  Online advertising fuels the (seemingly) free internet. However, although
users can access most of the web services free of charge, they pay a heavy
coston their privacy. They are forced to trust third parties and
intermediaries, who not only collect behavioral data but also absorb great
amounts of ad revenues. Consequently, more and more users opt out from
advertising by resorting to ad blockers, thus costing publishers millions of
dollars in lost ad revenues. Albeit there are various privacy-preserving
advertising proposals (e.g.,Adnostic, Privad, Brave Ads) from both academia and
industry, they all rely on centralized management that users have to blindly
trust without being able to audit, while they also fail to guarantee the
integrity of the per-formance analytics they provide to advertisers.
  In this paper, we design and deploy THEMIS, a novel, decentralized and
privacy-by-design ad platform that requires zero trust by users. THEMIS (i)
provides auditability to its participants, (ii) rewards users for viewing ads,
and (iii) allows advertisers to verify the performance and billing reports of
their ad campaigns. By leveraging smart contracts and zero-knowledge schemes,
we implement a prototype of THEMIS and early performance evaluation results
show that it can scale linearly on a multi sidechain setup while it supports
more than 51M users on a single-sidechain.
