setup or 153M users ona multi-sidechain
setup, thus proving that THEMIS scales linearly.
