Permissionless Consensus
Andrew Lewis-Pye, Tim Roughgarden

TL;DR
This paper introduces a framework for analyzing blockchain protocols in permissionless settings, revealing fundamental limitations and possibilities depending on the level of participant knowledge and synchronization assumptions.
Contribution
It defines a hierarchy of permissionless settings and demonstrates how protocol capabilities vary across these, highlighting inherent limitations and potential solutions.
Findings
Deterministic Byzantine agreement protocols cannot terminate in fully permissionless settings.
No protocol can reliably solve Byzantine agreement in dynamically available, partially synchronous settings.
State machine replication is achievable in quasi-permissionless, partially synchronous settings with bounded Byzantine players.
Abstract
Blockchain protocols typically aspire to run in the permissionless setting, in which nodes are owned and operated by a large number of diverse and unknown entities, with each node free to start or stop running the protocol at any time. This setting is more challenging than the traditional permissioned setting, in which the set of nodes that will be running the protocol is fixed and known at the time of protocol deployment. The goal of this paper is to provide a framework for reasoning about the rich design space of blockchain protocols and their capabilities and limitations in the permissionless setting. We propose a hierarchy of settings with different "degrees of permissionlessness", specified by the amount of knowledge that a protocol has about the current participants: These are the fully permissionless, dynamically available and quasi-permissionless settings. The paper also…
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Taxonomy
TopicsBlockchain Technology Applications and Security · Cryptography and Data Security · Distributed systems and fault tolerance
