Paper 2020/1122

The Velvet Path to Superlight Blockchain Clients

Aggelos Kiayias, Andrianna Polydouri, and Dionysis Zindros


Superlight blockchain clients learn facts about the blockchain state while requiring merely polylogarithmic communication in the total number of blocks. For proof-of-work blockchains, two known constructions exist: Superblock and FlyClient. Unfortunately, none of them can be easily deployed to existing blockchains, as they require consensus changes and at least a soft fork to implement. In this paper, we investigate how a blockchain can be upgraded to support superblock clients without a soft fork. We show that it is possible to implement the needed changes without modifying the consensus protocol and by requiring only a minority of miners to upgrade, a process termed a "velvet fork'' in the literature. While previous work conjectured that superblock clients can be safely deployed using velvet forks as-is, we show that previous constructions are insecure, and that using velvet techniques to interlink a blockchain can pose insidious security risks. We describe a novel class of attacks, called "chain-sewing'', which arise in the velvet fork setting: an adversary can cut-and-paste portions of various chains from independent temporary forks, sewing them together to fool a superlight client into accepting a false claim. We show how previous velvet fork constructions can be attacked via chain-sewing. Next, we put forth the first provably secure velvet superblock client construction which we show secure against adversaries that are bounded by 1/3 of the upgraded honest miner population. Like non-velvet superlight clients, our approach allows proving generic predicates about chains using infix proofs and as such can be adopted in practice for fast synchronization of transactions and accounts.

Available format(s)
Cryptographic protocols
Publication info
Published elsewhere. Minor revision. ACM Advances in Financial Technologies - AFT 2021
cryptographic protocolsblockchainssuperlight clientsvelvet fork
Contact author(s)
andriannapolyd @ gmail com
dionyziz @ di uoa gr
Aggelos Kiayias @ ed ac uk
2022-01-02: last of 2 revisions
2020-09-21: received
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Creative Commons Attribution


      author = {Aggelos Kiayias and Andrianna Polydouri and Dionysis Zindros},
      title = {The Velvet Path to Superlight Blockchain Clients},
      howpublished = {Cryptology ePrint Archive, Paper 2020/1122},
      year = {2020},
      doi = {10.1145/3479722.3480999},
      note = {\url{}},
      url = {}
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