Understand channel storage

The adjudicator contract stores certain information about any channel that it knows about. Specifically, it stores

  • uint48 turnNumRecord
  • uint48 finalizesAt
  • uint160 fingerprint

serialized, inside the following mapping (with channelId as the key):

mapping(bytes32 => bytes32) public channelStorageHashes;

The value of channelStorageHashes[someChannelId] is obtained by:

  • setting the most significant 48 bits to the turnNumRecord
  • setting the next most significant 48 bits to finalizesAt
  • setting the next most significant 160 bits to the fingerprint

The fingerprint uniquely identifies the channel's current state, up to hash collisions. It is the 160 least significant bits of keccak256(abi.encode(channelData)), where channelData is a struct of type

struct ChannelData {
uint256 turnNumRecord;
uint256 finalizesAt;
bytes32 stateHash; // keccak256(abi.encode(State))
address challengerAddress;
bytes32 outcomeHash // keccak256(abi.encode(Outcome));
}

When the adjudicator needs to verify the exact state or outcome, the data is provided in the function arguments, as part of the calldata. The chain will then check that the hydrated data hashes to the image that has been stored.

We provide a helper function to construct the appropriate hash from a javascript representation of ChannelData:

import {ChannelData, channelDataToChannelStorageHash} from '@statechannels/nitro-protocol';
const channelData: ChannelData = {
turnNumRecord: largestTurnNum,
finalizesAt: 0x0
};
const channelStorageHash = channelDataToChannelStorageHash(channelData);

We'll be using this in the next tutorial lesson.

turnNumRecord

turNumRecord is the highest turn number that is known to the chain to be supported by a full set of signatures. The exception to this rule is that it is set to 0 when the channel is concluded via a conclude call.

For example, the turnNumRecord might be increased by a submitted transaction including

  • a validTransition m-chain (i.e. an ordered list of m <= n states such that each state in the list is a valid transition from its predecessor), and
  • n signatures such that each participant has signed the state in the m-chain for which they are a mover (or a later one)

One example of this is a transaction including a single state signed by all n participants.

Note that a new validTransition m-chain may be implied by a single, signed state that is a validTransition from a state already supported on-chain: and hence the turnNumRecord can be incremented by a respond transaction.

Channel Modes

  • Open if and only if finalizesAt is null
    • implies that stateHash and challengerAddress are also null
  • Challenge if and only if finalizesAt < currentTime
    • implies that all other fields are not null
  • Finalized if and only if finalizesAt >= currentTime
    • implies that all other fields are not null

These states can be represented in the following state machine:

graph LR linkStyle default interpolate basis Open -->|forceMove| Challenge Open -->|checkpoint| Open Open-->|conclude| Finalized Challenge-->|forceMove| Challenge Challenge-->|respond| Open Challenge-->|checkpoint| Open Challenge-->|conclude| Finalized Challenge-->|timeout| Finalized
Last updated on by geoknee