Adaptively Secure Fast Settlement with Dynamic Participation and Self-Healing

An important property of blockchain systems is the time it takes for a block to enter the irreversible part of the chain, which is typically captured by the notions of finality or settlement. To date, only Nakamoto-style protocols are known to simultaneously offer (1) resilience against adaptive adversaries controlling up to less than half of the underlying resource (such as stake or computational power), (2) dynamic participation (where parties join and leave at will without announcement), and (3) self-healing, i.e., being able to recover from temporary periods of adversarial majority without external intervention. However, these protocols offer only probabilistic settlement that requires wait time linear in a security parameter. The following open question thus arises: can a protocol be designed that maintains these properties—50%-resilience to adaptive attacks, dynamic participation, and self-healing—while offering faster settlement?

This work answers the above question by proposing a novel Nakamoto-style proof-of-stake (PoS) protocol—Ouroboros Peras—that builds on Ouroboros, the proof-of-stake algorithm behind the Cardano Blockchain. Our protocol augments the chain-selection rule to incorporate a weight-based criterion and features a mechanism for certifying blocks, which increases their weight. Under favorable conditions, in the presence of an adversary controlling less than 25% of the stake, Peras continuously certifies new blocks in rapid succession, thereby significantly accelerating settlement time. In the presence of larger adversaries, Peras’ settlement guarantees gracefully fall back to those offered by Ouroboros. A unique feature of our approach is that, compared to prior approaches based on finality gadgets, our settlement acceleration mechanism retains the self-healing and dynamic availability properties of Nakamoto-style protocols.