Tardigrade - An Atomic Broadcast Protocol for Arbitrary Network Conditions.

We study the problem of atomic broadcast-the underlying problem addressed by blockchain protocols-in the presence of a malicious adversary who corrupts some fraction of the n parties running the protocol. Existing protocols are either robust for any number of corruptions in a synchronous network (where messages are delivered within some known time.) but fail if the synchrony assumption is violated, or tolerate fewer than n/3 corrupted parties in an asynchronous network (where messages can be delayed arbitrarily) and cannot tolerate more corruptions even if the network happens to be well behaved. We design an atomic broadcast protocol (Tardigrade) that, for any t(s) >= t(a) with 2t(s) + t(a) < n, provides security against t(s) corrupted parties if the network is synchronous, while remaining secure when t(a) parties are corrupted even in an asynchronous network. We show that Tardigrade achieves optimal tradeoffs between t(s) and t(a). Finally, we show a second protocol (upgrade) with similar (but slightly weaker) guarantees that achieves per-transaction communication complexity linear in n.