Blockchain protocols typically aspire to run in the permissionless ​setting, in which nodes are owned and operated by a potentially large number of diverse and unknown entities, with each node free to use multiple identifiers and 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. Andrew Lewis-Pye (LSE) discusses work that is aimed at developing a framework for reasoning about the rich design space of blockchain protocols and their capabilities and limitations in the permissionless setting.

The framework describes a hierarchy of settings with different "degrees of permissionlessness," specified by the amount of knowledge that a protocol has about the current participants:
* Fully permissionless​ setting: A protocol has no knowledge about current participation. Proof-of-work protocols are typically interpreted as operating in this setting.
* Dynamically available​ setting: A protocol knows a (dynamically evolving) list of IDs (e.g., public keys associated with locked-up stake), and the current participants are a subset of these IDs. Proof-of-stake longest-chain protocols are typically interpreted as operating in this setting.
* Quasi-permissionless​ setting: The current participants are the full set of the IDs above. Proof-of-stake BFT-style protocols are typically interpreted as operating in this setting.
* Permissioned setting​: The list of IDs above is fixed at the time of the protocol's deployment, with no dependence on the protocol's execution.

Andy also describes a number of impossibility results that can be established using this framework.

This is joint work with Tim Roughgarden

About the speaker
Andy is a mathematician and computer scientist, and is a Professor at the London School of Economics.

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