Wendy grows up

In recent years, blockchain applications have increased in complexity and utility, allowing more advanced financial tools such as exchanges and trading markets to be decentralised. The introduction of decentralised trading markets highlights a number of new challenges for consensus protocols [1, 2]. Classically, consensus layer protocols are only required to maintain consistency of the blockchain. While additional requirements have been investigated in the past – for example causal order or censorship resilience – very little attention has been given to the fairness of the order of events, making it possible to execute frontrunning or rushing attacks; several such attacks have been observed in the wild already, and there is evidence of bots systematically scanning unscheduled transactions vulnerable to frontrunning. Some blockchains attempt to make such attacks somewhat harder, for example through special protection for the leader, rapid leader change, or a completely leaderless approach, while others can be easily manipulated by a single corrupt validator or a well targeted denial of service attack. In addition to allowing questionable behavior, this can also be a potential regulatory issue, if exchanges are required to prevent some levels of fraud.