Network-Agnostic State Machine Replication

We study the problem of $\textit{state machine replication}$ (SMR) -- the underlying problem addressed by blockchain protocols -- in the presence of a malicious adversary who can corrupt some fraction of the parties running the protocol. Existing protocols for this task assume either a $\textit{synchronous}$ network (where all messages are delivered within some known time $\Delta$) or an $\textit{asynchronous}$ network (where messages can be delayed arbitrarily). Although protocols for the latter case give seemingly stronger guarantees, in fact they are incomparable since they (inherently) tolerate a lower fraction of corrupted parties. We design an SMR protocol that is $\textit{network-agnostic}$ in the following sense: if it is run in a synchronous network, it tolerates $t_s$ corrupted parties; if the network happens to be asynchronous it is resilient to $t_a \leq t_s$ faults. Our protocol achieves optimal tradeoffs between $t_s$ and $t_a$.