Abstract
Off-chain protocols (channels) are a promising solution to the scalability and privacy challenges of blockchain payments. Current proposals, however, require synchrony assumptions to preserve the safety of a channel, leaking to an adversary the exact amount of time needed to control the network for a successful attack. In this paper, we introduce Brick, the first payment channel that remains secure under network asynchrony and concurrently provides correct incentives. The core idea is to incorporate the conflict resolution process within the channel by introducing a rational committee of external parties, called wardens. Hence, if a party wants to close a channel unilaterally, it can only get the committee’s approval for the last valid state.
Additionally, Brick provides sub-second latency because it does not employ heavy-weight consensus. Instead, Brick uses consistent broadcast to announce updates and close the channel, a light-weight abstraction that is powerful enough to preserve safety and liveness to any rational parties. We formally define and prove for Brick the properties a payment channel construction should fulfill. We also design incentives for Brick such that honest and rational behavior aligns. Finally, we provide a reference implementation of the smart contracts in Solidity.
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Notes
- 1.
This censoring ability is encompassed by the chain-quality property [18] of blockchain systems which is rightly bound to the synchrony of the network.
- 2.
The part of the chain where the probability of fork is negligible hence there is transaction finality, e.g., 6 blocks in Bitcoin.
- 3.
Depending on the message delivery.
- 4.
Blinding the commitment to the state is not necessary for Brick, but we do it for compatibility with an auditable extension of Brick [5] where the hash of the state is given to the wardens along with the sequence number. Because the states of a channel may be limited, the salt \(r_i\) is used to prevent wardens from retrieving the state by simply hashing all possible states, effectively compromising privacy.
- 5.
We abuse the notation of signature \(\sigma \) to refer to the multisig of both A and B.
- 6.
The source code is available at https://github.com/dionyziz/brick.
- 7.
Of course if a party crashes we cannot provide liveness, but safety holds.
- 8.
Persistence states that once a transaction is included in the permanent part of one honest party’s chain, then it will be included in every honest party’s blockchain.
- 9.
We assume the channel to be created long before these attacks take place, so the adversary cannot fork the transaction that creates the channel.
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Acknowledgments
We would like to thank Kaoutar Elkhiyaoui for her valuable feedback as well as Jakub Sliwinski for his impactful contribution to this work.
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Avarikioti, Z., Kokoris-Kogias, E., Wattenhofer, R., Zindros, D. (2021). Brick: Asynchronous Incentive-Compatible Payment Channels. In: Borisov, N., Diaz, C. (eds) Financial Cryptography and Data Security. FC 2021. Lecture Notes in Computer Science(), vol 12675. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-64331-0_11
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