  Online portals include an increasing amount of user feedback in form of
ratings and reviews. Recent research highlighted the importance of this
feedback and confirmed that positive feedback improves product sales figures
and thus its success. However, online portals' operators act as central
authorities throughout the overall review process. In the worst case, operators
can exclude users from submitting reviews, modify existing reviews, and
introduce fake reviews by fictional consumers. This paper presents ReviewChain,
a decentralized review approach. Our approach avoids central authorities by
using blockchain technologies, decentralized apps and storage. Thereby, we
enable users to submit and retrieve untampered reviews. We highlight the
implementation challenges encountered when realizing our approach on the public
Ethereum blockchain. For each implementation challange, we discuss possible
design alternatives and their trade-offs regarding costs, security, and
trustworthiness. Finally, we analyze which design decision should be chosen to
support specific trade-offs and present resulting combinations of decentralized
blockchain technologies, also with conventional centralized technologies.
