library ECDSA {
    /**
     * @dev Recover signer address from a message by using their signature
     * @param hash bytes32 message, the hash is the signed message. What is recovered is the signer address.
     * @param signature bytes signature, the signature is generated using web3.eth.sign()
     */
    function recover(bytes32 hash, bytes memory signature)
        internal
        pure
        returns (address)
    {
        bytes32 r;
        bytes32 s;
        uint8 v;

        // Check the signature length
        if (signature.length != 65) {
            return (address(0));
        }

        // Divide the signature in r, s and v variables with inline assembly.
        assembly {
            r := mload(add(signature, 0x20))
            s := mload(add(signature, 0x40))
            v := byte(0, mload(add(signature, 0x60)))
        }

        // Version of signature should be 27 or 28, but 0 and 1 are also possible versions
        if (v < 27) {
            v += 27;
        }

        // If the version is correct return the signer address
        if (v != 27 && v != 28) {
            return (address(0));
        } else {
            // solium-disable-next-line arg-overflow
            return ecrecover(hash, v, r, s);
        }
    }

    /**
     * toEthSignedMessageHash
     * @dev prefix a bytes32 value with "\x19Ethereum Signed Message:"
     * and hash the result
     */
    function toEthSignedMessageHash(bytes32 hash)
        internal
        pure
        returns (bytes32)
    {
        return
            keccak256(
                abi.encodePacked("\x19Ethereum Signed Message:\n32", hash)
            );
    }
}
function recover(bytes32 hash, bytes memory signature)
    internal
    pure
    returns (address)
{
    bytes32 r;
    bytes32 s;
    uint8 v;

    // Check the signature length
    if (signature.length != 65) {
        return (address(0));
    }

    // Divide the signature in r, s and v variables with inline assembly.
    assembly {
        r := mload(add(signature, 0x20))
        s := mload(add(signature, 0x40))
        v := byte(0, mload(add(signature, 0x60)))
    }

    // Version of signature should be 27 or 28, but 0 and 1 are also possible versions
    if (v < 27) {
        v += 27;
    }

    // If the version is correct return the signer address
    if (v != 27 && v != 28) {
        return (address(0));
    } else {
        // solium-disable-next-line arg-overflow
        return ecrecover(hash, v, r, s);
    }
}

/**
 * @dev Recover signer address from a message by using their signature
 * @param hash bytes32 message, the hash is the signed message. What is recovered is the signer address.
 * @param signature bytes signature, the signature is generated using web3.eth.sign()
 */