Evaluation of PISA and BRICK (#12)

* Give state channels its own folder

* Summary of PISA and BRICK protocols

evaluations/statechannels/disputes.md

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+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.