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States & Channels

A state channel can be thought of as a set of data structures (called "states") committed to and exchanged between a fixed set of actors (which we call participants), together with some execution rules.

Info

In Nitro, participants "commit to" a state by digitially signing it.

A state channel controls funds which are locked up -- either on an L1 blockchain or on some other ledger such as another state channel.

State channel execution may always be disputed on-chain via a contract we call the Adjudicator, although this not necessary.

States

In Nitro protocol, a state is broken up into fixed and variable parts:

import {ExitFormat as Outcome} from '@statechannels/exit-format/contracts/ExitFormat.sol';

struct FixedPart {
    uint256 chainId;
    address[] participants;
    uint48 channelNonce;
    address appDefinition;
    uint48 challengeDuration;
}

struct VariablePart {
    Outcome.SingleAssetExit[] outcome; // (1)
    bytes appData;
    uint48 turnNum;
    bool isFinal;
}
  1. This composite type is explained in the section on outcomes.
    import * as ExitFormat from '@statechannels/exit-format';
    // (1)
    import {Address, Bytes, Bytes32, Uint256, Uint48, Uint64} from '@statechannels/nitro-protocol';

    export interface FixedPart {
        chainId: Uint256;
        participants: Address[];
        channelNonce: Uint64;
        appDefinition: Address;
        challengeDuration: Uint48;
    }

    export interface VariablePart {
        outcome: ExitFormat.Exit; // (2)
        appData: Bytes;
        turnNum: Uint48;
        isFinal: boolean;
    }
  1. Bytes32, Bytes, Address, Uint256, Uint64 are aliases to the Javascript string type. They are respresented as hex strings. Uint48 is aliased to a number.
  2. This composite type is explained in the section on outcomes.
import (
    "github.com/statechannels/go-nitro/channel/state/outcome"
    "github.com/statechannels/go-nitro/types" // (1)
)

type (
    FixedPart struct {
        ChainId           *types.Uint256
        Participants      []types.Address
        ChannelNonce      uint64
        AppDefinition     types.Address
        ChallengeDuration uint32
    }

    VariablePart struct {
        AppData types.Bytes
        Outcome outcome.Exit // (2)
        TurnNum uint64
        IsFinal bool
    }
)
  1. types.Address is an alias to go-ethereum's common.Address type. types.Bytes32 is an alias to go-ethereum's common.Hash type.
  2. This composite type is explained in the section on outcomes.

Info

States are usually submitted to the blockchain as a single fixed part and multiple (signed) variable parts. This is known as a "support proof".

Let's take each property in turn:

Fixed Part

Chain id

This needs to match the id of the chain where assets are to be locked (i.e. the 'root' of the funding graph for this channel). In the event of a mismatch, the channel cannot be concluded and funds cannot be unlocked.

Participants

This is a list of Ethereum addresses, each derived from an ECDSA private key in the usual manner. Each address represents a participant in the state channel who is able to commit to state updates and thereby cause the channel to finalize on chain.

Warning

Before joining a state channel, you (or your off-chain software) should check that it has length at least 2, but no more than 255, and include a public key (account) that you control. Each entry should be a nonzero ethereum address.

ChannelNonce

This is a unique number used to differentiate channels with an otherwise identical FixedPart. For example, if the same participants want to run the same kind of channel on the same chain as a previous channel, they can choose a new ChannelNonce to prevent state updates for the original channel from being replayed on the new one.

Warning

You should never join a channel which re-uses a channel nonce.

AppDefinition

This is an Ethereum address where a Nitro application has been deployed. This is a contract conforming to the ForceMoveApp and defining application rules.

Warning

You should have confidence that the application is not malicious or suffering from security flaws. You should inspect the source code (which should be publically available and verifiable) or appeal to a trusted authority to do this.

ChallengeDuration

This is duration (in seconds) of the challenge-response window. If a challenge is raised on chain at time t, the channel will finalize at t + ChallengeDuration unless cleared by a subqsequent on-chain transaction.

Warning

This should be at least 1 block time (~15 seconds on mainnet) and less than 2^48-1 seconds. Whatever it is set to, the channel should be closed long before 2^48 - 1 - challengeDuration. In practice we recommend somewhere between 5 minutes and 5 months.

Variable Part

Outcome

This describes how funds will be disbursed if the channel were to finalize in the current state. See the section on Outcomes.

AppData

The AppData is optional data which may be interpreted by the Nitro application and affect the execution rules of the channel -- see the section on application rules. For example, it could describe the state of a chess board or include the hash of a secret.

TurnNum

The turn number is the mechanism by which newer states take precedence over older ones. The turn number usually increments as the channel progresses.

Warning

The turn number must not exceed 281,474,976,710,655 because then it will overflow on chain. It should not exceed 4,294,967,295 because it may then overflow off-chain. It is very unlikely a channel would ever have this many updates.

IsFinal

This is a boolean flag which allows the channel execution rules to be bypassed and for the channel to be finalized "instantly" without waiting for the challenge-response window to lapse.

Warning

As soon as an isFinal=true state is enabled (that is to say, you cannot prevent it from becoming supported) it is not safe to continue executing the state channel. It should be finalized immediately.

Channel IDs

Channels are identified by the hash of the FixedPart of the state (those parts that may not vary):

  bytes32 channelId = keccak256(
      abi.encode(
          fixedPart.chainId,
          fixedPart.participants,
          fixedPart.channelNonce,
          fixedPart.appDefinition,
          fixedPart.challengeDuration
      )
  );

State commitments

To commit to a state, a hash is formed as follows:

 bytes32 stateHash = keccak256(abi.encode(
        channelId,
        variablePart.appData,
        variablePart.outcome,
        variablePart.turnNum,
        variablePart.isFinal
    ));

and this hash is signed using an dedicated Ethereum private key generated solely for the purpose of executing state channel(s) and not otherwise controlling funds on chain. The signature has the following type:

    struct Signature {
        uint8 v;
        bytes32 r;
        bytes32 s;
    }
    import { Signature } from "ethers";
    type Signature struct {
        R []byte
        S []byte
        V byte
    }

You can sign a state using these utilities:

    import {signData} from '@statechannels/nitro-protocol;

    const signature = signData(stateHash, privateKey);
import  nc "github.com/statechannels/go-nitro/crypto"

signature := nc.SignEthereumMessage(stateHash.Bytes(), secretKey)

SignedVariableParts

Signatures on a state hash by different participants are often bundled up with the variable part of the state when being submitted to chain.

struct SignedVariablePart {
    VariablePart variablePart;
    Signature[] sigs;
}
export interface SignedVariablePart {
    variablePart: VariablePart;
    sigs: Signature[];
}
    // INitroTypesSignedVariablePart is an auto generated low-level Go binding around an user-defined struct.
    type INitroTypesSignedVariablePart struct {
        VariablePart INitroTypesVariablePart
        Sigs         []INitroTypesSignature
    }

Support proofs

A support proof is any bundle of information sufficient for the chain to verify that a given channel state is legitimate. They usually consist of FixedPart, plus a singular SignedVariablePart named candidate, plus an array of SignedVariableParts named proof.

The trivial support proof is a state with IsFinal: true signed by every participant. For an intuition around more complicated support proofs, see Putting the 'state' in state channels.

RecoveredVariableParts

The adjudicator smart contract will recover the signer from each signature on a SignedVariablePart, and convert the resulting list into a signedBy bitmask indicating which participant has signed that particular state. The bitfield is bundled with the VariablePart into a RecoveredVariablePart:

struct RecoveredVariablePart {
    VariablePart variablePart;
    uint256 signedBy;
}
export interface RecoveredVariablePart {
    variablePart: VariablePart;
    signedBy: Uint256;
}
    // not yet implemented

before being passed to the application execution rules (which do not need to do any signature recovery of their own).

For example, if a channel has three participants and they all signed the state in question, we would have signedBy = 0b111 = 7. If only participant with index 0 had signed, we would have signedBy = 0b001 = 1.