  Cryptocurrencies redefined how money can be stored and transferred among
users. However, independent of the amount being sent, public blockchain-based
cryptocurrencies suffer from high transaction waiting times and fees. These
drawbacks hinder the wide use of cryptocurrencies by masses. To address these
challenges, payment channel network concept is touted as the most viable
solution to be used for micro-payments. The idea is exchanging the ownership of
money by keeping the state of the accounts locally. The users inform the
blockchain rarely, which decreases the load on the blockchain. Specifically,
payment channel networks can provide transaction approvals in seconds by
charging a nominal fee proportional to the payment amount. Such attraction on
payment channel networks inspired many recent studies which focus on how to
design them and allocate channels such that the transactions will be secure and
efficient. However, as payment channel networks are emerging and reaching large
number of users, privacy issues are becoming more relevant that raise concerns
about exposing not only individual habits but also businesses' revenues. In
this paper, we first propose a categorization of the existing payment networks
formed on top of blockchain-backed cryptocurrencies. After discussing several
emerging attacks on user/business privacy in these payment channel networks, we
qualitatively evaluate them based on a number of privacy metrics that relate to
our case. Based on the discussions on the strengths and weaknesses of the
approaches, we offer possible directions for research for the future of privacy
based payment channel networks.
