Blockchain CAP Theorem Allows User-Dependent Adaptivity and Finality

Suryanarayana Sankagiri, Xuechao Wang, Sreeram Kannan, Pramod Viswanath

Research output: Chapter in Book/Report/Conference proceedingConference contribution

6 Scopus citations


Longest-chain blockchain protocols, such as Bitcoin, guarantee liveness even when the number of actively participating users is variable, i.e., they are adaptive. However, they are not safe under network partitions, i.e., they do not guarantee finality. On the other hand, classical blockchain protocols, like PBFT, achieve finality but not adaptivity. Indeed, the CAP theorem in the context of blockchains asserts that no protocol can simultaneously offer both adaptivity and finality. We propose a new blockchain protocol, called the checkpointed longest chain, that offers individual users the choice between finality and adaptivity instead of imposing it at a system-wide level. This protocol’s salient feature is that it supports two distinct confirmation rules: one that guarantees adaptivity and the other finality. The more optimistic adaptive rule always confirms blocks that are marked as finalized by the more conservative rule, and may possibly confirm more blocks during variable participation levels. Clients (users) make a local choice between the confirmation rules as per their personal preference, while miners follow a fixed block proposal rule that is consistent with both confirmation rules. The proposed protocol has the additional benefit of intrinsic validity: the finalized blocks always lie on a single blockchain, and therefore miners can attest to the validity of transactions while proposing blocks. Our protocol builds on the notion of a finality gadget, a popular technique for adding finality to longest-chain protocols.

Original languageEnglish (US)
Title of host publicationFinancial Cryptography and Data Security - 25th International Conference, FC 2021, Revised Selected Papers
EditorsNikita Borisov, Claudia Diaz
PublisherSpringer Science and Business Media Deutschland GmbH
Number of pages20
ISBN (Print)9783662643303
StatePublished - 2021
Externally publishedYes
Event25th International Conference on Financial Cryptography and Data Security, FC 2021 - Virtual, Online
Duration: Mar 1 2021Mar 5 2021

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume12675 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Conference25th International Conference on Financial Cryptography and Data Security, FC 2021
CityVirtual, Online

All Science Journal Classification (ASJC) codes

  • Theoretical Computer Science
  • General Computer Science


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