BasicMetaTransaction.sol

pragma solidity 0.6.0;

import "@openzeppelin/contracts/math/SafeMath.sol";

contract BasicMetaTransaction {

    using SafeMath for uint256;

    event MetaTransactionExecuted(address userAddress, address payable relayerAddress, bytes functionSignature);
    mapping(address => uint256) private nonces;

    function getChainID() public pure returns (uint256) {
        uint256 id;
        assembly {
            id := chainid()
        }
        return id;
    }

    /**
     * Main function to be called when user wants to execute meta transaction.
     * The actual function to be called should be passed as param with name functionSignature
     * Here the basic signature recovery is being used. Signature is expected to be generated using
     * personal_sign method.
     * @param userAddress Address of user trying to do meta transaction
     * @param functionSignature Signature of the actual function to be called via meta transaction
     * @param sigR R part of the signature
     * @param sigS S part of the signature
     * @param sigV V part of the signature
     */
    function executeMetaTransaction(address userAddress, bytes memory functionSignature,
        bytes32 sigR, bytes32 sigS, uint8 sigV) public payable returns(bytes memory) {

        require(verify(userAddress, nonces[userAddress], getChainID(), functionSignature, sigR, sigS, sigV), "Signer and signature do not match");
        nonces[userAddress] = nonces[userAddress].add(1);

        // Append userAddress at the end to extract it from calling context
        (bool success, bytes memory returnData) = address(this).call(abi.encodePacked(functionSignature, userAddress));

        require(success, "Function call not successful");
        emit MetaTransactionExecuted(userAddress, msg.sender, functionSignature);
        return returnData;
    }

    function getNonce(address user) external view returns(uint256 nonce) {
        nonce = nonces[user];
    }

    // Builds a prefixed hash to mimic the behavior of eth_sign.
    function prefixed(bytes32 hash) internal pure returns (bytes32) {
        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
    }

    function verify(address owner, uint256 nonce, uint256 chainID, bytes memory functionSignature,
        bytes32 sigR, bytes32 sigS, uint8 sigV) public view returns (bool) {

        bytes32 hash = prefixed(keccak256(abi.encodePacked(nonce, this, chainID, functionSignature)));
        address signer = ecrecover(hash, sigV, sigR, sigS);
        require(signer != address(0), "Invalid signature");
		return (owner == signer);
    }

    function msgSender() internal view returns(address sender) {
        if(msg.sender == address(this)) {
            bytes memory array = msg.data;
            uint256 index = msg.data.length;
            assembly {
                // Load the 32 bytes word from memory with the address on the lower 20 bytes, and mask those.
                sender := and(mload(add(array, index)), 0xffffffffffffffffffffffffffffffffffffffff)
            }
        } else {
            return msg.sender;
        }
    }
}