### State Channel + Create2 #### Advanced Smart Contract <div class="author">Yuet Loo Wong</div>

### Topics - State Channel - Counterfactual - Create2 - EIP 1014

### Blockchain Scalability Problem One of the major problem with blockchain is scalability problem - Bitcoin: <a href="https://lightning.network/lightning-network-paper.pdf" target="_blank">7</a> tps - Ethereum: <a href="https://bravenewcoin.com/insights/vitalik-ethereum-en-route-to-a-million-transactions-per-second" target="_blank">15</a> tps - Visa: <a href="https://www.visa.com/blogarchives/us/2013/10/10/stress-test-prepares-visanet-for-the-most-wonderful-time-of-the-year/index.html" target="_blank">47,000</a> transactions per second (tps)

### State Channel - A blockchain scaling solution - Conduct blockchain transactions off-chain - Uses cryptographic signatures - to allow secure, instantaneous transactions - <a href="https://www.jeffcoleman.ca/state-channels/" target="_blank">State channels by Jeff Coleman</a>

### Payment Channel - A specific type of state channel - An <a href="https://solidity.readthedocs.io/en/v0.6.3/solidity-by-example.html#micropayment-channel" target="_blank">example</a> in solidity document - Uses cryptographic signatures - Make repeated transfers of Ether off-chain - Each message specifies total Ether owed

### Payment Steps - Alice funds a smart contract with Ether - This “opens” the payment channel - Alice signs payment messages to Bob - This is repeated for each payment - This is done off-chain - Bob “closes” the payment channel - This withdraws Ether

### Creating signatures - What to Sign - The amount to be transferred - Protection against replay attacks - contract address

### Create Signature (Javascript) ``` function signPayment(amount, contractAddress) { const digest = ethers.utils.solidityKeccak256( ["uint256", "address"], [amount, contractAddress] ); const arrayDigest = ethers.utils.arrayify(digest); const signature = wallet.signMessage(arrayDigest); return signature; } ```

### Verifying Signature (Contract) ``` function isValidSignature(uint256 amount, uint8 v, bytes32 r, bytes32 s) public view returns(bool) { // this recreates the message that was signed on the client bytes32 hash = keccak256(abi.encodePacked(amount, contractAddress)); bytes32 message = keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash)); address signer = ecrecover(message, v, r, s); return( signer == sender); } ```

### Closing Channel - Bob closes the channel to receive funds - Bob uses last signed message to close channel - ``` function close(uint256 amount, uint8 v, bytes32 r, bytes32 s) public { require(msg.sender == recipient); require(isValidSignature(amount, v, r, s)); recipient.transfer(amount); selfdestruct(sender); } ```

### Channel Timeout - A way for Alice to recover escrowed funds - An expiration is set at contract creation - After expiry, Alice can recover fund - Bob cannot receive fund after channel closure - ``` function claimTimeout() public { require(now >= expiration); selfdestruct(sender); } ```

### Create2 - A low level way to create a contract - create2(v, p, n, s) - v is contract creation value in wei - p is the memory location of the contract code - n is the length of contract code - s is the salt - <a href="https://eips.ethereum.org/EIPS/eip-1014" target="_blank">EIP 1014 Skinny CREATE2</a>

### Create2 Motivation Allows state channel interactions to be made with addresses that do not exist yet on-chain but can be relied on to eventually contain code

### Counterfactual Things that could happen on the blockchain, but don't

### Contract Address - If create is used, the contract address is: - keccak256(rlp([caller, nonce]))[12:] - If create2 is used, the contract address is: - keccak256(0xff + caller + salt + keccak256(init_code))[12:]

## Contract Code - need assembly to use extcodesize and extcodecopy ``` function GetCodeAt(address _addr) public view returns (bytes memory o_code) { assembly { let size := extcodesize(_addr) // code size o_code := mload(0x40) // free memory pointer mstore(0x40, add(o_code, and(add(add(size, 0x20), 0x1f), not(0x1f)))) mstore(o_code, size) // code length extcodecopy(_addr, add(o_code, 0x20), 0, size) // copy code } } ```

## Contract Hash - EXTCODEHASH - <a href="https://eips.ethereum.org/EIPS/eip-1052" target="_blank">Introduced by EIP-1052</a> - Openzepplin <a href="https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/Address.sol#L24" target="_blank">isContract()</a> utility function - ``` function isContract(address account) internal view returns (bool) { bytes32 codehash; bytes32 accountHash = 0xc5d2...ad8045d85a470; assembly { codehash := extcodehash(account) } return (codehash != accountHash && codehash != 0x0); } ```