Is naked addition and subtraction safe in transfer and transferFrom?

Question

I'm implementing my own ERC20 token and trying to figure out best practices by looking at existing implementations. I found that DAI has addition and subtraction functions that appear to prevent integer overflow in the transfer and transferFrom methods.

function add(uint x, uint y) internal pure returns (uint z) {
    require((z = x + y) >= x);
}
function sub(uint x, uint y) internal pure returns (uint z) {
    require((z = x - y) <= x);
}

Whereas the Consensys reference implementation just subtracts and adds balances without performing any such check.

    balances[_to] += _value;
    balances[_from] -= _value;

My intuition is that my token doesn't need these methods, as it has a fixed supply far less than MAX_UINT256. Am I missing an attack vector? Is there a more authoritative source to copy?

Answer 1

That contract you mentioned is 3+ years old and an Integer overflow & underflow attack was discovered after it (https://github.com/SmartContractSecurity/SWC-registry/blob/master/entries/SWC-101.md), you shouldn't trust that repo, it is unmaintained. Use openzeppelin's SafeMath for arithmetic operations if you need it