Permissionless Blockchains

The rise of Bitcoin [193] and other cryptocurrencies [57, 244] led to the research and design of several new BFT consensus protocols. These protocols were designed with following two goals in mind: (1) running BFT consensus among a massively large set of replicas and (2) protecting identities of the participating replicas. The former goal is a consequence of PBFT’s design, which necessitates multiple phases of quadratic communication among the replicas. Hence, even though PBFT is safe, it cannot be scaled to large distributed applications. The latter goal is a key ingredient for expressing permissionless blockchains [126, 251].