  In the field of distributed consensus and blockchains, the synchronous
communication model assumes that all messages between honest parties are
delayed at most by a known constant $\Delta$. Recent literature establishes
that the longest-chain blockchain protocol is secure under the synchronous
model. However, for a fixed mining rate, the security guarantees degrade with
$\Delta$. We analyze the performance of the longest-chain protocol under the
assumption that the communication delays are random, independent, and
identically distributed. This communication model allows for distributions with
unbounded support and is a strict generalization of the synchronous model. We
provide safety and liveness guarantees with simple, explicit bounds on the
failure probabilities. These bounds hold for infinite-horizon executions and
decay exponentially with the security parameter. In particular, we show that
the longest-chain protocol has good security guarantees when delays are
sporadically large and possibly unbounded, which is reflective of real-world
network conditions.
