  We outline two dishonest strategies that can be cheaply executed on the
Ethereum 2.0 beacon chain, even by validators holding less than one-third of
the total stake: malicious chain reorganizations ("reorgs") and finality
delays. In a malicious reorg, an attacker withholds their blocks and
attestations before releasing them at an opportune time in order to force a
chain reorganization, which they can take advantage of by double-spending or
front-running transactions. To execute a finality delay an attacker uses
delayed block releases and withholding of attestations to increase the mean and
variance of the time it takes blocks to become finalized. This impacts the
efficiency and predictability of the system. We provide a probabilistic and
cost analysis for each of these attacks, considering a validator with 30% of
the total stake.
