Paper 2019/589

A${^2}$L: Anonymous Atomic Locks for Scalability in Payment Channel Hubs

Erkan Tairi, Pedro Moreno-Sanchez, and Matteo Maffei

Abstract

Payment channel hubs (PCHs) constitute a promising solution to the inherent scalability problems of blockchain technologies, allowing for off-chain payments between sender and receiver through an intermediary, called the tumbler. While state-of-the-art PCHs provide security and privacy guarantees against a malicious tumbler, they do so by relying on the scripting-based functionality available only at few cryptocurrencies, and they thus fall short of fundamental properties such as backwards compatibility and efficiency. In this work, we present the first PCH protocol to achieve all aforementioned properties. Our PCH builds upon A${^2}$L, a novel cryptographic primitive that realizes a three-party protocol for conditional transactions, where the tumbler pays the receiver only if the latter solves a cryptographic challenge with the help of the sender, which implies the sender has paid the tumbler. We prove the security and privacy guarantees of A${^2}$L (which carry over to our PCH construction) in the Universal Composability framework and present a provably secure instantiation based on adaptor signatures and randomizable puzzles. We implemented A${^2}$L and compared it to TumbleBit, the state-of-the-art Bitcoin-compatible PCH. Asymptotically, A${^2}$L has a communication complexity that is constant, as opposed to linear in the security parameter like in TumbleBit. In practice, A${^2}$L requires $\sim33$x less bandwidth than TumleBit, while retaining the computational cost (or providing $2$x speedup with a preprocessing technique). This demonstrates that A${^2}$L (and thus our PCH construction) is ready to be deployed today. In theory, we demonstrate for the first time that it is possible to design a secure and privacy-preserving PCH while requiring only digital signatures and timelock functionality from the underlying scripting language. In practice, this result makes our PCH backwards compatible with virtually all cryptocurrencies available today, even those offering a highly restricted form of scripting language such as Ripple or Stellar. The practical appealing of our construction has resulted in a proof-of-concept implementation in the COMIT Network, a blockchain technology focused on cross-currency payments.

Metadata
Available format(s)
PDF
Category
Applications
Publication info
Published elsewhere. Major revision. IEEE Symposium on Security and Privacy - S&P 2021
Keywords
blockchainmixingbitcoin
Contact author(s)
erkan tairi @ tuwien ac at
pedro sanchez @ tuwien ac at
matteo maffei @ tuwien ac at
History
2021-03-23: last of 7 revisions
2019-05-30: received
See all versions
Short URL
https://ia.cr/2019/589
License
Creative Commons Attribution
CC BY

BibTeX

@misc{cryptoeprint:2019/589,
      author = {Erkan Tairi and Pedro Moreno-Sanchez and Matteo Maffei},
      title = {A${^2}$L: Anonymous Atomic Locks for Scalability in Payment Channel Hubs},
      howpublished = {Cryptology ePrint Archive, Paper 2019/589},
      year = {2019},
      note = {\url{https://eprint.iacr.org/2019/589}},
      url = {https://eprint.iacr.org/2019/589}
}
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