Finalize a channel (sad)

When cooperation breaks down, it is possible to finalize a state channel without requiring on-demand cooperation of all n participants. This is the so-called 'sad' path to finalizing a channel, and it requires a supported (but not necessarily isFinal) state(s) being submitted to the chain.

The forceMove function allows anyone holding the appropriate off-chain state(s) to register a challenge state on chain. It is designed to ensure that a state channel can progress or be finalized in the event of inactivity on behalf of a participant (e.g. the current mover).

The required data for this method consist of between 1 and n states, along with n signatures that imply a support proof for the challenge state. Once these are submitted (in an optimized format), and once relevant checks have passed, an outcome is registered against the channelId, with a finalization time set at some delay after the transaction is processed. This delay allows the challenge to be cleared by a timely and well-formed respond or checkpoint transaction. We'll get to those shortly. If no such transaction is forthcoming, the challenge will time out, allowing the outcome registered to be finalized. A finalized outcome can then be used to extract funds from the channel (more on that below, too).

Call forceMove

note

The challenger needs to sign this data:

keccak256(abi.encode(challengeStateHash, 'forceMove'))

in order to form challengerSig. This signals their intent to forceMove this channel with this particular state. This mechanism allows the forceMove to be authorized only by a channel participant.

We provide a handy utility function signChallengeMessage to form this signature.

// In lesson7.test.ts
import {signChallengeMessage} from '@statechannels/nitro-protocol';
const participants = [];
const wallets: ethers.Wallet[] = [];
for (let i = 0; i < 3; i++) {
wallets[i] = ethers.Wallet.createRandom();
participants[i] = wallets[i].address;
}
const chainId = '0x1234';
const channelNonce = 0;
const channel: Channel = {chainId, channelNonce, participants};
/* Choose a challenger */
const challenger = wallets[0];
/* Construct a progression of states */
const largestTurnNum = 8;
const isFinalCount = 0;
const appDatas = [0, 1, 2];
const states: State[] = appDatas.map((data, idx) => ({
turnNum: largestTurnNum - appDatas.length + 1 + idx,
isFinal: idx > appDatas.length - isFinalCount,
channel,
challengeDuration: 1,
outcome: [],
appDefinition: process.env.TRIVIAL_APP_ADDRESS,
appData: HashZero
}));
/* Construct a support proof */
const whoSignedWhat = [0, 1, 2];
const signatures = await signStates(states, wallets, whoSignedWhat);
/* Form the challengeSignature */
const challengeSignedState: SignedState = signState(
states[states.length - 1],
challenger.privateKey
);
const challengeSignature = signChallengeMessage([challengeSignedState], challenger.privateKey);
/* Submit the forceMove transaction */
const variableParts = states.map(state => getVariablePart(state));
const fixedPart = getFixedPart(states[0]);
const tx = NitroAdjudicator.forceMove(
fixedPart,
largestTurnNum,
variableParts,
isFinalCount,
signatures,
whoSignedWhat,
challengeSignature
);
Last updated on by geoknee