Supporting Meta Transactions
All Uniswap V2 pool tokens support meta-transaction approvals via the permit function. This obviates the need for a blocking approve transaction before programmatic interactions with pool tokens can occur.
In vanilla ERC-20 token contracts, owners may only register approvals by directly calling a function which uses
msg.senderto permission itself. With meta-approvals, ownership and permissioning are derived from a signature passed into the function by the caller (sometimes referred to as the relayer). Because signing data with Ethereum private keys can be a tricky endeavor, Uniswap V2 relies on ERC-712, a signature standard with widespread community support, to ensure user safety and wallet compatibility.
keccak256( abi.encode( keccak256('EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)'), keccak256(bytes(name)), keccak256(bytes('1')), chainId, address(this) ));
keccak256('Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)');`