  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. 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. Furthermore, we consider permissioned
blockchains, where the additional property of auditability might be desired for
regulatory purposes. We introduce Brick+, an off-chain construction that
provides auditability on top of Brick without conflicting with its privacy
guarantees. We formally define the properties our payment channel construction
should fulfill, and prove that both Brick and Brick+ satisfy them. 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.
