  Online trust systems are playing an important role in to-days world and face
various challenges in building them. Billions of dollars of products and
services are traded through electronic commerce, files are shared among large
peer-to-peer networks and smart contracts can potentially replace paper
contracts with digital contracts. These systems rely on trust mechanisms in
peer-to-peer networks like reputation systems or a trustless public ledger. In
most cases, reputation systems are build to determine the trustworthiness of
users and to provide incentives for users to make a fair contribution to the
peer-to-peer network. The main challenges are how to set up a good trust
system, how to deal with security issues and how to deal with strategic users
trying to cheat on the system. The Sybil attack, the most important attack on
reputation systems is discussed. At last match making in two sided markets and
the strategy proofness of these markets are discussed.
