2.7 KiB
State Channel Disputes
A problem with state channels is that participants have to remain "online"; they need to keep an eye on latest state of the underlying blockchain and be able to respond to any disputes about the final state of the state channel. Ironically this problem stems from the mechanism that allows a state channel to be closed when a participant goes offline. Closing a channel unilaterally is allowed, but there is a period in which the other participant can dispute the final state of the channel. Therefore participants should be monitoring the blockchain so that they can respond during a dispute period.
Pisa
https://www.cs.cornell.edu/~iddo/pisa.pdf
The PISA protocol enables a participant to outsource monitoring of the underlying blockchain to an untrusted watchtower. The main idea is that a hash of the latest state channel update is sent to the watchtower. The watchtower responds with a signed promise to use this information to settle any disputes that may arise. Should the watchtower fail to do so, then the signed promise can be used as proof of negligence and it will lose its substantial collateral.
A potential problem with this scheme is that the collateral is shared among all state channels that the watchtower is monitoring, which could lead to bribing attacks.
Brick
https://arxiv.org/pdf/1905.11360.pdf
The BRICK protocol provides an alternative to the dispute period based on byzantine consistent broadcast. Participants in a state channel assign a committee that is allowed to sign off on channel closing in case they are not able to do so themselves. Instead of waiting for a period of time before unilaterally closing the channel, with BRICK you wait for a threshold number of committee members to confirm the latest state of the channel. This is much faster.
Each state channel update contains a sequence number, which is signed by the channel participants and sent to the committee members. For a channel to be closed unilaterally, the BRICK smart contract requires a signed state update, and signed sequence numbers provided by a majority of the committee. The highest submitted sequence number should match the submitted state update. Committee members that submit a lower sequence number lose the collateral that they provided when the channel was opened.
A potential problem with the implementation of BRICK as outlined in the paper is that the collateral scheme is vulnerable to sybil attacks; committee members can attempt to steal their own collateral by providing proof of their own wrongdoing.
Unilateral closing is also rather heavy on blockchain transactions; each committee member has to separately perform a transaction on chain to supply their sequence number.