  The HashGraph Protocol is a Byzantine fault tolerant atomic broadcast
protocol. Its novel use of locally stored metadata allows parties to recover a
consistent ordering of their log just by examining their local data, removing
the need for a voting protocol. Our paper's first contribution is to present a
rewritten proof of security for the HashGraph Protocol that follows the
consistency and liveness paradigm used in the atomic broadcast literature. In
our second contribution, we show a novel adversarial strategy that stalls the
protocol from committing data to the log for an expected exponential number of
rounds. This proves tight the exponential upper bound conjectured in the
original paper. We believe that our proof of security will make it easier to
compare HashGraph with other atomic broadcast protocols and to incorporate its
ideas into new constructions. We also believe that our attack might inspire
more research into similar attacks for other DAG-based atomic broadcast
protocols.
